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The Mono Basin Ecosystem: Effects of Changing Lake Level (1987)

Chapter: 4. Biological System of Mono Lake

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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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Suggested Citation:"4. Biological System of Mono Lake." National Research Council. 1987. The Mono Basin Ecosystem: Effects of Changing Lake Level. Washington, DC: The National Academies Press. doi: 10.17226/1007.
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4 Biological System of Mono Lake INTRODUCTION Mono Lake is a productive aquatic ecosystem but with very few species. The lake has two major habitats--an open water pelagic region and a nearshore littoral region. Trophic structure, the linkages within the food web, is different in these two habitats. In the pelagic waters, phytoplankton are the primary producers, using sunlight to reduce inorganic carbon to organic matter. These algae are grazed by the brine shrimp, Artemia monica, which are preyed upon mainly by eared grebes (Podiceps nigricoZlis) and California gulls (Laws californicus). No fish live in Mono Lake. The cur- rent combination of high salinity and alkalinity makes it impossible for fish to survive. Inputs of organic material to the profundal sediments, those sediments under the pelagic zone, consist largely of fecal pellets and cysts of brine shrimp and detritus. No zoobenthos has been record- ed in the profundal sediments, which are anoxic much or all of the year. The role of protozoans and bacteria as food for brine shrimp or as decomposers of organic matter remains undetermined, but is likely to be of importance. In the littoral region, the overlying waters have the same planktonic organisms as the pelagic zone and an intermittent complement of organisms associated with the bottom. The benthic habitat is highly variable as a func- tion of depth and substrate (Herbs", 1986; Pelagos Corpora- tion, 1987~. The principal constituents are a microbial 69

70 The Mono Basin Ecosystem community, an algal flora, and a brine fly, Ephydra hians, which feeds upon the benthic algae and probably bacteria and detritus derived from a number of sources, some likely to be terrestrial. The brine fly is prey to a variety of birds including phalaropes, and to a lesser extent eared grebes and gulls. Although the trophic structure of Mono Lake is simple in comparison with that in many aquatic ecosystems, the lack of sufficient information on key components such as bacteria and protozoans precludes the formulation of a complete, quantitative description of carbon or nitrogen _~ ~ ~ . Two trophic links that have received some quantitative attention are the algae- brine shrimp and brine shrimp-bird links. Grazing by brine shrimp contributes to a decline in phytoplankton during the spring and maintains a low algal abundance during the summer (Lenz, 1982, Jellison, 1985~. The regeneration of ammonium by the brine shrimp, in turn, sustains the growth of the phytoplankton (Jellison and Melack, 1986~. The decline in brine shrimp in the autumn can be, in part, attributed to predation by the grebes (Cooper et al., 1984~. The remainder of this chapter discusses the ecological and physiological aspects of the components of the food web--and primary producers and decomposers (bacteria, phytoplankton, and phytobenthos), primary consumers (brine shrimp and brine fly), and secondary consumers (aquatic bird populations). flow through the whole food web ECOLOGICAL ASPECTS OF AQUATIC PELAGIC AND LITTORAL ORGANISMS Primary Producers and Decomposers Bacteria The abundance and significance of bacteria in alkaline, saline lakes are not well-known. Bacteria probably func- tion as both decomposers and primary producers in the food web of Mono Lake. Recent research by R. S. Oremland and his associates indicates that the same major processes that are carried

Biological System of Mono Lake 71 out by anaerobic bacteria in fresh water and marine habi- tats also occur in alkaline, saline lakes. They have exam- ined, for example, methanogenesis, sulfate reduction, and other anaerobic processes in Big Soda Lake, Nevada (Orem- land et al., 1982, 1985; Iversen et al., in press). In Mono Lake, R. S. Oremland (U.S. Geological Survey, Menlo Park, personal communication) has discovered that large quanti- ties of methane are leaving the sediments even though relatively small amounts of methane are produced at the sediment-water interface. He argues that most of the methane-rich gas seeps in the lake produce biogenic meth- ane that is derived from the anaerobic decomposition of fossil organic matter by bacteria. However, the methane from one seep associated with a hot spring had a more thermogenic character, indicating a chemical process that does not involve bacteria. The presence of numerous gas seeps on the floor of Mono Lake is supported by the dis- covery that large areas of bottom sediments are disturbed by gas bubbles (Pelagos Corporation, 1987~. Pelagic, aerobic bacteria are often abundant in alkaline, saline lakes. In freshwater lakes and in the ocean, con- centrations of pelagic bacteria between 105 and 1 o6 bac- teria/ml are commonly observed. . . . , . ~ . . . .. However, alkaline, saline lakes In east At~r~ca contain from 107 to 108 bacteria/ml (Kilham, 1981~. These large concentrations presumably rep- resent a balance between the availability of organic sub- strates in these highly productive lakes and the abundance of heterotrophic organisms that consume bacteria (e.g., ciliates). Pyramid Lake in Nevada is the only alkaline, saline lake in the Great Basin in which a detailed study of bacteria has been carried out. Hamilton-Galat and Galat (1983) found from 5.1 x 105 to 2.5 x 107 bacteria/ml in Pyramid Lake. Bacterial numbers more or less tracked periods of algal production. One reason that bacterial numbers are not higher in Pyramid Lake is that the lake is only moderately productive (i.e., mesotrophic). For Mono Lake, R. S. Oremland (personal communication) and R. W. Harvey (U.S. Geological Survey, Menlo Park, personal com- munication) have observed bacterial concentrations of between 1.4 and 2.0 x 107 bacteria/ml. On average, these concentrations are considerably higher than most found in

72 The Mono Basin Ecosystem Pyramid Lake and generally similar to those observed in the lakes in east Africa. Phyto plank ton and Phyto b e nthos The algal community of Mono Lake includes few species, as is typical of hypersaline waters. The phytoplankton is dominated by a coccoid green alga, NannochZoris sp., cyan- obacteria, and diatoms (Mason, 1967; Lovejoy and Dana, 1977; Melack, 1983~. The benthic algae are composed of Nitzschia frustum, other less common diatoms, filamentous cyanobacteria, and the green alga, Ctenocladus circinnatus (Herbs", 1986~. The seasonal dynamics of the phytoplankton in Mono Lake are unusual (Mason, 1967; Lovejoy and Dana, 1977; Melack, 1983, 1985; Jellison and Melack, in press) (Figure 4. 1~. During the winter, the phytoplankton are abundant throughout the lake, and after the onset of the seasonal thermocline in early spring, the algae increase in the upper water. This increase was reduced during 1984, 1985, and 1986 after the initiation of meromixis. As described in chapter 3, the chemical stratification reduced vertical mix- ~ -- ~ ~ ~ ~~ ~~ nutrient, nitrogen, to the euphotic zone. A rapid decrease in algal abundance occurs in late May and June above the thermo- cline. During the summer, the phytoplankton are sparse in the upper waters and abundant in the deeper, cold and dim or dark waters. In midsummer, higher chlorophyll concen- trations occur in a layer coinciding with the chemocline. In autumn, algal concentrations increase in the upper waters as thermal stratification weakens and brine shrimr, sing, wn~cn reoucecl the supply of the limiting numbers decline (Figure 4.1~. Primary productivity measurements spanning the period from 1983 to 1985 vary from 340 to 540 g carbon/m2/yr (Jellison and Melack, in press). Mono Lake would thus be classified as eutrophic. Production was higher during the spring of 1983 than in 1984 and 1985; the difference may be at least partially attributed to meromixis.

Biological System of Mono Lake 60~ - CL O 30 o I O 82 /~` 86 73 , i, . , i, I `, , I 1 1 1 _ 'v' WN I~ · ·- I I~ at/ 1 J F M A M J J A S O N D TIME (months) FIGURE 4.1 Mean mixolimnetic chlorophyll a for 1982, 1983, 1984, 1985, and 1986. Primary Consumers Zooplankton The Mono Lake brine shrimp, Artemia monica, is the major zooplankton species (Mason, 1967; Lenz, 1980, 1982~. A. monica, a member of the A. franciscana superspecies, is now considered a sibling species (Bowen et al., 1985~. The zooplankton also includes protozoans and has included roti- fers (Mason, 1967~. The abundance of brine shrimp in Mono Lake varies seasonally (Lenz, 1982, 1984; Figure 4.2~. The brine shrimp hatch from overwintering cysts from January through May. By mid-May, the first adult brine shrimp are present. For

74 The Mono Basin Ecosystem 50 45 40 35 - Standard Enora 55 ~ Naupill ~ ~ ~ I I I~Adults O~ in J F M A M J J A S O N MONTH . , D FIGURE 4.2 Seasonal abundance of brine shrimp at Mono Lake in 1985. Lakewide mean of 10 stations (three vertical net tows per station). approximately one month females bear live young, which mature rapidly in the warm upper mixed layer. In June females switch to oviparous reproduction. The diapause eggs lie dormant on the bottom of the lake until the fol- lowing winter. During the summer, brine shrimp are abun- dant in the oxygenated upper waters and very sparse or absent in the anoxic deeper waters. By September, the brine shrimp begin to decline in numbers and are almost absent from the plankton by December. Studies conducted with similar methods since 1978 per- mit interannual comparisons of brine shrimp abundances and reproductive characteristics (Lenz, 1984; G. Dana, R. Jellison, and J. M. Melack, University of California, Santa Barbara, unpublished). Statistically significant interannual

Biological System of Mono Lake 75 differences in abundances of first-generation adults (late May to June populations) occurred. In 1979, 1984, and 1986 brine shrimp numbered between 19,000 and 31,000 ani- mals/m2. whereas from 1980 to 1983 numbers were only . . ~ · · . . 2,400 to 5,700 an~ma~s/m~. dances (first- and second-generation adults) were much higher in 1981 and 1982 than in other years. A number of factors are associated with these variations. First- generation adult abundances denend on the number of cysts Maximum midsummer aoun- . — available for hatching, hatching success, and survival to adulthood. In laboratory experiments, Dana and Lenz (1986) determined that salinities in the period from 1979 to 1986 are not indicated as a cause for changes in hatching success. Emergence trap trials in spring 1985 showed that ~ ~ . ~ . ~ . ~ . . ~7 ~ J very low hatching occurred In sediments In anoxlc water below the chemocline (Dana et al., in press). In contrast, large numbers of cysts lying in sediments under the oxy- genated mixolimnion hatched. The number of cysts available depends on the production of cysts during the previous year and possibly past years and on the viability of the cysts. Cyst production is related to brood size, numbers of ovigerous females, the percentage of those females producing cysts, and the time interval between broods. Brood size varied from 30 to 140 eggs per brood from 1983 to 1986 and is explained primarily by differences in female length and algal abundance. Second-generation abundances depend on the abundance, percent ovoviviparity, and fecundity of the first-generation females. Recruitment to adults depends on survival of naupliar and juvenile stages. Differences in all these factors occurred from 1982 to 1986. The switch from ovoviviparity (live bearing) to oviparity (cyst production) occurred at the time of dec- reasing phytoplankton in all years studied. In years with a substantial spring hatch, the first generation dominates the population. When the spring hatch is relatively low, first- generation adults are less abundant, algal densities remain higher later into the spring, and a large second generation can occur. The spatial distribution of brine shrimp is heterogeneous on large (square kilometers) and small (square meters) spa- tial scales and varies on time scales from hours to days to seasons (Lenz, 1982; Melack, 1985; Lenz et al., 1986; Conte

