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6 Ecological Responses to Changes in Lake Level INTRODUCTION Changes in the level of Mono Lake from the current level of 6380 ft can be expected to affect the Mono Basin ecosystem in various ways. Other wildlife scenic tUt a ~ ormat~ons, The aquatic biota, birds, and ~ ~ shoreline and ripa- rian vegetation, lake water chemistry, and air quality will all be affected in some way by a rising or falling of the lake level. Because these aspects of the basin can be expected to respond differently to changes in lake level, the committee assessed the consequences to each of these components for a range of lake levels. In this chapter, the ecological re- sponses to changes in lake level are assessed for levels in 1 0-ft intervals from 6430 ft. the approximate historic high stand, to 6330 ft. the approximate stabilization level--i.e., the level at which inflows equal outflows and equilibrium occurs--assuming exports of 100,000 acre-ft/yr and climatic conditions similar to those in the previous 40 years (LADWP, 1987; Vorster, 1985~. If the climate were to be- come drier, the stabilization level would be lower. The ecological consequences of these lower levels were not considered because the effects would be much the same as those that would occur if the lake level fell to 6330 ft. Figure 6.1 illustrates the appearance of the lake and shoreline at representative lake elevations of 6400, 6380 (current lake level), 6360, and 6340 ft above sea level. Fig- ure 6.2 is a composite representation of the shoreline at 179

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180 EIeva1Ion Bonn e EIev~on ~0 ant Me ~~o B~ Ero~~ age IN FEW n Ale IN FEW 1 0~ Do_ FIGURE 6.1 Sbore~ne of Mono Lake at lake elevations of 6400 and 6380 f1 above sea level

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Ecological Responses to Changes in Lake Level .,. ~ . ; .............. ; ;.; .. . . ....................... . . .. .. .. .......... .. Elevation 6360 ft 1 N .. ~ ............. ~ .. . , , , ., , ...................... ,,, ............................. .. ............... .... .............. , . .......... ........................ , .. ~ ..................... -; ......................... \ . l .. .. .. ~ ... ... ....W .. ......... .............. ,.,. , , , , . .. , ., ,,, , .,, , ., ......... .... , , , ,,,, ,.,, , , , .. . . . . . . .. , . . . . , . ~ .... . ................ . , \ ........ ,, , , 10000 000 0 10000 20000 SCALE IN FEET Elevation 6~0 ~ N 10000 5000 0 10000 SCA E IN FEE 181 aim ,., ] A..,--.... ..... . ~ :.:~:.:~:.:: :. :~:: :~: :~: :~: :-:-:-:::-\ E: ~~ - ~ ~ :::::::-::: :-:-:: :-:~::3 :~:.:~: :~:-:-:~-:~:: :::: :~: ::::-:::. :~:-::: - :-:.::.:-: :~: ~ ~ ~ :' A:.::-::: :-:-:-::: :.:::.: c. :~:.:~:.:-~:~:~:-:~:~:-~:.: ~ Am: :-: :-:: :-: :-:: ::::::::::::::::::::::::::::::::::::::::::: ~ :-: :::: -:-:-: :-:-:-:-:-: :~: :~::: :-:::: :~: :: W:~::-:-:-:-:::::::::~::-:::-:.:::::::. W~ :::::::::-::. `::::~:~:~:::-:-::-:::-::-::-:::::::. N:-.:~:.:~:::-:-::::::::::~:::::::-:: \::::::::::::::::::::::: :-: :-:-::: :-:-:- \:::: :-:-:::::: - - ~ .:: I-: :.:~:-~::::: :- - - : :-: (.:::: :-:: :-: ~ :-: A-: ::::::: - ~ ::: i~ :~ .-::::- ::::-:-: :-::: ::: ::. A:: :~:~:-~:~:~:~:~:.:~:~:~:~:~:. ~ ~ ~ .:-:.:~:~:~:~:~:.:~:~:-: /:.-:::.::-:-:: :-:-: :-:::::::::: :-:.:-::: :-:-.:::. :::: :~: :~: :-:: ::: :-:-:.-.: :~ : :---- /::-:-::~::::::::::::-:-:::- - ::- ~::-::-::-:::::::::::::::: - -:-- ':-:~:~:-:-: - -::::: :~:: :-:-:: - - - :-- /: :-::: :~ :-- c' 2~ 2-2~ 2e2-~ r ............... \ $ ............................ \ ............................ . a . ~, . ....................................... .~ ~.,. ~ ~2"22222"2"'"2"222"~ t" - :::::::::::::::::::::::::::::::::::1 1::::::::::::::::::::::::::::::::::' \..................... \........................... \..................... ................................ \............................. t........................... Y......................... t............................. 1.......................... I......... I........................ A................ .~ ... - - . :--. ..~ of: FIGURE 6.1 (continued) Shoreline of Mono Lake at lake elevations of 6360 and 6340 ft above sea level.

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182 The Mono Basin Ecosystem .~ ~ i.... ~ .. ~ ~ ~ ~~ ........... :.:.: :.::.:.: ~ 0 it:. 6400 ::::::::::::::::::: ::: ::: - - - ~ ~ 6 60 ~.:::::: :. ..... ::::::::::: ;~ ~ `\ ~ ::: .:::::::: "''''' ~ \~g' ~~ ) at' ' '. ~ ) ~ ~2 '-' 2.' '.2'2.2--'''''.'2' //~.''"''.2.2-'.'..'.2.'...'.'...2'' j~....................................................... / ~ F 2" : ' : - 2 ',, :. //~ ~ 2.2 -: /? f,~ ..... 1 MONO ~ SCALE IN FEET FIGURE 6.2 Composite representation of shoreline of Mono Lake at elevations of 6400, 6380, 6360, and 6340 ft above sea level. these elevations. If the lake level were to rise above or fall below the current level, the shoreline would be either inundated or exposed. These changes have implications for the snowy plover that use the alkali flats for nesting, the shoreline vegetation, the air quality in and around the basin, and the tufa formations. If the lake level were to drop, the islands would become peninsulas, affecting the ability of California gulls to use the islands for nesting. At lower lake levels the gently sloping incline, particularly on the eastern and northern portions of the lake, would be exposed and the nearshore region of the lake bottom would be more steeply inclined. This result of the bathymetry decreases the availability of the shallow littoral habitat necessary for the maintenance of the brine fly population. Most importantly, the bird populations would be unable to rely on Mono Lake if the lake level decreased to the point where the concomitant increase in salinity eliminated their food source, the brine shrimp and brine fly. Changes in

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Ecological Responses to Changes in Lake Level 183 lake level are also associated with changes in the flow of the streams feeding the lake, with consequences for stream biota, the riparian vegetation, and the wildlife populations relying on the riparian habitat. Lake level is controlled not only by the amount of water exported from the basin, but also by natural climatic conditions. Regardless of the lake level that may be es- tablished as necessary to preserve the ecosystem, fluctua- tions in lake level will occur because of natural changes in weather from year to year. Therefore, predictions about the ecological consequences that would occur at a parti- cular lake level must take into account the possibility of fluctuations around that level. The effects of changes in lake level on the individual components of the ecosystem--salinity and chemical strat- bird populations, ification of the lake, aquatic biology, shoreline environment, and upland environment--are dis- cussec} below. Later in the chapter, the overall ecological consequences of changes in lake level on the resources of the Mono Basin are summarized. RESPONSES OF ECOSYSTEM COMPONENTS TO CHANGES IN LAKE LEVEL Salinity and Chemical Stratification of Lake Water Because there are no outlets from Mono Lake, ions dis- solved in the streamflow and groundwater that feed the lake become concentrated as evaporation causes the lake level to fall. If the lake level rises, the concentration of ions would decrease with dilution from the incoming fresh water. Using both historical measurements and calculated data from hydrological models, one can estimate the salinity of Mono Lake at specific lake levels, as described in chapter 2. Historical measurements of lake level and salinity are presented in Table 6.1. Predicted values for the salinity of the lake at levels below the current lake level (approxi- mately 6380 ft) are given in Table 6.2. These values were calculated by using the LADWP hydrologic model and the