76 The Mono Basin Ecosystem et al., in press). These differences in concentrations of brine shrimp result in variable profitability of foraging for Small- spr~ngs where upwelling varies widely in strength and hence in the entrainment of brine shrimp. Mason ( 1966) hypothesized that the very dense plumes that formed near shore, but not in association with springs, result from thermal currents and behavioral responses of the animals. Foam lines con- tain concentrations of living ant! dead brine shrimp as well as other debris and can stretch for hundreds of meters. These features seem to delimit water masses. Large-scale patchiness has been documented by sampling transects and lakewide grids. Abundance differences between the eastern and western halves of the lake are common. The degree of variability differs seasonally and appears greater during transition periods such as spring and autumn. the birds (see section on bird populations below). scale patches are associated with sublacustrine . . . · ~ . - - ., Current sampling programs are designed to assess the lakewide abundance of brine shrimp and include biweekly or monthly samples from 10 pelagic stations. Regular sampling is not performed in water overlying the littoral region or at sites of aggregation such as springs. Therefore, while providing statistically sound estimates of the overall abun- dance of brine shrimp, the sampling does not include sites that may be of particular importance to some birds some of the time. No efforts are in progress to sample zooplankton other than brine shrimp. Zoobenthos The benthic community of Mono Lake includes several species of dipteran insects, as is typical of hypersaline waters. The predominant dipteran is the brine fly, Ephydra hians, but other species are present, such as the deer fly (Chrysops spy and the long-legged fly (Hydrophorus plum- beus). The biting midge (Culicoides occidentalis) is also found among the macroinvertebrates (Herbs", 1986~. The seasonal dynamics of the macroinvertebrates are not well-known. However, recent research on brine flies by Herbst (1986) using the third instar and pupae as popula- tion~ indices showed a phase of rapid population growth

Biological System of Mono Lake 77 occurring in the spring (May and June), a summer maximum (July through September), a gradual decline in the autumn, _ _ : ~1 ~ ~ late winter through early spring. Since seasonal dynamics of the phytobenthic bac- terial and algal populations (diatoms and filamentous algae) are unknown, one cannot determine if the zoobenthic com- munity, as reflected by numbers of brine flies, is tracking periods of algal production. The spatial distribution of brine flies is heterogeneous on large (square kilometers) and small scales (Herbs", in press). . ~ · ~ and minimal abundance ~ rom , (square meters) Small-scale patches are associ- ated with tufa pinnacles and nearshore grasses, which are excellent substrates for larval and pupal attachment. Large-scale patchiness has recently been documented by video and lakewide bathymetric transects (Pelagos Corpora- tion, 1987~. Large mats of pupae have been found on dead submerged grasses along the eastern shore and on under- water tufa and hard-surface sediments at depths sometimes greater than 10 m in the central and eastern provinces, as shown in Figure 4.3. Abundance differences observed be- tween tufa-hard rock shoal regions and soft mud-sand lake bottom sediments common to the eastern and central prov- inces are probably due to the larvae's inability to attach to smooth surfaces. The placement of eggs by brine fly females at depths greater than a few meters has been ob- served (Pelagos Corporation, 1987~. This observation raises questions about the typical mechanism of oviposition re- ported for other ephydran flies, including whether females utilize respiratory mechanisms other than gas bubble entrapment for vertical descent. If the lake level dropped, the loss of hard-surface sediments would reduce brine fly habitat. PHYSIOLOGICAL ASPECTS AND SALINITY TOLERANCES OF AQUATIC PELAGIC AND LITTORAL ORGANISMS Primary Producers Two kinds of evidence are available to evaluate the effects of increased salinity on phytoplankton: (1) algal responses to experimental increases in the salinity of Mono

78 The Mono Basin Ecosystem /~ EphydrahiansMats ' 1 Paoha _\ z / \ \` Island I / \~ \\ ~/O~,N<O ~.~ ~, ~5 LEE DINING \~\ \ ~ J FIGURE 4.3 Locations of mats of brine fly (Pelagos Cor- poration, 1987~. Lake water and (2) salinity tolerances of phytoplankton known to occur in saline waters. Melack (1985) and Melack et al. (1982) studied microcosms containing Mono Lake water and microflora, some with brine shrimp and some without. These experiments, initiated in March 1981, gradually increased the salinity to 1.5 times the lake's salinity at the time (97 g/1) over a 3-month period. The results indicate a 10 percent decrease in gross primary productivity of microcosms with each 10 percent increase in salinity over the range of approximately 97 to 140 g/1 TDS. Chapman (1982) reported results from laboratory experiments with isolates of the dominant algae--a green alga and a diatom--in synthetic and concentrated Mono Lake water. At 150 g/1, diatom growth was 95 percent of that at 97 g/1. Diatoms did not survive at a salinity of 185 g/1. At 150 g/1, the green alga growth was 74 percent of that at 97 g/1. The green alga continued to show growth beyond 185 to at least 237 g/1. Total salinity,

Biological System of Mono Lake 79 arsenic, and fluoride contribute to these reductions in growth. Overall, Chapman's results indicate that a salinity of about 175 g/1 results in fairly large decreases in growth of the two currently dominant algae. A number of species of phytoplankton, not currently extant or dominant in Mono Lake, are known to grow well at salinities reaching 200 g/1 (Hammer, 1986; Melack, in press). In particular, a green alga, DunalieZ/a sp., now occurs in low numbers in Mono Lake, and a related species, D. parka, grew best in Mono Lake concentrated to 150 g/1 . · . and still grew slowly at 235 g/1 (Chapman, 1982). Herbst (1986) isolated a clone of Ctenocladus circinnatus from Mono Lake and determined its growth rate and yield at salinities of 25, 50, 75, 100, and 150 g/1. The solutions were obtained by dilution or low-temperature evaporation of Mono Lake water. The experiments indicated decreased growth and yield at 75 and 100 g/1 and no growth at 150 g/l. Primary Consumers · — A fundamental requirement for aquatic organisms living in saline lakes is to have sufficient "free water," or water that is available to sustain vital cellular activities. (In aquatic organisms, water molecules forming hydration shells are termed "bound water"; water molecules not associated with these shells are termed "free water.") This biological axiom is most evident in the osmotic effects of salinity upon the growth and larval development of brine shrimp and brine flies. If the salinity of the medium in which the organisms reside is sufficiently high, the thermodynamic forces responsible for the osmotic gradient will not allow sufficient free water to remain inside the organism. The loss of free water will in turn cause an inhibition or ces- sation of metabolic processes. This physical relationship sets an absolute upper limit on the salinity of salt lakes in which a self-sustaining population of halophilic organisms such as brine shrimp and brine flies can persist. Physiological solutions within and surrounding cells are primarily dilute aqueous solutions. These solutions contain a large amount of free water and behave in a manner

80 The Mono Basin Ecosystem similar to pure water (PO), which has predictable physical and chemical properties. Environmental salinities can cause organisms to alter their physiological solutions such that many of the internal fluids tend to become concentrated and the available free water diminishes. This is caused by free water molecules becoming associated with cellular solutes. Measurements of free water in both biotic and abiotic saline solutions have been made by the determination of water activity (aw) as reflected in the vapor pressure of saturated solutions relative to that of pure water (P/PO). Table 4.1 consists of values for aw, expressed as water vapor sorption ratios for various saturated salt solutions (Rockland, 1960; Winston and Bates, 1960~. Additional values for solutions below saturation i, especially sodium chloride, have been taken from studies by Lang (1967) and Clegg (1976a). The amount of free water needed to sustain vital cel- lular and subcellular activities in A. salina has been estab- lished by Clegg (1974, 1976a,b, 1978~. Upon analyzing the internal water content of various embryos, Clegg demon- strated that embryos required 0.65 g H2O/g dry weight to be viable (Table 4.2~. Activation of dormant embryos (cyst stage) required that environmental salinities have water activities (a. ~ of 0.95. ~ If water activities were lower, for instance below 0.93, only partial developmental activities were restored. The embryo could achieve only a degree of hydration that was equivalent to the aw of the environ- ment and subsequently would have to remain quiescent, as shown in Figure 4.4. This value of 0.95 is also where the volume of tissue liquid gained by passive equilibrium pro- cesses is sufficient to sustain vital life functions needed in organ system development. If environmental water activity values go above aw = 0.97, the rate of water uptake is greater than predicted by passive equilibrium conditions with environmental aw and represents another water uptake mechanism. This latter mechanism Is an active transport system and requires the coupling of energy with ion trans- port. The movement of free water is coupled with transit of monovalent ions, sodium, and potassium. The free water requirements for larval and adult stages of brine shrimp living in various environmental salinities

Biological System of Mono Lake 81 TABLE 4.1 Water Activity, aw, of Saturated Salt Solutions and Equilibrium Hydration Levels for Individuals of CaSO4- dried A. salina Cysts Incubated in the Vapors of NaC1 Solutions Water Activity aw (P/Po) Individuals Measured (,~1 H2O/cyst x 103 + S.D.) (n = 20) Saturated Salt Solutions at 29°C LiC1 0.116 2.247 + 0.223 Na2Cr2o7 0.525 1.619 + 0.301 NaC1 0.755 1.284 + 0.143 KC1 0.845 0.992 + 0.218 KNO3 0.913 0.901+0.138 NaC1 Solutions at 23°C 88 g/1 117 g/1 176 g/1 234 g/1 293 g/1 0.95 0.93 0.89 0.35 0.31 SOURCES: Winston and Bates, 1960; Rockland, 1960; Lang, 1967; Clegg, 1976a. have been established by the studies of Croghan ~ 1958) and Conte et al. (1972, 1977~. Brine shrimp maintain an uptake of water by metabolically controlling osmotic desiccation. Osmotic desiccation is caused by the loss of free water through semipermeable membranes surrounding living

82 The Mono Basin Ecosystem TABLE 4.2 Metabolic Activities in Cysts as a Function of Hydration Level NaC} Solution (g/l) Hydrationa (g H2O/g dry weight cysts) Metabolic Statusb 0.65 117 0.50 205 0.42-0.33 293 0.28 Conventional metabolism, same events occur as in fully hydrated cysts in the presence of oxygen. In the absence of oxygen, no carbohydrate metabolism occurs, slow catabolism of diguanosine - tetraphosphateC. PHi decreases by >1 unita7 No respiration, some amino acid metabolism, decrease in glycogen No data Decrease in active transport system aClegg (1974, 1976a). bClegg (1978~. CStocco et al. (1972~. dBusa et al. (1982~. Organisms. This outward toss of water is offset by coupling the passive inward diffusion of ions carrying water with selective removal of sodium and chloride ions (Come, 1977~. Figure 4.5 compares the osmotic potential of changing environmental salinities in terms of the move- ment of free water molecules being translocated across a membrane along with each ion transported into the

Biological System of Mono Lake 83 1.2 1.0 In In 0.8 o :C 0.6 0.4 0 1 I .5 m It To l .5 J 1 _ l - NONVIABLE CYST HYDRATION — is l - ~Y I At/ 0~2 0.8 a 0.9 w 1.0 FIGURE 4.4 A comparison of the hydration behavior of viable (~) and nonviable (o) cysts. The cysts were ren- dered nonviable by exposure to ammonia vapors. The inset shows the relationship between the hydration of viable and nonviable cysts at specific water activities (aw) (Clegg, 1978~.