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184 The Mono Basin Ecosystem TABLE 6.1 Historical Measurements of Lake Elevation and Salinity for Mono Lake (LADWP, 1986) Lake Elevation (ft) Salinity (g/1 total dissolved solids) 6417 6414 6410 6407 6403 6380 6378 6377 6376 6375 6373 6372 51.3 54.0 56.3 58.1 60.2 89.3 86.8 91.6 89.3 93.4 97.7 99.4 TABLE 6.2 Predicted Salinity Values for Lake Elevations Below the Present-day Level of Mono Lake (LADWP, 1986) Lake Elevation (ft) Salinity (g/1 total dissolved solids) 6370 6365 6360 6355 6350 6345 6340 6335 6330 6325 101.6 110.9 121.4 133.6 147.8 164.6 184.5 208.2 237.2 273.1

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Ecological Responses to Changes in Lake Level 185 area capacity information from the recent bathymetric sur- vey of the lake (Pelagos Corporation, 1987~. The predicted values in Table 6.2 should be treated as upper bounds for the salinity because the LADWP hydro- logic model assumes that mineral precipitation does not occur over the range of lake levels modeled. However, geochemical models (R. J. Spencer, University of Calgary, personal communication, 1986) indicate that minerals such as bona, mirabilite, and natron will begin to precipitate at low temperatures (near 0C) once the salinity of the lake exceeds approximately 125 g/1. Therefore, surface water salinities should be less than the models predict at lake levels below the level at which these minerals begin to precipitate. This occurs because minerals precipitate throughout the water column at low temperatures in the winter and then redissolve near the bottom at higher tem- peratures in the summer. Consequently, a highly saline layer may develop at the bottom of the lake that is sup- plied by salts precipitated from the upper regions. In addition to the chemical stratification caused by pre- cip~tation of minerals, a less intense stratification occurs with large inputs of fresh water. The less dense fresh water does not completely mix with the denser saline lake water, creating a condition known as meromixis. The ob- servation that Mono Lake became meromictic following the large freshwater inflows in the spring of 1983 raises the possibility that periods of meromixis have occurred in the past and can occur in the future. Historical records of snow pack sizes indicate that years with large runoff oc- curred in 1862, 1890, 193S, 1952, 1967, and 1969, as well as in 1983 and 1986. The probability of meromixis depends upon the relative volumes of lake water and inflows and on the density difference between the two waters. Therefore, at higher stands when the lake is less saline and larger in volume, the likelihood of meromixis would be reduced. In contrast, as level and volume decline and salinity increases, the probability that a large runoff will cause persistent chemical stratification increases. Measurements of limnological and biological conditions in the years following the onset of meromixis in l9X3 (chapters 3 and 4) can be used to generalize the implica- tions to the aquatic ecosystem of periods of meromixis.

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186 The Mono Basin Ecosystem The persistent and strong stratification in the meromictic lake reduces rates of vertical mixing and results in increasing concentrations of reduced compounds such as methane, ammonium, and hydrogen sulfide in the water below the chemocline. The vertical flux of substances, such as ammonium, across the chemocline depends upon the coefficient of eddy conductivity, which is reduced by the strong density gradient, and upon the concentration gra- dient of the substance. Hence, while reduced vertical flux of ammonium was observed in 1984-1985 in the initial stages of meromixis, if the density gradient weakens and the concentration gradient of ammonium increases, the ver- tical flux may increase. Furthermore, if entrainment of water within the chemocline or complete turnover occurred, a large pulse of anoxic water rich in reduced compounds would mix into the upper water. In contrast, the forma- tion of a highly saline layer near the bottom of the lake as a result of the settling and redissolution of minerals should cause the lake to remain meromictic for a very long time. This dense, anoxic layer would trap nutrients within it. Several biological responses to meromixis are possible. Reduced supply of ammonium will reduce phytoplankton growth and abundance in the upper mixed layer. However, it is possible that photoautotrophic and chemoautotrophic bacteria may grow in the chemocline and saline bottom waters and augment the lake's primary productivity, though probably not sufficiently to replace the loss in algal growth. With lower algal abundance the food available to the first generation of brine shrimp would be reduced, the fecundity of the females would decline, and the switch to cyst production from live bearing could occur earlier each year. A smaller second generation of brine shrimp would result. If a sudden mixing event that injected high con- centrations of reduced substances toxic to brine shrimp occurred, an abrupt, major decline in brine shrimp abun- dance would be likely. Such an event would also add am- monium, which would enhance algal growth. Another consequence of meromixis is a more dilute upper mixed layer. Hence, if the salinity of the lake had reached a concentration that was adversely affecting the

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Ecological Responses to Changes in Lake Level 187 biota, the freshening of the surface waters could improve conditions. Aquatic Biology Falling lake levels in Mono Lake would result in increasing salinity with concomitant effects on the aquatic biota. A summary of the impacts on planktonic and ben- thic algae is provided in Table 6.3 and on brine fly and brine shrimp populations in Table 6.4. Biological responses to meromixis, which would become more likely if lake lev- els fell, might have synergistic effects with those related to salinity per se. A gradual decrease in phytoplankton productivity would be expected as the salinity increased to approximately 150 g/l. A greater decrease in growth at salinities above ap- proximately 175 g/1 would occur if the current species re- tained dominance. If more salt-tolerant genera such as Dunaliella attained dominance, which is likely, a lesser decrease in phytoplankton productivity at salinities greater than 150 g/1 would be probable. One major component of the benthic alga assemblage, Ctenocladus circinnatus, would become less abundant and less productive at a salinity of approximately 100 g/1. A much greater decline in overall phytobenthic productivity is expected at approximately 150 g/l. . . .. . ~ If lake levels were to fall and expose highly erodible sediments, the comnloatlon action could transport these . . _ ~ . ~ of upland runoff and wave sediments into the lake. The potential influence of these particles on the plankton is complex (Melack, 1985~. Increased suspended sediments would decrease transparency, which would decrease the _ locate Oreo and filter-feeding brine shrimp. region where primary productivity can occur and may re- tard feeding by predators such as grebes, who use vision to Suspended . ~ .~ , _ Limed can adsorb or desorb nutrients such as nitrogen _ ~ ~ . ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ A ~ or phosphorus, organic compounds, ana tOXlC SUDSIaIlCeS. Much more information is required about the sediments of Mono Lake and the quantity expected to into the pelagic region before these evaluated. be transported possibilities can be

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188 The Mono Basin Ecosystem TABLE 6.3 Predicted Effects of Lake Elevation and Sali- nity on Aquatic Plants Lake Elevation Salinity (ft) (g/l) Effects on Aquatic Plants 6430-6380 <50-89 Phytoplankton and phytobenthic algae flourish. (current level) 6370 102 Phytobenthic algal production reduced for some spe- c~es. 6360 121 Reduction in phytoplankton productivity. 6350 148 Decrease in phytoplankton and phytobenthic productivity. Shift in phytoplankton species composition expected. 6340-6330 1 85-237 Large decreases in all types of algal productivity. Further changes in species composition. Because of bioenergetic demands placed on larval growth and development, brine shrimp populations could be expected to gradually decrease in abundance if salinity increased above approximately 120 g/1. A decrease in hatching of dormant brine shrimp embryos would be ex- pected if the salinity increased to approximately 130 g/1. This effect is attributable to a reduction in the "free water" of hydration of the embryos. Rapid decreases in the overall abundance of brine shrimp populations could be expected if salinity increased to approximately 150 g/1. This rapid decrease is predicted on the basis of postulated reductions in primary phytoplankton productivity, which translates into less food, slower growth of all larval stages, and less reproductive output of the adults and lack of cyst hatching. Similarly, a gradual decrease in primary phyto- benthic productivity through losses of hard rock-mu~flat surfaces and elevated salinities would alter the quantity ant! composition of benthic foodstuff. Reduction of benthic foodstuff along with the bioener- getic demands of osmotic stress caused by increases in sa- linity suggests that salinities of 130 to 150 g/1 would result