84 400 300 c, O O ~ 200 O ~ , O IL o The Mono Basin Ecosystem - 0.54 1.08 1.61 2.15 2.69 3.23 3.76 CONCENTRATION OF NaCI (M) FIGURE 4.5 Water transport per ion translocated across the membrane as a function of solute concentration (Come, 1977~. Organism. The bioenergetic cost for maintaining the coup- led ion-water exchange increases dramatically when an en- vironmental water activity of 0.93 (equivalent to a salinity of 117 g NaCl/l) is reached (Come, 1984~. In summary, the lower limit of environmental water activities (aw) appears to be O.9S for creating the internal fluid level of 0.65 g H2O/g dry weight needed for resump- tion of vital cellular processes of brine shrimp. The effects of embryonic hydration on the other biological parameters are shown in Table 4.2. Zooplankton Reproduction and Development. The dynamics of larval brine shrimp populations can be strongly influenced by the physiological mechanisms that control the development of

Biological System of Mono Lake 85 individual embryos. The most influential physical factors are salinity and temperature. The Mono Lake brine shrimp species. Artemia monica. is dioloid and bisexual. ~~~ _~ ~ Fertilization occurs when sperm enter the ovisac after copulation. Work on strains of brine shrimp from San Francisco and Utah indicates that sperm are not stored within females (Bower, 19621. Subsequently, ~ ~ . ~ . ~ . ~ ~ . ~ ~ , . i. cleavage ot the zygote begins, and embryonic Development can proceed along either of two lines. The female brine shrimp can retain the developing embryos within the ovisac and give birth to live, fully swimming naupliar larvae (the ovoviviparous pathway), or the female can interrupt embryogenesis by covering the embryos with a thick pro- tein shell that inhibits development and then release dor- mant embryos as cysts (oviparous pathway). The control of the maternal shell gland and the factors that make the shell "impermeable," which allow the formation of a devel- opmentally arrested embryo (cyst), are not completely understood. However, In some cysts ot A. montca the breaking of dormancy appears to be triggered at release, since nauplli are produced shortly after cysts enter the water. In other circumstances, the cysts need to be "ac- tivated" before dormancy is broken. The activation process appears to depend upon the amount of free water and oxy- gen in the environment. Because the cysts of A. monica, unlike those of other brine shrimp, sink (Dana and Lenz, 1986), factors influencing environmental conditions near the bottom are of critical importance in the activation of these cysts. Embryos that have become dormant because of develop- mental inhibition live in one of two distinct conditions: a diapause state or a quiescent state. Embryos under dia- pause conditions are kept dormant by endogenous factors (Drinkwater and Crowe, 1986) that prevent further resump- tion of development even under favorable environmental conditions. ~~~ne o~apauseo embryo requires an activation stimulus that releases the embryo from the endogenous constraints and allows favorable environmental conditions to initiate the resumption , %, ., . lo, ., . . . , ~ , · .— ~ - ~ . ~ ~ ,~ .. , — of development. The quiescent embryo has usually been delayed by simple environmental constraints (lack of water, lack of oxygen, or low tempera- ture) that prevent development. It does not require any

86 The Mono Basin Ecosystem auxiliary activation signal. In ephemeral saline lakes, osmotic desiccation and anoxia are examples of environ- mental conditions found that produce quiescence. When environmental conditions become satisfactory, such that the quiescent embryo can be fully hydrated and oxygenated, quiescence is usually broken and the embryo resumes full development to hatching. Mono Lake, being a large, deep salt lake, does not fluc- tuate widely in salinity or dry up completely. Therefore, environmental conditions under which cyst development and termination occur are unlike those found in shallow saline lakes. For instance, because cysts from A. monica sink, they are always under conditions of hydration. If cysts reside on the lake bottom in deep water, they have the additional environmental condition of remaining in a state of anoxia. Thus, such cysts will never hatch unless the lake mixes down to the bottom. Diapause Cysts and the Role of Intracellular pH f pHi). The environmental requirements limiting the processes that con- trol emergence (splitting of the shell) and hatching (escape of the nauplius) of A. salina are reviewed by Clegg and Conte (1980~. Recent studies of A. monica diapause cysts show no difference from A. salina in the role of pHi in activation of arrested embryos. Previously published data (Busa et al., 1982; Busa and Crowe, 1983) had suggested that pHi in dormant cysts of A. salina was depressed. However, the cysts they used were not really in diapause; truly diapaused A. salina cysts, like those of A. Monica, may have elevated pHi (Drinkwater and Crowe, 1987), Intracellular pH affected critical preemergent metabolic pathways, and pHi may be involved in the breaking of dia- pause in A. salina and in A. monica (Drinkwater and Crowe, 1987~. Diapause Cysts and the Role of Temperature. An important feature of diapause regulation in A. monica is that cold creates conditions that allow typical preemergence carbo- hydrate and nuclectide metabolism to occur, which in other Artemia species is normally inhibited at lower temperatures.

Biological System of Mono Lake 87 Dana (1981) showed that release from diapause could be achieved by a long cold-hydration period (>90 days) at 5°C under anoxic conditions, and this release was not depen- dent on either a cycle of hydration/dehydration or a cycle of oxygenation (although oxygen is still required to end quiescence). The mechanism of this release from diapause is not understood. Thun and Starrett (1986) confirmed these findings and found that at 4°C and 90 g/1 maximal hatching was achieved after 90 days. If cold-treated cysts were removed before the 90-day period and allowed to con- tinue development at elevated temperatures (10°C) in the presence of oxygen at identical salinity, there was no increase in the number of nauplii that emerged or hatched. If the salinities of the media after the cold-hydration period were increased from 90 to 140 g/1 and 90-day cold- treated CYStS were reintroduced into these higher salinities .. . . .. ~ . . . . .. Osmotic desiccation occurred and emergence and hatching of embryos diminished. Drinkwater and Crowe (1986) reported that carbohydrate metabolism occurring during the cold-hydration period prepares A. monica embryos for preemergence in a manner similar to that reported for A. franciscana at elevated tem- peratures (Clegg, 1964~. These biochemical changes include the breakdown of trehalose into glycerol, which normally occurs only under aerobic conditions and at higher temper- atures in those cysts that are terminating diapause. In addition, cold-hydrated A. monica cysts can synthesize ~~ ~ ~ ~ ~~ to those found In other species of Artemia at higher temperatures and Therefore, it Would seem that the Mono Lake brine shrimp has evolved controls that act differently from those of other Artemia species in avoiding the inhibiting effects of low temperatures on pre- emergence mechanisms. 7 ~ .~ —- -- - - -- — - _ _ _ _ _ __ cysts organic acids and glycogen at levels similar · . ~ · ~ ~ . · . ~ · ~ oxygen levels (Conte et al., 1980~. Fluidity and Circulation. One functional prerequisite for the survival of brine shrimp is the formation of a circula- tory system. The circulatory system provides a transport fluid containing essential nutrients and growth factors vital to the rapidly growing and differentiating embryonic cells. Additionally, the toxic metabolic wastes created by these

88 The Mono Basin Ecosystem cells are removed by the circulatory fluids because the fluids are constantly flushing the spaces surrounding the cells with large quantities of free water. Drinkwater and Crowe ( 1986) have measured the maximal amount of bound water in A. monica cysts. In a compari- son with other populations within the superspecies A. fran- ciscana, both species showed that the population of cells in the cysts contained 0.297 g ti2~/g . ~~ dry weight of cells. This is the limiting value of the water of hydration that protects the viability of brine shrimp cells. One can pre- dict that increasing the amount of fluidity, by increasing the content of free water within tissue and cellular com- partments up to a value of 0.60 g H2O/g dry weight of cells, will allow restricted metabolism to occur and will allow the physiological functions of termination of diapause, gastrulation differentiation, and formation and preemergence of the nauplius. The mechanism for provid- ing the entrance of free water in the restricted metabolism stage of the quiescent embryo is dependent upon internal glycerol gradients that offset osmotic desiccation. This mechanism remains successful as long as the membranes surrounding the embryo do not leak glycerol. Hatching of cysts does not occur unless the content of free water reaches 0.60 g H2O/g dry weight of the cells. Hatching destroys the glycerol-impermeable membranes and with it the passive mechanism of water balance in the growing embryo. A new active mechanism of ionic (Na+) excretion via an enzymatic Na,K-ATPase membrane pump is initiated in the free swimming nauplius (Come, 1984~. It is this ATP-utilizing enzymatic mechanism that transports water into the nauplius against the osmotic gradient. This sodium pump provides the additional free water that accounts for the elevation of embryonic water content (>1.2 g H2O/g dry weight) above aw values predicted from equi- librium measurements of the saline medium (Clegg, 1978~. The increase in prenaupliar water content provides for more tissue fluidity and allows unrestricted oxidative metabolism. In turn, naupliar circulation and transport of food substrates are enhanced, furnishing optimal conditions for the initiation of other physiological activities, such as locomotion, feeding, and digestion.

Biological System of Mono Lake 89 Enzymatic assays for the existence of the sodium pump in A. monica have not been made. However, indirect evi- dence for the existence of a sodium pump in A. monica nn~,nlli ~nr1 adults comes from the studies by Dana and Lenz (1986) during larval growth at various salinities of Mono Lake water. Their findings, based upon survival, do_ _ ~ ~ _ _ _ growth, and the development of the nauplius to the juven- ile stage, show that salinities above 133 g/1 (TDS) signif- icantly depress these life processes. Nauplii living in lower salinities were affected less and were capable of continued postabdominal development of swimming legs and subsequent metamorphosis into juveniles and adults. Since feeding of nauplii occurred! as part of the experimental treatment, it would appear that food capture, assimilation, and digestion were not adversely affected. Bioenergetics and Fecundity. The metabolism of food pro- vides the developing embryo with the energy necessary to balance the various energy-utilizing reactions. The produc- tion of a circulatory fluid having a low salt content requires distillation of ingested brines, a process that has a high energetic cost. Dana and Lenz (19X6) evaluated the reproductive poten- tial of laboratory-reared A. monica, Mono Basin brine shrimp, with unlimited food supply. Young reproductive adults were paired to assess the effect of salinity on reproduction. The number of eggs per brood and hatching rate of cysts appeared to be reduced at salinities above 1 18 g/l- (TDS). Low hatching was observed at a salinity of 133 g/l, and no hatching at a salinity of 159 g/1. Reproductive potential as judged by egg formation, egg fertilization, egg number, mode of development of fertilized egg, and female mortality appears to be adequate until increased salinity (osmotic desiccation) lowers water content (aw) below 0.95 g H2O/g dry weight. Above these salinities, the amount of energy available for reproduction is reduced because egg formation and development are lower priorities than the need for free water for maintaining the circulatory system. ~ ~ ~ ~ . . .