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Ecological Responses to Changes in Lake Level TABLE 6.4 Predicted Effects of Lake Elevation and Sali- nity on. Aquatic Animals Lake Elevation (ft) 6430-6380 (current level) 6370 6360 6350 6340 6330 Salinity (g/l) Effects on Aquatic Animals <50-89 Brine shrimp and brine fly populations flourish. 102 121 148 185 237 189 Brine shrimp populations unimpaired. Brine fly populations unimpaired by physiological effects of salinity. Loss of' about 40% of submerged hard substrate relative to 6380 ft. Brine shrimp experience slight impairment of hatch. Brine fly larvae show modest decrease in growth. Brine shrimp: no hatch of cysts; decreased naupliar growth and juvenile metamorphosis; and reduced female fecundity. Brine fly: no growth of eggs; reduced female fecundity and reproductive potential. Brine shrimp: nonviability of dormant cysts; inhibition of preemergence mechanism of diapaused embryo; high mortalities in naupliar lifestage; and large reductions in adult populations. Brine fly: high mortalities in larval brine fly; reduced adult populations. Brine shrimp: loss of populations except for small populations located near freshwater spring inflows. Brine fly: loss of populations ex'cept for small populations located at shorelines where fresh water is present. in severe reductions in the growth and development of brine fly larval and adult populations. The hard substrate required for larval grazing on benthic algae would be reduced dramatically if the lake level dropped below 6380 ft (Table 6.5~. Although it has not been possible to quantitatively estimate the brine fly larval population at the current lake level, a reduction of 40 percent of the hard substrate area that would result if lake level fell to

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Ecological Responses to Changes in Lake Level 203 had a major part of their instream flows diverted for over 40 years. Riparian vegetation below diversions on those streams has suffered and little remains. Maintenance of various lake levels above the stabiliza- tion level will require input of various amounts of surface water into these stream systems as shown in Table 6. 12. The response of riparian species to any given input is de- pendent on water requirements for germination, successful establishment of seedlings, and survival of plants during the maturing process. Streamflow apparently does not have to be perennial to maintain a riparian community; many intermittent streams in the semiarid Southwest support riparian vegetation of varying densities. Therefore, some streamflow should main- tain some riparian vegetation, although limited flow may never create the environment that will stimulate recruit- ment or regeneration of some riparian species. With high flows in Rush Creek in 1983 and periodically since, and with minimum flows of 19 cfs since 1985, the surviving riparian vegetation has been rejuvenated. Iso- lated willow and Populus plants have established on the gravel bars along the stream. This surface flow regime in Rush Creek has apparently been sufficient to stimulate re- establishment of woody riparian vegetation. Below the water diversion points, Rush and Lee Vining creeks cross alluvial deposits. That material is so porous that it does not hold groundwater near enough to the sur- face and is inadequate to maintain riparian plants if the upstream input is low. Of the 19 cfs released into Rush Creek at Grant Lake, some but not all reaches Mono Lake. Some of the flow recharges below-surface aquifers that could be used by riparian vegetation. Evidence that the released flow is adequate to recharge the channel alluvium is seen in the fact the water reaches the lake via the stream channel. In 1986, a continual flow of 10 cfs was released into the lower reaches of Lee Vining Creek. That amount is apparently adequate to recharge the channel al- luvium and maintain enough flow to reach the lake. Again, the flow reaching the lake was undoubtedly less than 10 cfs, but, as at Rush Creek, it was not measured. The streamflows given in Table 6.12 would be measured at the point of release and do not represent the amount of