JO Zoobenthic Organisms The Mono Basin Ecosystem Reproduction and Development. The principal species of zoobenthos at Mono Lake is the brine fly, Ephydra hians. The population is diploid and bisexual, as are other popula- tions of ephydrids found in the Great Salt Lake and Great Basin drainage area (Collins, 1975, 1 980a,b). The female has a spermatheca for storing sperm after copulation. The female provides special nutrients for the oocytes via acces- sory nurse cells, allowing vitellogenesis, the deposition of yolk in the lower parts of the ovariole, and a very large egg. She also provides a chorion for attaching the eggs to the hard substrates where they are laid. The vitelline membrane surrounds the outer layer of the oocyte and is ~ . ~ ~ . . ~ ~ ~ . - - . ~~ ~ ~_ _~_ _ . . 1 ne memorane serves as the protective barrier between the environment and embryo. Oviposition (egg-laying) behavior by female brine flies Influences their spatial distribution. The newly laid, fer- tilized egg is a hydrated embryo covered with a nonsticky, opalescent, semipermeable shell. If the egg is not kept fully hydrated, it will die. Therefore, the selection by the female of a suitable benthic site for the egg is of critical importance. The embryo must be adequately protected by its environment, and a food source must be available to the larva when it emerges. The ovipositing female must also be able to breathe while she is underwater. The female fly obtains the needed oxygen by forming an air bubble that surrounds most of her body. The deposited eggs adhere to the substratum surface. When in place, the deposited egg is continuously bathed by the saline water, which furnishes oxygen while removing waste from the growing embryo. Most importantly, embryonic growth processes can proceed only if the hydration state of the early fertilized egg is maintained. The osmoregulatory mechanisms used by the fertilized egg to combat osmotic desiccation are unknown. 1ala Down at the end ot vltellooenesls - . Fluidity and Circulation. Loss of free water by osmotic desiccation in early brine fly embryogenesis results in in- creased mortality of adult and juvenile life stages. Herbst (1981) found that Mono Lake water at a salinity of 120 g/1

Biological System of Mono Lake 91 (TDS) would cause larval mortality and prolong time of development of first, second., and third insta~rs. The pathology of saline-induced larval death is not known. It may be assumed that the lack of circulating fluids for growth processes impairs development. Bioenergetics. The embryo of the brine fly uses stored yolk material far growth up to the first instar, after which the larva begins to graze upon the microbial-algal mat cov- ering the substrate. Herbst (19X6) found that a reduction of 50 percent of "normal" food rations significantly inhi- bited larval development beyond the third instar and pre- vented pupation and adult emergence. Development was best when larvae were fed microbial-algal-diatom mixtures rather than a single-algal diet. Most life history traits of the brine fly are adversely affected as osmotic stress of ~ environment the environment increases, especially when the salinities exceed 120 to 130 g/1 TDS. The detrimental effect of osmotic stress is related to the energy debt created by excretory organs working against the osmotic constraints. The energy required by the excretory organs to maintain osmotic regulation reduces the total energy available for the growth and development of any life stage. Adaptation of Brine Shrimp and Brine Fly to Changes in Salinity If brine flies and brine shrimp could adapt genetically to increased salinity and alkalinity, a lowered lake level would have a smaller effect on their populations. There are two lines of evidence suggesting that the probability that brine flies and brine shrimp would adapt to higher salinities (> 150 g/~) is remote. First, although alkaline, saline lakes in North America, Africa, Asia, and Australia with salinities greater than 150 g/1 support macroinverte- brate populations, none are as alkaline as Mono Lake. The combination of high alkalinity and salinity appears to be very difficult to adapt to. Second, whenever it has been examined, the stoichiometry of the sodium pump, which is the critical mechanism that organisms use to maintain

92 The Mono Basin Ecosystem adequate free water against strong osmotic gradients, is the same for both vertebrates and invertebrates. Thus it appears that in its function, at least, the sodium pump has little or no variability available on which natural selection can act. BIRD POPULATIONS: SECONDARY CONSUMERS At saline and alkaline lakes, birds are often conspicuous top consumers in relatively short food chains. Birds are attracted to lakes such as Mono because they provide an unusual abundance of food. The harsh chemical conditions of the lake preclude the existence there of many kinds of aquatic predators, particularly fish. Without aquatic preda- tors, populations of some aquatic invertebrates reach extra- ordinarily high densities. Nonaquatic predators, such as birds that exploit these populations, show typical numerical and functional responses to the higher concentrations of prey. Impressive concentrations of birds can result, as at Mono Lake, where several species exploit the lake's pro- duction of brine shrimp and brine fly. Some bird species exploit these abundant invertebrate In other cases birds may have a invertebrate populations as a crucial food source during portions of their annual cycle. If these food resources were dimin- ished or lost and alternative sources of food could not compensate, dependent bird populations would probably experience major limitations at the crucial stage in their life cycle when they formerly relied on the lake's resources. At Mono Lake, three species--eared grebes Wilson's populations facultatively. more obligatory reliance on lee abundant . , .. . .. phalaropes, and California gulls--may have a more or less obligatory dependence on the lake's seasonally abundant invertebrates. Each of these three species is present in extraordinarily high numbers, and each exploits brine shrimp and brine fly populations. The grebes and phala- ropes rely on Mono Lake's aquatic resources during stop- overs on their fall migrations. The gulls nest on islands in Mono Lake. Abundant food is, however, a common attrac- tion for these birds as well as others that visit the lake in

Biological System of Mono Lake 93 smaller numbers. The numbers of eared grebes, Wilson's phalaropes, and California gulls that visit Mono Lake rep- resent substantial proportions of the North American popu- lations of each species. Our main challenge is to predict the responses of these bird populations to a possible reduc- tion in their food supply at Mono Lake. If these birds are dependent on Mono Lake's food resources, changes in the lake's aquatic community could have major consequences for North American populations of all three species. If, on the ether hand the birds could switch to alternative food supplies at other locations, they could, · . — . — . ~ in the long run, Show only minor a~srup~ons In the normal dynamics of their populations. Because of the central importance of understanding the relationships between Mono Lake and the eared grebes, Wilson's phalaropes, and California gulls that visit it, sepa- rate, detailed accounts of each of these species are presen- ted. In these accounts, particular attention is paid to the estimated numbers of individuals that visit Mono Lake and their patterns of habitat and resource utilization while at the lake. On the basis of the population data available, an attempt is made to determine the relationships between the birds visiting Mono Lake and the rest of their regional and continental populations. Brief discussions of the other aquatic bird species that are present in smaller numbers or that use Mono Lakers resources more opportunistically and the nonaquatic birds that visit the Mono Basin are presented in chapter 5. Eared Grebes Eared grebes (PodFiceps nigricollis) use Mono Lake pri- marily as a stopover site during fall migration; much smal- ler numbers of grebes use the lake in other seasons. In North America, only Great Salt Lake and the Salton Sea attract as many grebes as Mono Lake. The seasonal cycle of grebe occurrence at the lake is shown in Figure 4.6. Peak numbers of grebes occur during late September, Octo- ber, and November (Winkler, 1977; Jehl, 1982a; Cooper et al., 1984), a time when grebe predation contributes to the annual decline in brine shrimp densities (Cooper et al.,

94 1,000,000 100,000 10,000 1000 _ coot J , J F M A M J J A TIME (month) The Mono Basin Ecosystem / l S O N D FIGURE 4.6 Estimates of the number of eared grebes on Mono Lake each month in a typical year (based on Winkler, 1977; Jehl, 1982a, Cooper et al., 1984~.

Bioltogical System of Mono Lake 95 1984~. The departure of grebes from Mono Lake is precipitated by the seasonal collapse of the lake's popula- tion of adult brine shrimp. Grebes would probably prolong their stay at Mono Lake, perhaps even overwintering as they do in the Salton Sea, if their food remained abundant. Annual estimates of peak grebe numbers are usually in the range of 600,000 to 900,000 (Jehl, 1982a; Cooper et al., 1984~. There are, however, major difficulties in arriving at a precise estimate of these peak numbers. Although census methods have improved since 1976, when the earliest sys- tematic efforts took place, they still fail to provide con- vincing estimates of variances. Confidence intervals around the population estimates remain unknown, making statistical comparisons between time periods difficult. Furthermore, estimates of peak numbers do not necessarily reflect the total number of grebes that visit the lake during the year. There is certain to be some turnover of grebes during the fall, so that total numbers exceed peak numbers by some unknown percentage. Information on individually recogniz- able birds suggests, however, that turnover rates may be relatively low under present lake conditions (Jehl, 1983c). The origins and destinations of the migrant grebes that visit Mono Lake each fall are not well known, and at pres- ent there are too few banding data to clarify the issue (Jehl and Yochem, 1986~. Estimates of the size of the entire grebe population in North America are much less precise than estimates of grebe numbers at Mono Lake. Totaling the more or less contemporaneous counts of grebes at major fall migration stopovers, J. R. Jehl, Jr. (Hubbs-Sea World Research Insti- tute, personal communication) estimates a continent-wide fall population of over 2.5 million, but there is no way of knowing what proportion of these birds are counted more than once as they move between the major concentration points. Fewer estimates are available for the scattered birds that do not visit the major stopover sites. Although these population figures certainly leave much to be desired, the conclusion that Mono Lake is visited by one quarter to one third of the eared grebes in North America each year seems to be justified. While at Mono Lake, grebes feed on adult brine shrimp and brine fly larvae, these two food items constituting over

96 The Mono Basin Ecosystem 95 percent of their diet from August to November (Winkler and Cooper, 1986~. Ingestion of salt while feeding on these invertebrates is not a physiological problem for grebes, nor is it likely to be over the range of salinities that brine shrimp can tolerate (Mahoney and Kohl, 1985~. Grebes swallow minimal amounts of saltwater while inges- ting their food and apparently can meet their water requirements with water obtained from their food. Eared grebes are gregarious migratory birds; they typ- ically concentrate at a few major stopover sites, especially during the autumn migration (Palmer, 1962; Cramp and Sim- mons, 1977~. While at these autumn stopover sites, grebes gain weight and, in some instances, undergo a more or less i extensive molt. It seems that the degree to which an ex- tensive molt takes place is correlated with the abundance of food at a particular stopover site. When food is very abundant, as it is at Mono Lake, the grebes remain at the stopover site for an extended period of time ant! undergo a major molt in which they replace most of their body feath- ers in about 6 weeks (Storer and Jehl, 1984~. In contrast, at less productive sites, the stopover may be shorter and the molt less extensive. It is likely that massive, rapid molting is triggered by the birds reaching a threshold in general body condition; J. R. Jehl, Jr. (personal communica- tion) has suggested this threshold may be about 50 g above the weight on arrival at a stopover point. If the bird fails to reach this threshold while at a stopover site, the molt will be delayed until the wintering area is reached. The body weight of a grebe is a good indicator of the amount of fat the bird has accumulated (Winkler and Cooper, 1986), and the weight of its fat reserves deter- mines its flight range. While visiting Mono- Lake, all eared grebes gain weight--some may more than double their weight--by adding body fat. Jehl (1982a) reports that body weight of starving grebes averages about 250 g; this sug- gests an average fat-free weight of about 225 g because 10 percent of a bird's body weight represents unusable fat reserves (Bleary, 1980~. The average grebe probably must weigh at least 275 g in order to undertake a long-distance flight. The weights of some of the earliest migrants to arrive at Mono Lake, particularly of juveniles, indicate they have essentially no usable fat reserves. They may be

Biological System of Mono Lake 97 in such poor condition that they would be incapable of successfully extending their migration very far beyond Mono Lake. Birds arriving later weigh more (>385 g), hav- ing accumulated fat earlier in their migration. Winkler and Cooper (1986) report that peak weight for eared grebes at Mono Lake reaches about 500 g. Birds of this weight have usable fat reserves far in excess of what is required for a typical migratory flight to wintering areas. The Salton Sea, the Pacific coast, and the Gulf of Cali- fornia--areas where many grebes spend the winter (Palmer, 1962--are within reach of grebes leaving Mono Lake with at least 45 to 85 g of usable fat (Figure 4.7~. The major- ity of the grebes that visit Mono Lake do not, therefore, need to become as fat as they do to successfully reach wintering areas, but deprived of food resources of Mono Lake, an unknown but smaller number of grebes would have difficulty continuing their migration. It is not known whether or not food resources during the fall in the Salton Sea, Pacific Ocean, and Gulf of California would be able to support all of the grebes that visit Mono Lake if they visited these areas earlier in the autumn than they do now. The primary feature of Mono Lake of importance to eared grebes is the abundance of brine shrimp. During August through October, eared grebes gain weight at a rate of about 3 g/day (Jehl, 1982a), and Cooper et al. (1984) estimate a consumption rate of up to 70,000 shrimp/day; these crude figures are, however, based on population av- erages and not measurements of individual weight gains or consumption rates. The critical density of brine shrimp below which grebes would completely abandon Mono Lake as a stopover site is difficult to predict, but the yearly exodus of birds coincides approximately with the time when mean lakewide densities of brine shrimp drop below about 25,000/m2 (Cooper et al., 1984~. Furthermore, the distribu- tion of grebes over the takers surface is patchy, perhaps reflecting local variations in brine shrimp densities (Lenz et al., 1986~. Grebes are particularly concentrated in re- gions of the lake with brine shrimp densities over 20,000/m2. Both of these independent measures of the as- sociation between grebes and brine shrimp densities suggest a threshold density of about 20,000 to 25,000 shrimp/m2 below which grebes may find brine shrimp to be a