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204 The Mono Basin Ecosystem water entering the lake. With little or no riparian vegeta- tion along the lower reaches of Rush and Lee Vining creeks, the relationship between a flow at the diversion points and the flow entering the lake will remain constant; however, once riparian vegetation becomes reestablished, the flow into the lake will decrease because of increased loss from evapotranspiration. If the 7-ml stretch of Rush Creek between Grant Lake and Mono Lake had a riparian strand averaging 750 ft wide (approximate average width measured from pre-1941 from aerial photos) and was composed of two-thirds Populus spe- cies and one-third willow, the annual evapotranspiration loss would be about 3500 acre-ft based on information in chapter 5. That loss is equivalent to a reduction in streamflow of about 4.S cfs, an amount that may maintain a riparian strand of this size if none of the streamflow were lost into the porous substrate. The minimum flows of 19 and 10 cEs currently maintained in Rush and Lee Vining creeks should be ade- qua~e to maintain r~par~an strands equivalent to those ex- isting in 1941. According to the model results discussed in chapter 2, these flows (29 cfs) would be the average flow to maintain the lake level at 6360 ft as predicted by the LADWP (1987) model. Vorster's mode! (Vorster, 1985) pre- clicts that the flows would be the average to maintain the lake level at approximately 6330 ft (Table 6.12~. In light of the uncertainties in the model predictions and the com- mitteets conservative approach to predicting effects of changes in streamflow on the riparian strand, the commit- tee concludes that flows necessary to maintain lake levels above 6360 ft. regardless of which model is used, should maintain riparian strands on Rush and Lee Vining creeks. From another perspective, if minimal releases of 19 cfs for Rush Creek and 10 cfs for Lee Vining Creek are maintained, the composite (29 cfs) is predicted to maintain lake levels of 6360 or 6330 ft. depending on which model is used (Table 6. 12~. Intermittent dry periods and changes in flow probably would have little negative effect, especially in the fall and winter. However, maintenance of riparian vegetation would also require periodic heavy releases in the spring to enhance the recruitment potential of riparian species. .

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Ecological Responses to Changes in Lake Level 205 Releases of less than 29 cfs may not be sufficient to maintain a riparian community. If released solely in Rush Creek, the flow may be sufficient to maintain a riparian strand. If the flow were to be divided between Rush and Lee Vining creeks, however, it would probably be inade- quate to maintain a healthy riparian strand. The role of a riparian strand becomes important in cal- culating the flow required to maintain any given lake level. If the fully reestablished riparian strand along Rush Creek were to consume 3500 acre-ft/yr of water, the lake levels that could be maintained by a given release would be re- duced by about 2 ft. Establishment of a riparian strand along Lee Vining Creek would lead to yet further reduction in lake level. Fish Because no fish live in Mono Lake itself, the water level in the lake has no direct effect on any fish popula- tions in the basin. However, the water level in Mono Lake is affected by the amount of water flowing in the streams that drain the basin (Table 6. 12~. Although it is difficult to predict the relationship between flow at the diversion points and the suitability of the stream for fish if the var- iation in flow is not known, it is certain that sustained moderate releases spread throughout the year would- be more beneficial to fish populations and the animals that they rely on for food than a single major release in spring followed by a complete lack of flow during the remainder of the year. Similar considerations apply to mammalian wildlife that uses the riparian systems. At present, minimal flows are being maintained of 19 cfs in lower Rush Creek and 10 cfs in lower Lee Vining Creek. These flows are adequate to support reproducing populations of brown trout in the two streams, and some rainbow trout may be reproducing in lower Lee Vining Creek. It is probable that increasing the flows (up to a point) would increase the sizes of the trout populations and the number of trout species that reproduce in Lee Vining and Rush creeks.

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206 The Mono Basin Ecosystem SUMMARY AND CONCLUSIONS The resources of the Mono Basin ecosystem--the aquatic biological community, bird populations, and shoreline and upland environments--are affected by changes in lake level in different ways. Some would be adversely affected if lake level rose above the current level (6380 ft), and others would be adversely affected by lower lake levels. In using the conclusions of this report to determine how the ecosystem should be managed, decisions about the rela- tive importance of different resources will have to be made. It is important to keep in mind that the responses of the various resources to changes in lake level would occur gradually over a range of levels. A precise lake level at which an impact would occur cannot generally be pinpointed. Rather, the impacts would increase in intensity with changes in lake level to a point where the impact becomes severe. The effects of changes in lake level are summarized in Table 6. 13, and the range of levels over which the impacts occur are shown in Figure 6.3. The major ecological con- cerns with changes in lake level involve the ecological ef- fects of salinity and habitat availability. If the lake level were to drop, salinity would increase and reduce the water available for metabolism ("free water") for the dominant aquatic organisms: algae, brine shrimp, and brine flies. This reduction would increase the physiological costs of reproduction and growth to the or- ganisms. At salinities around 120 g/1, populations of these organisms would begin to show negative responses, and at 150 g/1 acute ecological effects--drastic population reduc- tions--are predicted. These salinities are associated with lake levels of approximately 6360 and 6350 ft. respectively. The chemical stratification of the lake, known as mero- mixis, that currently occurs and can be expected to occur in the future, particularly if the lake level were to fall, results in an upper layer of water that is less saline than the bottom layer. Therefore, the adverse effects of in- creases in salinity would be alleviated to some degree. If salinity increased, precipitation of salts would occur and would increase the probability and persistence of