98 120 - ~ 100 u, 80 an ~ 40 m oh 60 20 O The Mono Basin Ecosystem Mono Lake--Santa Barbara Channel I Mono Lake--Salton Sea Mono Lake-- Gul' of Calltomia 1 ~ 11 1 1 1 1 i 1 1 100 200 300 400 500 600 700 800 900 1000 NONSTOP FLIGHT RANGE (km) FIGURE 4.7 The potential nonstop flight range of an eared grebe as a function of usable fat reserves. Calcu- lated using the most conservative estimates available for migration energetics (9.0 kcal/g of fat, 1.05 kcal/k of non- stop flight) according to Them (1980~. marginally profitable food on which they could not easily gain weight. These measurements of shrimp abundance refer to the number of shrimp caught with a plankton net lifted vertically over the entire water column. Most sam- ples collected from discrete depths in early autumn contain 1 to 10 brine shrimp per liter in the upper 10 to 15 m (Lenz, 1980; Conte et al., in press). One might, therefore, expect that grebes arriving in good body condition (with >45 to 85 g of usable fat) would not stay long at Mono Lake or molt heavily if mean lakewide brine shrimp densities were below 25,000/m2 or if similarly high local concentrations of shrimp could not be located. On the other hand, grebes arriving in poorer con- dition might stay for a while and attempt to slowly fatten themselves, even if brine shrimp densities were lower. Grebes are known to stop at lakes with invertebrate

Biological System of Mono Lake 99 densities below those at which they leave Mono Lake (Winkler, 1982), but at present we cannot predict with cer- tainty the critical density of brine shrimp below which it becomes difficult for a grebe to gain weight. If the invertebrate populations of Mono Lake were to decline, the committee hypothesizes the following sequence of responses by the eared grebes. As invertebrate numbers began to drop below recent levels, grebes would continue to visit Mono Lake in current numbers, but they would not stay as long as they do now because they would deplete shrimp populations earlier in the season. The lower the number of shrimp became, the sooner grebes would depart. At some point, the shortened stay at Mono Lake would be incompatible with the type of rapid, heavy molting that now occurs. Grebes would then be forced to interrupt or delay their molt until they reached wintering areas, as they apparently do in eastern North America (Palmer, 1962~. If brine shrimp densities reached a level so low that it would be impossible for grebes to rapidly fatten themselves (prob- ably somewhere below 20,000 shrimp/m2), individuals arriv- ing at Mono Lake with sufficient fat reserves to continue migrating would probably do so. Only grebes arriving in relatively poor condition would remain, but only if Mono Lake's food resources were superior to those offered by other lakes in the Great Basin. At this level of food availability, the total number of grebes visiting the lake during the fall might remain essentially unchanged from current numbers, but turnover rates would be high and numbers present at any one time would be much lower than under present conditions. When Mono Lake's invertebrate populations became no different from those at other poten- tial stopover sites, grebes would eventually redistribute themselves among other lakes, perhaps occupying lakes that currently receive little or no use (Winkler, 1982) because their food resources compare unfavorably with Mono Lake's. The impact that this hypothesized sequence of events would have on the dynamics of the North American eared grebe population can only be roughly assessed, but specula- tions in the popular press about a possible devastation of the grebe population seem unwarranted. Shortening the length of stay at Mono Lake and interrupting or delaying

100 The Mono Basin Ecosystem the molt would probably not result in a rise in mortality rates. When grebes began to leave Mono Lake with fat reserves that were only marginally above the minimal levels needed to reach wintering areas, mortality rates could be expected to rise. Migrating grebes can be grounded by inclement weather, and extra fat resources would be needed to sustain the birds during these crises. Lacking these extra reserves, a portion of the population would, each year, be vulnerable to unpredictable but catastrophic losses, such as those described by Jehl and Bond (1983~. The impact on the North American population would probably be manifested in greater year-to-year variations in population size than occur now. If grebes were largely redistributed among other stopover sites when Mono Lake's food re- sources became comparable to those at other lakes, there might be a reduction in the North American population, but its magnitude is impossible to predict. The eventual popu- lation might be determined by the unknown carrying capac- ities of lakes that are, for the most part, not now sup- porting anywhere near the numbers of grebes that they might be capable of supporting because birds are so con- centrated at Mono Lake. Under these circumstances, the autumn migration period could potentially become the limit- ing season of the year for grebes. Mono Lake's rich food resources have so far kept this from happening. Mortality rates among grebes at Mono Lake are low (lehl, 1981 b), and major population limitations are probably now associated with the grebes' wintering or breeding areas. Phalaropes Three species of phalaropes use Mono Lake during migration, Wilson's phalarope (Phalarop1ls tricolor), red phalarope (Phalaropus fulicarius), and red-necked or north- ern phalarope (Phalaropus lobatus). Red phalaropes visit Mono Lake in very small numbers (Jehl recorded three in 3 years) during their fall migration (lehl, 1986), and essen- tially nothing is recorded of their use of the lake. Moderate numbers of red-necked phalaropes use Mono Lake as a brief migratory stopover. Jehl (1986) estimates the total population using the lake each year to be 2.5

Biological System of Mono Lake 101 times his maximum count for that year, although the com- mittee feels that this multiplier is inadequately justified. Additionally, his estimates have no confidence intervals, which are needed to assess the statistical significance of year-to-year changes in numbers. However, accepting his figures, 43,000 red-necked phalaropes used Mono Lake in 1981, 54,000 in 1982, and 33,000 in 1983. There is no rea- son to suppose that these apparent changes in numbers reflect events at Mono Lake. Jehl speculates that the 1983 drop in numbers reflects mortality on the oceanic wintering grounds due to the 1982 E1 Nino. Other year-to-year vari- ations may reflect the use of alternative lakes as stopping points, but data are inadequate to test this hypothesis. There are no data useful for estimating the portion of the North American population of red-necked phalaropes using Mono Lake, but it is probably fairly small given the num- bers using other western saline lakes (Jehl, 1986~. How- ever, of the Great Basin lakes in northern California, Mono Lake supports one of the highest peak populations of red- necked phalaropes, and is thus of local importance (Winkler, 1982; J. R. Jehl, Jr., personal communication, 1986). Wilson's phalaropes are the most abundant phalarope using Mono Lake, with peak populations of 93,000 to 100,000 or more (Winkler, 1982; Mahoney and Jebl, 1984~. I. R. Jehl, Jr. (personal communication, 1986) estimates a peak of 70,000 with a total population use of 100,000 to 125,000. Unlike red-necked phalaropes, Wilson's phalaropes make an extended stop at Mono Lake, using it as a staging area before commencing what may be a nonstop migration to South America (Jehl, 1981 a). While at Mono Lake they complete their molt and put on fat. It is unknown what percentage of the North American population uses Mono Lake, but lehl (l98la) suggests that Mono Lake is the lar- gest staging area for Wilson's phalaropes migrating through western North America. Certainly, Mono Lake is of major importance to this species in California (Winkler, 1982~. The time of arrival of phalaropes at Mono Lake coin- cides with the presence of immense numbers of brine shrimp and brine flies. Post-breeding red-necked phalaropes begin to arrive at Mono Lake in early to mid- July (Iehl, l981c, 1982b, 1986~. Numbers build until early

102 The Mono Basin Ecosystem August, remain high until early September, and then drop steadily until most birds are gone in early or mid-October. Wilson's phalaropes begin to arrive in middle to late June. After a period of 35 to 40 days needed to replace body plumage and lay on fat, the birds begin their southward migration near the end of July, with females preceding males (Jehl, 1981 a, personal communication). Wilson's phalaropes are gone from the lake by September. The principal prey of red-necked phalaropes at Mono Lake is the brine fly (Jehl, 1986~. The phalaropes take adult flies, as well as pupae and larvae; they will eat brine shrimp when brine flies are not available. Red-necked phalaropes at Great Salt Lake also apparently prefer brine flies to brine shrimp (Wetmore, 1925~. Brine flies are like- ly preferred to brine shrimp because of their larger size and presumed higher nutritional content. There are no data concerning critical densities of prey needed, nor is there an estimate of prey consumption by red-necked phal- aropes at Mono Lake. The Wilson's phalarope at Mono Lake takes both brine shrimp and brine flies (Jehl, 1981a; Mahoney and Jehl, 1984~. There is, however, a surprising lack of quantitative information on the diet of Wilson's phalaropes at Mono Lake, particularly in light of the importance of the lake as a migratory staging area for this species. Winkler (1977) reports on the examination of eight specimens from Mono Lake, which contained 93 percent brine flies and 7 percent brine shrimp. There are insufficient data available to esti- mate the total food consumption of Wilson's phalaropes and the partitioning of diet between prey species at Mono Lake. At Mono Lake, red-necked phalaropes concentrate over shallow submerged rock formations and submerged vegeta- tion mats near shore where pupating brine flies are abun- dant, such as near tufa towers and the smaller islands (Jehl, 1981 c, 1 982b, 1986~. Late in the season, possibly in preparation for migration, birds congregate in small flocks at the center of the lake. Access to fresh water may not be required by red- necked phalaropes, but they visit freshwater sources around the lake to drink and bathe (Jehl, 1986~. Although data on the physiological tolerances of red-necked phalaropes to

Biological System of Mono Lake 103 salt and alkali concentrations are lacking, it is reasonable to assume that Mono Lake water exceeds their tolerances for salt ingestion (Mahoney and Jehl, 1984, 1985~. As is true for the other Mono Lake birds, the red-necked phala- ropes undoubtedly avoid salt loading by taking hyposmotic prey and by minimizing their intake of lake water. Habitat use by Wilson's ohalarooes at Mono Lake has not been described in detail. ~- However, Wilson9s phalaropes make extensive use of emergent tufa towers and nearshore sandbars for roosting (J. R. Jehl, Jr., personal communica- tion) and they forage over much of the lake's surface for brine shrimp and brine flies. Given the virtual absence of any quantitative data on this species at Mono Lake, it is difficult to evaluate habitat requirements, except that it is clear that the lake provides a foraging habitat and a loca- tion for molting while staging in preparation ~ or migration to South America (Jehl, 1986~. Wilson's phalaropes at Mono Lake apparently avoid incurring a salt load by minimizing their ingestion of Mono Lake water and by taking hyposmotic prey (Mahoney and Jehl, 1984~. Just before migration, when feeding rates are maximized for fat accumulation, Wilson's phalaropes at Mono Lake show enlarged salt glands and at this time visit freshwater sources morning and evening (Mahoney and Jehl, 1984~. These data suggest that the availability of fresh water for 2 weeks prior to migration from the lake may be important for coping with excess salts ingested during the period of heavy foraging associated with fat accumulation. The use of Mono Lake as a staging area by Wilson phalaropes has important implications for the population involved. The use of staging areas is often traditional in shorebirds (Pitelka, 1979), and most individuals show great fidelity to a given site once a migration pattern is estab- lished (Pienkowski and Evans, 1985; Evans and Townshend, in press). Adults with site attachment might be reluctant to leave a staging area such as Mono Lake before molting and putting on fat reserves. Many shorebirds arrive at staging areas with relatively little reserve tat and In an ~~^' Weight data from Wilson's phalaropes arriving at Mono Lake show low, but not depleted fat reserves (J. R. Jehl, Jr., per- sonal communication). Additionally, Jehl (personal . ~ exhausted condition (Dick and rlen~owslcl, 1Y /~). · -

104 The Mono Basin Ecosystem communication) has some data that indicate that Wilson's phalaropes at Mono Lake may be able to shift to alterna- tive sites. At present it is difficult to predict the response of adult Wilson's phalaropes if they arrived at Mono Lake and found their expected food sources severely diminished, or the population consequences if they attempt- ed to use alternate sites for staging. California Gulls California gulls (Laws californic~cs) nest on several of the islands and islets in Mono Lake. In addition, Califor- nia gulls nesting elsewhere in the Great Basin may visit Mono Lake during migration (J. R. Jehl, Jr., personal com- munication). A variety of reports and publications provide estimates of the number of California gulls nesting at Mono Lake (Winkler, 1979, 1983a; Power et al., 1980; Jehl, 1983a,b, 1984, 1985; Jehl et al., 1984; Shuford et al., 1984, 1985; Shuford, 1985; D. Winkler and W. D. Shuford, Cornell University, and Point Reyes Bird Observatory, respectively, personal communication). The extent of use of Mono Lake by California gulls in migration is not known. The methods for assessing the numbers of California gulls nesting at Mono Lake have varied considerably over historical times. Early records of numbers are generally based on crude estimates by naturalists or even on hearsay (Ieh! et al., 1984, D. Winkler and W. D. Shuford, personal communication). Since 1979, observers have estimated breeding populations of gulls based on counts of chicks seen in the colonies multiplied by correction factors for chicks produced per pair and the visibility of chicks on each colony. The accuracy of this census method has been questioned by Jeh! (1983a, 1984, 1985), as it has produced variations from his counts of up to 170 percent for some islets (Jehl, 1985~. The population of California gulls nesting at Mono Lake has increased from an historical low of 3,000 to 4,000 birds in the early 1900s to 50,000 by 1976, since when it has been relatively constant at between 40,000 and 50,000 birds (Jehl et al., 1984; Shuford, 1985~. Jehl et al. (1984) thought that the population was more or less constant in

Biological System of Mono Lake 105 size from 1916 to the early l950s, after which they hypo- thesized a rapid expansion of the population, driven by im- migration. According to their view, the provision of new nest sites by the lowering lake level made the population increase possible. In contrast, D. Winkler and W. D. Shu- ford (personal communication) believe that the Mono Lake gull population has been recovering from excess egg col- lecting in the 1 SOOs and has undergone an exponential in- crease to its former numbers. In their estimation, the gull population in the past has not been constrained by insuffi- cient nesting space. The California gull nests on islands in lakes throughout much of the western United States and central Canada (Power et al., 1980~. Despite variation between California gulls on Mono Lake and some other populations (Winkler, 1977, 1985; J. R. Jehl, Jr., personal communication), there is no evidence for treating the Mono Basin population as a separate local race or subspecies (Zink and Winkler, 1983~. Several authors have estimated the significance of the Mono Lake California gull popula- tion in relation to regional and North American (world) populations of the species. Power et al. (1980) estimated a world population of about 220,000 California gulls, of which the Mono Lake population of about 47,000 in 1976 to 1978 made up 21 percent. However, this species is expanding its population in California and in the Great Basin (Conover and Conover, 1981; Conover, 1983), and estimates of the relative importance of various colonies or subpopulations are quickly out of date. Given the lack of precision in estimates of the world population, the Mono Lake popula- tion is likely to be between 15 and 25 percent of the breeding population of California gulls. The Mono Lake colony is the second largest known California gull colony in North America (Conover, 1983~. From almost any per- spective, the Mono Lake colony supports an important seg- ment of this species' population. Mono Lake is used by California gulls primarily between early April and early August. California gulls arrive at Mono Lake as early as the first week of March (lehl, 1983b), with peak numbers arriving probably in early to mid-April (Power et al., 1980~. Egg-laying begins in late April, peaks in mid-May, and is mostly completed by the first week of June (lehl, 1983b; Shuford et al., 1985~.

106 The Mono Basin Ecosystem Chicks begin hatching in late May, hatching peaks in early to mid-June, and is essentially complete by the end of June ~ ~ ~ Peak fledging occurred between July (Shuford et al., 1985~. 10 and 21 in 1983 and 1984. California gulls begin to leave Mono Lake about mid-July. with a major nortinn of the .~ 7 ,, — ~ — ~ _ - ~ population gone by mid-August (Power et al., 1980; J. R. Jehl, Jr., personal communication). Egg-laying takes place while brine shrimp are relatively scarce, but chick hatching and growth usually coincide with a period when this prin- cipal source of food is at or near peak abundance (Winkler, 1983b, 1985). Several aspects of the breeding biology of California gulls at Mono Lake are notentiallv sensitive to chances in lake levels. The production of young per pair depends upon clutch size, hatching success, the number of chicks surviving to fledging at about 42 days of age, and freedom from disturbance (Power et al., 1980; Winkler, 1983b). California gulls nesting at Mono Lake produce smaller eggs and smaller clutches than do birds in other colonies of this species (Winkler, 1985~. Winkler (1985) argues that this smaller clutch size (and reduced egg volume) is caused by a scarcity of brine shrimp early in the season at the time of egg formation. Energy for egg production is ap- parently not sufficient for the female gulls to produce a clutch of three full-sized eggs. Adult gulls at Mono Lake have less food in their stomachs, and they courtship-feed less frequently during the pre-egg stage than gulls at Great Salt Lake, where clutches are larger (Winkler, 1985~. Both of these findings support the hypothesis that food is limit- ed early in the breeding season. Hatching and fledging success of gulls at Mono Lake vary between years. Survival to hatching has been as low 1982 (Winkler, 1 983a,b), 5 percent of chicks sur- year, between 30 and 35 ~ ,, O as 40 percent of eggs laid, as in and in 1981 ann~r~ntiv f~.wer then vived to fledging. In a "normal" percent of chicks fledge (Winkler, 15~83b), and in a "good" year, up to 70 percent of chicks may survive (J. R. Jehl, Ir., personal communication). .. . . . . v arlatlons In methodology and incomplete data sets make year-to-year comparisons and estimates of chick pro- duction difficult. An average of about 0.5 chicks fledged per pair in the period 1983 through 1985. A portion of the

Biological System of Mono Lake 107 interannual variation in chick production may be the result of infestation of chicks by ticks (Shuford, 1985~. Post- fledging mortality of independent young prior to departure also varies greatly between years, in 1981 approximately 70 percent of fledged chicks died before migration, while in 1982 only about 2 percent were lost (Jehl, 1981 b; Jehl and Jehl, 1981, 1982~. Year-to-year variability in the production of chicks is common among most species of gulls and by itself does not indicate a local population is likely to decline. In their lifetime a pair must produce but two young that survive to breed if a population is to remain stable. Power et al. ( 1980) have estimated, based on banct returns, that most California gulls live about 9 years, with a few surviving to 13 years. These estimates may well be conservative, as band loss and its effect on the underestimation of age of survival were not considered (Kadlec and Drury, 1969~. The average of 0.5 chicks fledged per pair in the period 1983 through 1985 may be sufficient for maintaining a stable or even moderately increasing population size, but we have insufficient data to know. The data base is cur- rently very incomplete, particularly in the area of adult mortality rates, and the years sampled for chick productiv- ity and post-fledging survival are few. Hence, it is prema- ture to draw conclusions about the stability and growth potential of this population based on the reproductive and mortality data available. California gulls at Mono Lake have three requirements of their habitat. They need predator- and disturbance-free areas on which to breed, they require a plentiful, nearby source of food; and they need access to fresh water. The first two requirements must be met in the immediate vicin- ity of Mono Lake if the present breeding population is to be maintained there. It is possible that alternate fresh- water sources within several tens of kilometers could be substituted for the present use of springs along the edge of the lake. The importance of nesting areas free of predators and disturbance is underscored by the history of use of various islands in Mono Lake and changes in reproductive output on those islands. Paoha Island has been an important nesting area in the past but more recently was deserted

108 The Mono Basin Ecosystem either due to a population increase of feral goats or pos- sibly due to coyotes (Cants latrans) (Jehl et al., 1984; D. Winkler and W. D. Shuford, personal communication). Although coyotes apparently did not disturb gull nesting on Negit Island in 197S, the first breeding season with a land bridge to the island, on July 10, 1978, canid scat and tracks were found on the island (D. Winkler and W. D. Shuford, personal communication). The next season ( 1979), breeding was disrupted: on May 12 and 13 Winkler (D. Winkler and W. D. Shuford, personal communication) found "normal" numbers of gulls nesting, but "on 27 May he found only scattered broken eggshells. No human tracks were seen, and the sandy soil of the colony site was criss- crossed with canid tracks." Given contradictory statements in Jehl et al. (1984) and D. Winkler and W. D. Shuford (personal communication), it is not clear whether birds deserting Negit Island settled elsewhere to breed in 1979 or simply did not breed that year. However, both sources agree that the Paoha Islets were used for the first time in 1979, and Jehl et al. (1984) state "while it is plausible that the arrival of coyotes prompted the gulls' relocation, we point out that there were no significant barriers to prevent the coyotes' arrival several years earlier." The fact that the canids took one or more years to discover the colony on Negit Island after the land bridge was formed is less important than the fact that the colony was broken up and deserted once canids arrived. Similarly, large colonies on Twain and Jarvis islets were deserted in 1982 after land bridges connected them to the mainland in 1981 (D. Winkler, personal communication). On the basis of experi- mental evidence, we know that breeding activities will be disrupted in gull colonies invaded by large terrestrial mam- malian predators (Kadlec, 1971~. The type and amount of cover, whether vegetation, rock, or other material, may affect the desirability of a given area as nesting habitat. Pugesek and Diem ( 1983) studied habitat selection by California gulls nesting in Wyoming and found that gulls nesting in shrubbery had higher nesting success than those nesting in the open. Gull chicks can die from overheating, and in some cases vegetation or rock crevices provide protection from insola- tion (Salzman, 1982; Chappell et al., 1984~. On the small

Biological System of Mono Lake 109 islets, temperatures frequently exceed the thermoregulatory abilities of chicks, and shading by parent gulls appears critical for chick survival (Chappell et al., 1984~. Dense vegetation also may block cooling breezes, and on Negit Island temperatures near the ground in areas of dense vegetation were higher than those found on Coyote Islet in the open near the shore (Jehl and Mahoney, in press). Cover in some cases may be important for providing protection from avian predators, and at Mono Lake, great horned owl (Bubo virginianus) predation apparently caused the gulls to abandon nesting on several small unvegetated islets in 1982 and 1983 (Jehl et al., 1984~. However, in other instances, gulls nesting in dense cover may be more susceptible to predation if predators can approach undetec- ted, or if gull escape is impaired (Jehl and Chase, in press). Gulls are generalist foragers and will take a wide vari- ety of foods. California gulls are no exception; they eat many types of natural prey and foods obtained from humans (Power et al., 1980~. Early reports of prey of California gulls at Mono Lake include brine fly larvae (Nichols, 1938; Young, 1952), trout (Nichols, 1938), and brine shrimp (Young, 1952; Winkler, 1977~. More recently, Jehl and Mahoney (1983) reported on prey found in 18 adults, 73 chicks, and 20 fledglings in 1982. The vast majority of the prey was brine shrimp, with small amounts of brine flies, fish, and freshwater insects. Garbage had been eaten as well. Substantial numbers of cicadas (no taxon given) were eaten late one evening (see also Winkler, 1983b). In spite of their dietary Catholicism, foods avail- able to Mono Lake gulls other than brine shrimp appear to be very limited. Winkler (1983b) states that brine shrimp are not present in "profitable" densities until the chick-rearing period, and early in the season the gulls forage away from the lake (Winkler, 1985; Jehl and Mahoney, 1983~. There are, how- ever, no data on what constitutes a "profitable" density and how the overall density of brine shrimp in the lake affects the presence of swarms or patches of higher den- sity. This information is critical for determining the effect of reductions of brine shrimp populations on gull popula- tions. Unlike the situation at Great Salt Lake, with a