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Ecological Responses to Changes in Lake Level 207 meromixis. Although the precise effects of- precipitation on salinity cannot be quantified with the current state of un- derstandina about the geochemistry of Mono Lake, the salts O con ~ , . ~ . . . , , ~ . - . . would negln to precipitate at sallnlues above approxlma~ely 125 g/1. The precipitated salts would reduce the salinity but would generate a dense layer toward the lake bottom that would resist mixing with the overlying water column. This layer would act as a sink for nutrients that would no longer support biological productivity through vertical mix- ~ng. The committee concludes that, in balance, the relation- ship between lake level and salinity predicted assuming a constant quantity of evenly distributed salt in the lake results in an upper limit on the estimate of salinity for a given lake level. Consequently, predictions about the ef- fects of falling lake levels on the organisms assume the worst case. This conservative prediction is appropriate when considering the uncertainties of the data and the severity of the consequences of increases in salinity for the Mono Basin ecosystem. If the lake level dropped, there would be a loss of lit- toral habitat that is essential to the chYtobenthic oroduc- tivity and the brine flies. ~ , , If lake level fell 10 ft from its current level, the amount of hard substrate available as littoral habitat for brine flies would be reduced by 40 per- cent. The nesting and migratory bird populations at Mono Lake are affected by changes in lake level through changes in food chain productivity and availability of nesting habitat. The decreases in availability of brine shrimp for food would begin to have adverse effects on those bird species relying on brine shrimp--eared grebes and Califor- nia gulls--at salinities of 120 g/1 (6360 ft). The impacts would be acute at 150 g/1 (6350 ft). For those birds rely- ing on brine flies--the phalaropes--impacts would begin to become apparent at 6370 ft and would be acute at 6360 ft. The nesting habitat for the California gulls is adversely affected by emerging land bridges and positively affected by new island emergence as water level decreases. A lake level of 6340 ft represents the breakpoint where further reductions are associated with major losses in nesting hab- itat.

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208 The Mono Basin Ecosystem TABLE 6.13 Predicted Major Effects of Changing Lake Elevation on Resources of Mono Basin (Effects are rela- tive to current elevation of 6380 ft) Resource Major Effect of Changing Lake Elevation Aquatic biology Algae Brine shrimp Brine fly Bird populations Eared grebe Phalaropes California gull Snowy plover Shoreline environment Vegetation Air quality Tufa Large reductions in productivity, caused by reduced nutrient supply associated with meromixis, increa- singly likely at salinities above 125 g/1 (corresponding to lake elevation of approximately 6358 ft). Large reductions in productivity and species shifts related to salinity increases above 150 g/1 (corre- sponding to lake elevation of 6350 ft) for benthic algae and above 185 g/1 for phytoplankton. Physiological effects of increase in salinity slight at 120 g/1 (corresponding to lake elevation of 6360 ft) and severe at 150 g/1 (corresponding to lake elevation of 6350 ft). Hard substrate required for habitat substantially reduced if lake elevation falls below 6370 ft. Physiological effects of increase in salinity severe at 150 g/1. Affected by decreases in availability of brine shrimp at 6350 to 6360 ft. Affected by decreases in availability of brine fly at 6360 to 6370 ft. Affected by decreases in availability of brine shrimp at 6350 to 6360 ft. Island area for nesting severely affected at elevations below 6350 ft. Gradual reduction in shoreline nesting area if lake elevation rises. Nesting population area approaches maximum size at 6360 ft. Increases in lake elevation gradually inundate saltgrass. Decreases in lake elevation would extend exposed lake beds. Vegetation would be established only in area with springs. Increases in lake elevation would inundate alkali flats and reduce dust problem. Decreases in lake elevation would expose alkali flats and exacerbate problem. Increases in lake elevation would gradually inundate tufa groves to 6430 ft. Sand tufa gradually destroyed with increases in elevation from 6410 to 6430 ft. Decreases in lake elevation would expose