110 The Mono Basin Ecosystem major refuse dump within 15 km of the colonies and exten- sive agricultural fields nearby, Mono Lake has only a few small refuse dumps nearby and has no nearby agricultural activity that would sustain a large gull population twinkler, 1 983b). There is a lack of information on the types and availability of prey taken by gulls early in the breeding season, and additional sampling is required to determine the importance of foods other than brine shrimp throughout the season. Summarizing the information on birds, large numbers of eared grebes, red-necked phalaropes, Wilson's phalaropes, and California gulls use the lake and depend on the brine shrimp and brine flies for their food. Additionally, the gulls require safe refuges on which to nest. Present cen- sus techniques are statistically inadequate to detect small but possibly biologically significant changes in avian num- bers or temporal patterns of use of Mono Lake. Likewise, we do not know critical food densities below which the invertebrates taken by birds are not sufficient to provide needed body weight, or how the density of available prey will change with overall prey densities. These data are needed if we are to quantitatively predict how bird popula- tions will respond to changing food supplies with changing lake salinity. Clearly, however, if the invertebrate popula- tions are drastically reduced, there will be reductions in the number of birds visiting and using Mono Lake. REFERENCES Them, C. R. 1980. The energetics of migration. Pp. 175- 224 in Animal Migration, Orientation, and Navigation, S. Gauthreaux, Jr., ed. New York: Academic Press. Bowen, S. T. 1962. The genetics of Artemia satina. I. The reproductive cycle. Biol. Bull. 122:25-32. Bowen, S. T., E. A. Fogarino, K. N. Hitchner, G. L. Dana, V. H. S. Chow, M. R. Buoncristiani, and I. R. Carl. 1985. Ecological isolation in Artemia: population differ- ences in tolerance of anion concentrations. J. Crusta- cean Biol. 5:106-129.

Biological System of Mono Lake 111 Busa, W. B., and J. H. Crowe. 1983. Intracellular pH regulates the dormancy and development of brine shrimp (Artemia salina) embryos. Science 221:366-368. Busa, W. B., J. H. Crowe, and G. B. Matson. ~ 982. Intracellular pH and the metabolic status of dormant and developing Artemia embryos. Arch. Biochem. Bio- phys. 216:711-718. Chapman, D. J. 1982. Investigations on the salinity toler- ance of a diatom and green algae isolated from Mono Lake. Abstract from Mono Lake Symposium, Santa Barbara, Calif., May 5-7, 1982. Chappell, M. A., D. L. Goldstein, and D. W. Winkler. 1984. Oxygen consumption, evaporative water loss, and tem- perature regulation of California gull chicks (Laws cali- fornicus) in a desert rookery. Physiol. Zool. 57:204-214. Clegg, J. S. 1964. The control of emergence and metabo- lism by external osmotic pressure and the role of free glycerol in developing cysts of Artemia salina. J. Exp. Biol. 41:879-892. Clegg, J. S. 1974. Interrelationships between water and metabolism in Artemia salina cysts: hydration-dehydra- tion from the liquid and vapour phases. J. Exp. Biol. 61:291-308. Clegg, J. S. 1976a. Hydration measurements on individual Artemia cysts. J. Exp. Zool. 198:267-272. Clegg, J. S. 1976b. Interrelationships between water and cellular metabolism in Artemia cysts. II. Carbohydrates. Comp. Biochem. Physiol. 53A:83-87. Clegg, I. S. 1976c. Interrelationships between water and cellular metabolism in Artemia cysts. III. Respiration. Comp. Biochem. Physiol. 53A:89-93. Clegg, I. S. 1978. Interrelationships between water and cellular metabolism in Artemia cysts. VIII. Sorption iso- therms and derived thermodynamic quantities. J. Cell. Physiol. 94:123- 137. Clegg, J. S., and F. P. Conte. 1980. A review of the cellular and developmental biology in Artemia. Pp. 11- 54 in The Brine Shrimp Artemia. Vol. 2. Physiology, Biochemistry, Molecular Biology. G. Persoone, P. Sorgeloos, O. A. Roels, and E. Jaspers, eds. Wetteren, Belgium: Universa Press.

112 The Mono Basin Ecosystem Collins, N. C. 1975. Population biology of a brine fly (Diptera: Ephydridae) in the presence of abundant algal food. Ecology 56:1139- 1148. Collins, N. C. 1980a. Population ecology of Ephydra cinerea Jones (Diptera Ephydridae), the only benthic metazoan of the Great Salt Lake, USA. Hydrobiologia 68:99- 112. Collins, N. C. 1980b. Developmental responses to food limitation as indicators of environmental conditions for Ephydra cinerea Jones (Diptera). Ecology 61:650-661. Conover, M. R. 1983. Recent changes in ring-billed and California gull populations in the western United States. Wilson Bull. 95:362-383. Conover, M. R., and D. O. Conover. 1981. A documented history of ring-billed and California gull colonies in the western United States. Colonial Waterbirds 4:37-43. Conte, F. P. 1977. Molecular mechanisms in the bran- chiopod larval salt gland (Crustacea). Pp. 143- 1 59 in Water Relations in Membrane Transport in Plants and Animals, A. M. Jungreis, T. K. Hodges, A. Kleinzeller, and S. G. Schultz, eds. New York: Academic Press. Conte, F. P. 1984. Structure and function of the crus- tacean larval salt gland. Pp. 45-106 in Membranes, J. F. Danielli, ed. International Review of Cytology, Vol. 91. Orlando, Fla.: Academic Press. Conte, F. P., S. R. Hootman, and P. I. Harris. 1972. Neck organ of Artemia salina nauplii: a larval salt gland. J. Comp. Physiol. 80:239-246. Conte, F. P., P. C. Droukas, and R. D. Ewing. 1977. Development of sodium regulation and de nova synthesis of Na+K-activated ATPase in the larval brine shrimp Artemia salina. I. Exp. Zool. 202:339-361. Conte, F. P., J. Lowy, J. Carpenter, A. Edwards, R. Smith, and R. D. Ewing. 1980. Aerobic and anaerobic metabo- lism of Artemia nauplii as a function of salinity. Pp. 126- 136 in The Brine Shrimp Artemia. Vol. 2. Physiol- ogy, Biochemistry, Molecular Biology. G. Persoone, P. Sorgeloos, O. A. Roels, and E. Jaspers, eds. Wetteren, Belgium: Universa Press. Conte, F. P., R. S. Jellison, and G. L. Starrett. In press. Nearshore and pelagic abundances of Artemia monica in Mono Lake, Calif. In Saline Lakes, J. M. Melack, ed.

Biological System of Mono Lake Developments in Hydrobiology. Dr W. Junk Publishers. 113 Dordrecht, Netherlands: Cooper, S. D., D. W. Winkler, and P. H. Lenz. 1984. The effect of grebe predation on a brine shrimp population. I. Anim. Ecol. 53~1~:51-64. Cramp, S., and K. E. L. Simmons, eds. 1977. Handbook of the Birds of Europe, the Middle East and North Africa, Vol. 1. Oxford, England: Oxford University Press. 722 PP. Croghan, P. C. 1958. The osmotic and ionic regulation of Artemia salina. I. Exp. Biol. 35:219-233. Dana, G. L. 1981. Comparative Population Ecology of the Brine Shrimp Artemia. Master's thesis, San Francisco State University. 125 pp. Dana, G. L., and P. H. Lenz. 1986. Effects of increasing salinity on an Artemia population from Mono Lake, Cali- fornia. Oecologia 68:428-436. Dana, G. L., C. Foley, G. Starret, W. Perry, and J. M. Melack. In press. In situ hatching rates of Artemia monica cysts in hypersaline Mono Lake. In Saline Lakes, J. M. Melack, ed. Developments in Hydrobiology. Dordrecht, Netherlands: Dr W. Junk Publishers. Dick, W. J. A., and M. W. Pienkowski. 1978. Autumn and early winter weights of waders in north-west Africa. Ornis Scand. 10: 117- 123. 1986. Physiological , Effects of Salinity on Dormancy and Hatching in Mono Lake Artemia Cysts. Los Angeles, Calif.: Los Angeles Department of Water and Power. Drinkwater. L. E.~ and J. H. Crowe. Drinkwater, L. E., and ]. H. Crowe. 1987. Regulation of embryonic diapause in Artemia: env~ronmental and phys- iological signals. I. Exp. Zool. 241:297-307. Evans, P. R., and D. J. Townshend. In press. Site faith- fuIness of waders away from the breeding grounds: how individual migration patterns are established. In Proceedings of the Congress, Ottawa. Hamilton-Galat, K., and D. L. Galat. 1983. Seasonal variation of nutrients, organic carbon, ATP, and micro- crops in a vertical profile of Pyramid Lake, Nevada. Hydrobiologia 105:27-43. 19th International Ornithological bial standing .

114 The Mono Basin Ecosystem Hammer, U. T. 1986. Saline Lake Ecosystems of the World. Monographiae Biologicae 59. Dordrecht, Netherlands: Dr W. Junk Publishers. 616 pp. Herbst, D. B. 1981. Ecological physiology of the larval brine fly, Ephydra hians, an alkaline-salt lake inhabiting Ephydrid. Master's thesis, Oregon State University, Corvallis. 65 pp. Herbst, D. B. 1986. Comparative Studies of the Population Ecology and Life History Patterns of an Alkaline Salt Lake Insect: Ephydra (Hyd~ropyrus) hians Say (Diptera: Ephydridae). Ph.D. dissertation, Oregon State Univer- sity, Corvallis. 222 pp. Herbst, D. B. In press. Comparative population ecology of Ephydra hians Say (Diptera: Ephydridae) at Mono Lake (California) and Abert Lake (Oregon). In Saline Lakes, J. M. Melack, ed. Developments in Hydrobiology. Dor- drecht, Netherlands: Dr W. Junk Publishers. Iversen, N., R. S. Oremland, and M. J. Kulg. In press. Big Soda Lake (Nevada). 3. Pelagic methanogenesis and anaerobic methane oxidation. Limnol. Oceanogr. Jehl, I. R., Jr. 1981a. Mono Lake: A vital way station for the Wilson's phalarope. Natl. Geogr. 160:520-525. Jehl, J. R., Jr. 1981 b. Mortality of Waterbirds at Mono Lake, California. Technical Report 81-133. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. 1981 c. The Biology of Northern Phalaropes at Mono Lake, California, 1981. Technical Report 81- 134. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. 1 982a. Biology of Eared Grebes at Mono Lake. California. Technical Report 82-136. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. 1982b. The Biology of Northern Phalaropes at Mono Lake, California, 1982. Technical Report 82- 146. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. California Gull Chicks at Special Reference to Populations on Technical Report 83-156. San Diego, Calif.: Hubbs-Sea World Research Institute. 6 pp. 1 983a. Comments on the Annual Census of Mono Lake, California with the Paoha Islets.

Biological System of Mono Lake 115 Jehl, J. R., Ir. 1983b. Breeding Success of California Gulls and Caspian Terns on the Paoha Islets, Mono Lake, California, 1983. Technical Report 83-157. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. 1983c. The Biology of Eared Grebes at Mono Lake, California. Technical Report 83- 136. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, J. R., Jr. 1984. Comments on the Cooperative Gull Census of 1984, with Special Reference to the Paoha Islets. Technical Report 84- 167. Hubbs-Sea World Research Institute. lehl, I. R., Jr. 1985. The Cooperative Gull Census of 1985: Comments and Recommendations. Technical Report. 85- 183. San Diego, Calif.: Hubbs-Sea World Research Institute. 5 pp. Jehl, I. R., Jr. 1986. Biology of red-necked phalaropes (Phalaropus Zobatus) at the western edge of the Great Basin in fall migration. Great Basin Nat. 46:185-197. Jehl, I. R., Jr., and S. I. Bond. 1983. Mortality of eared grebes in winter of 1982-83. Am. Birds 37:832-835. Jehl, J. R., Jr., and C. Chase, III. In press. Foraging pat- terns and prey selection in avian predators: a compara- tive study in two colonies of California gull. Stud. Avian Biol. Jehl, I. R., Jr., and D. R. Jehl. 1981. Post-Fledging Mor- talitv of California Gulls. San Diego, Calif.: Technical Report ~ 1 - 135. San Diego, Calif.: Hubbs-Sea World Research Institute. Jehl, I. R., Ir., and D. R. Jehl. 1982. Post-Fledging Mor- tality of California Gulls, 1982. Technical Report 82- 147. ~ *, T Institute. Jehl, I. R., Ir., and S. A. Mahoney. 1983. Possible sexual differences in foraging patterns in California gulls and their implications for studies of feeding ecology. Colo- nial Waterbirds 6:2 ~ 8-220. Jehl, J. R., Jr., and S. A. Mahoney. In press. The roles of thermal environment and predation in habitat choice in the California gull (Laws californicus). Condor. Jehl, J. R., Jr., and P. K. Yochem. 1986. Movements of eared grebes indicated by banding recoveries. J. Field Ornithol. 57:208-212. San forego, cant.: nunos-~ea worm Research

116 The Mono Basin Ecosystem Jehl, J. R., Jr., D. E. Babb, and D. M. Power. 1984. His- tory of the California gull colony at Mono Lake, Cali- fornia. Colonial Waterbirds 7:94-104. Jellison, R. 1985. Zooplankton mediated nitrogen and phytoplankton dynamics in a hypersaline lake. In Abstracts of the 4Sth annual meeting of the American Society of Limnology and Oceanography, June 17-21, 1985, Minneapolis, Minn. Allison, R., and I. M. Melack. 1986. Nitrogen supply and primary production in hypersaline Mono Lake. Eos Trans. Am. Geophys. Union 67:974. Jellison, R., and J. M. Melack. In press. Photosynthetic activity of phytoplankton and its relation to environ- mental factors in hypersaline Mono Lake, California. In Saline Lakes, J. M. Melack, ed. Developments in Hydro- biology. Dordrecht, Netherlands: Dr W. Junk Publishers. Kadlec, J. A. 1971. Effects of introducing foxes and rac- coons on herring gull colonies. J. Wildl. Manage. 35:625-636. Kadlec, J. A., and W. H. Drury, Jr. 1969. Loss of bands from adult herring gulls. Bird-Banding 40:216-221. Kilham, P. 1981. Pelagic bacteria extreme abundances in African saline lakes. Naturwissenschaften 68:380-381. Lang, A. R. G. 1967. Osmotic coefficients and water potentials of sodium chloride solutions from 0 to 40°C. Austr. J. Chem. 20:2017-2023. Lenz, P. H. 1980. Ecology of an alkali-adapted variety of Artemia from Mono Lake, California, U.S.A. Pp. 79-96 in The Brine Shrimp Artemia. Vol. 3. Ecology, Cultur- ing, Use in Aquaculture, G. Persoone, P. Sorgeloos, O. Roels, and E. Jaspers, eds. Wetteren, Belgium: Universa Press. Lenz, P. H. 1982. Population Studies on Artemia in Mono Lake, California. Ph.D. dissertation, University of Cali- fornia, Santa Barbara. 230 pp. Lenz, P. H. 1984. Life-history analysis of an Artemia population in a changing environment. J. Plankton Res. 6:967-983. Lenz, P. H., S. D. Cooper, I. M. Melack, and D. W. Winkler. 1986. Spatial and temporal distribution patterns of three trophic levels in a saline lake. J. Plankton Res. 8:1051 -1064.

Biological System of Mono Lake 117 Lovejoy, C., and G. Dana. 1977. Primary producer level. Pp. 42-57 in An Ecological Study of Mono Lake, Cali- fornia, D. W. Winkler, ed. Institute of Ecology Publica- tion No. 12. Davis, Calif.: University of California, Institute of Ecology. Mahoney, S. A., and J. R. Jehl, Jr. ~ ~ . . ~ . ~ . ~ ~ ~ . ~ 1984. The Physiology of Migratory Birds on Alkaline Lakes: Wilson's Phala- rope and American Avocet. Technical Report 84- 172. San Diego, Calif.: Hubbs-Sea World Research Institute. 21 pp. Mahoney, S. A., and J. R. Jehl. 1985. Avoidance of salt loading by a diving bird at a hypersaline and alkaline lake: eared grebe. Condor 87:389-397. Mason, D. T. 1966. Density-current plumes. 152:354-356. Science Mason, D. T. 1967. Limnology of Mono Lake, California. University of California Publications in Zoology No. 83. Berkeley, Calif.: University of California Press. Melack, I. M. 1983. Large, deep salt lakes: a comparative limnological analysis. Hydrobiologia 105:223-230. Melack, J. M. 1985. The ecology of Mono Lake, Cali- fornia. Pp. 461-470 in National Geographic Society Res- earch Reports, Vol. 20. 1979 projects. Washington, D.C.: National Geographic Society. Melack, J. M. In press. Aquatic plants in extreme environments. In Aquatic Vegetation, J. I. Symeons, ed. New York: Elsevier. Melack, I. M., I. L. Stoddard, and D. R. Dawson. 1982. Acid precipitation and buffer capacity of lakes in the Sierra Nevada, California. Pp. 465-471 in Proceedings of the International Symposium on Hydrometeorology, Denver, Colo., June 13-17, 1982. A. I. Johnson and R. A. Clarke, eds. Bethesda, Md.: American Water Resources Association. Nichols, W. F. 1938. Some notes from Negit Island, Mono Lake, California. Condor 40:262. Oremland, R. S., L. Marsh, and D. J. DesMarais. 1982. Methanogenesis in Big Soda Lake, Nevada: an alkaline, moderately hypersaline desert lake. Appl. Environ. Microbiol. 43:462-468. Oremland, R. S., R. L. Smith, and C. W. Culbertson. 1985. Aspects of the biogeochemistry of Big Soda Lake,

118 The Mono Basin Ecosystem Nevada. Pp. ~ 1 -99 in Planetary Ecology, D. E. Caldwell, J. A. Brierley, and C. L. Brierley, eds. New York: Van Nostrand Reinhold. Palmer, R. S. 1962. Handbook of North American Birds, Vol. 1. New Haven, Ct.: Yale University Press. 567 pp. Pelagos Corporation. 1987. A Bathymetric and Geologic Survey at Mono Lake, California. Report prepared for Los Angeles Department of Water and Power. San Diego, Calif. Pienkowski, M. W., and P. R. Evans. 1985. The role of migration in the populations dynamics of birds. Pp. 331 -352 in Behavioural Ecology: Ecological Consequences of Adaptive Behaviour, R. M. Sibly and R. H. Smith, eds. British Ecological Society, Vol. 25. Palo Alto, Calif.: Blackwell. Pitelka, F. D. 1979. Introduction: the Pacific coast shore- bird scene. Stud. Avian Biol. (2~:1 - 11. Power, D. M., P. W. Collins, and K. W. Rindlaub. 1980. i, Pp. 1 S-286 in The Biology of Certain Water and Shore Birds at Mono Lake. California. D. M. Power and Associates. The California gull. Unpublished technical report for Los Angeles Department of Water and Power. Pugusek, B. H., and K. L. Diem. 1983. A multivariate study of the relationship of parental age to reproductive success in California gulls. Ecology 64:829-839. Rockland, L. B. 1960. Saturated salt solutions for static control of relative humidity between 5° and 40°C. Anal. Chem. 32:1375- 1376. Salzman, A. G. 1982. The selective importance of heat stress in gull nest location. Ecology 63:742-751. Shuford, W. D. 1985. Reproductive Success and Ecology of California Gulls at Mono Lake, California in 1985, with Special Reference to the Negit Islets: An Overview of Three Years of Research. Contribution No. 31 S. Stin- son Beach, Calif.: Point Reyes Bird Observatory. 50 pp. Shuford, D., E. Strauss, and R. Hogan. 1984. Population Size and Breeding Success of California Gulls at Mono Lake, California, in 1983. Contribution No. 126. Stin- son Beach, Calif.: Point Reyes Bird Observatory. Shuford, D., P. Super, and S. Johnston. 1985. Population Size and Breeding Success of California Gulls at Mono

Biological System of Mono Lake Lake, California, in 1984. Contribution No. son Beach, Calif.: Point Reyes Bird Observatory. Stocco, D. M., P. C. Beers, and A. H. Warner. feet of anoxia on nucleotide metabolism embryos of the brine shrimp. Dev. Biol. 27:479-493 119 294. Stin- 1972. Ef- in encysted Storer, R. W., and J. R. Jehl, Jr. 1984. Moult patterns and moult migration in the black-necked grebe Pod iceps nigricollis. Ornis Scand. 16:253-260. Thun, M. L., and G. L. Starrett. 1986. The effect of cold, hydrated dormancy and salinity on the hatching of Artemia cysts from Mono Lake, Calif. Report to Los Angeles Department of Water and Power. Wetmore, A. 1925. Food of American Phalaropes, Avocets, and Stilts. U.S. Department of Agriculture Bulletin 1359. Washington, D.C.: U.S. Government Printing Office. 20 pp. Winkler, D. W., ed. 1977. An Ecological Study of Mono Lake, California. Institute of Ecology Publication No. 12. Davis, Calif.: University of California, Institute of Ecology. Winkler, D. W. 1979. Deposition in the case: National Audubon Society et al., vs. Department of Water and Power of the City of Los Angeles. California Superior Court, Mono County, Civil No. 6429, Vols. I and II. Winkler, D. W. 1982. Importance of Great Basin Lakes in Northern California to Nongame Aquatic Birds, 1977. Wildlife Management Branch Administration Report 82-4. Sacramento, Calif.: State of California, The Resources Agency. 30 pp. Winkler, D. W. 1983a. California Gull Nesting at Mono Lake, California, in 1982: Chick Production and Breeding Biology. Unpublished final report to U.S. Fish and Wildlife Service, Arcata Field Station, Calif. Winkler, D. W. 1983b. Ecological and Behavioral Deter- minants of Clutch Size: The California Gull (Laws californicus) in the Great Basin. Ph.D. dissertation, University of California, Berkeley. 195 pp. Winkler, D. W. 1985. ~ , e ~ _ Factors determining a clutch size reduction In California gulls (Laws californicus): a multi-hypothesis approach. Evolution 39:667-677.

120 The Mono Basin Ecosystem Winkler, D. W., and S. D. Cooper. 1986. The ecology of migrant black-necked grebes at Mono Lake, California. Ibis 128:483-491. Winston, P. W., and D. H. Bates. 1960. Saturated solutions for the control of humidity in biological research. Ecology 41 :232-237. Young, R. T. 1952. Status of the California gull colony at Mono Lake, California. Condor 54:206-207. Zink, R. M., and D. W. Winkler. 1983. Genetic and morphological similarity of two California gull popula- tions with different life history traits. Biochem. Syst. Ecol. 11:397-403.

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Mono Basin is a closed hydrologic basin spanning the border between California and Nevada. Los Angeles has been diverting streams since 1941 that normally would flow into Mono Lake. It has been predicted that continued diversion will have major ecological consequences for the natural resources of the Mono Basin National Forest Scenic Area. This book studies the ecological risk assessment that considers the effects of water diversions on an inland saline lake. It examines the hydrology of the Mono Basin, investigates the lake's physical and chemical systems, studies the biological relationships, and predicts the effects of changes in lake levels on the ecosystem.

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