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Ecological Responses to Changes in Lake Level TABLE 6.13 (continued) 209 Resource Major Effect of Changing Lake Elevation Upland environment more tufa for tourism. Exposed tufa may be susceptible to vandalism. Riparian vegetation Currently maintained flows of 19 cfs in Rush Creek and 10 cfs in Lee Vining (approximate average flow at diversion points to maintain lake elevation at 6360 ft or 6330 ft depending on the model used) adequate to maintain healthy riparian strand. Periodic flooding needed for riparian species recruitment. Riparian habitat Currently maintained flows adequate to maintain healthy fish populations. The exposure of the lake bed with lower lake levels would generate an increase in the areal extent of the shoreline playas and a lower water table under much of the existing shoreline vegetation. Exposure of playas would increase the area available for the snowy plover to nest. The maximum nesting area would be approached at lake levels around 6360 ft. Inundation of salt flats if lake level rose would reduce the area available to the snowy plover. If lake levels dropped, existing tufa formations would be exposed to more erosion and human damage, though sub- merged tufa formations would be exposed for visual enioY- ment. inundate currently exposed On the other hand, increases in lake level would ., ~ tufa formations, though the formations would still be visible. A widely fluctuating lake level would lead to more damage from wave action than would a lake level with less fluctuation. The sand tufa would be gradually destroyed if the lake level were to in- crease from 6410 to 6430 ft. The increase in exposed lake bed would exacerbate existing downwind air quality prob- lems. . ~ ^. ~ . ~ ~ ~ , ~ , ~1 _ Alteratlons 1,, A, basin would be expected with changes in lake level, although it is not possible to predict the precise flow regime that would be associated with a particular lake level because the distribution of flows between the diverted streams depends on how the water is managed. Changes in "n the flow of one unversed streams In one

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210 6420 6410 6400 *: at 6390 o 6380 6370 - 6360 _ I : 6350 6340 6330 The Mono Basin Ecosystem ~ resource maintained --resource silghtly affected .... resource severely affected resource eliminated 1 1 1 1 1 1 S - 1 ~ ~ 1 1 1 1 1 1 1 1 1 1 1 1 1' 1 1 1 1 1 1 1 . I ! , l 1 1 ~ ~ a ~ . : i _ ; I 1 8 ~ 1 1 . ~ : 1 : I : 1 : ~ ~ ~ ~ ~ I I I ~ s . E , , 0 = e sC ~ C ~ m ~ ~ In In ~ ~ ' an 89 ~ - an 121 185 FIGURE 6.3 Ranges of lake levels affecting resources of the Mono Basin, with three salinities added for reference. streamflow have consequences for the riparian vegetation and habitat. The flows currently maintained in Rush and Lee Vining creeks, l9 and lO cfs, respectively, appear ade- quate to maintain a healthy riparian community. Depending on which of the two hydrologic models of the basin is

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Ecological Responses to Changes in Lake Level 211 used, these flows are estimated to be approximately the average flows at the diversion points necessary to maintain the lake level at 6360 or 6330 ft. Determination of lake levels that would preserve the integrity of the ecosystem must take into account the need for an increment of lake level higher (or lower) than the basic minimum (or maximum) that causes catastrophic dis- ruption of the ecosystem. This buffer is required for two reasons. First, the predictions in this report are based on data that in some cases are incomplete or uncertain, resulting in the need for a margin of safety against disrup- tion of the ecosystem. Second, natural fluctuations in cli- mate can cause fluctuations around a particular lake level that could not be controlled. The predictions included in this report are the most accurate that the committee could achieve based on the available information. These predictions need to be veri- fied by continued and expanded research and monitoring, as pointed out throughout the report. REFERENCES Los Angeles Department of Water and Power. 1986. Report on Mono Lake Salinity. Los Angeles, Calif. Los Angeles Department of Water and Power. 1987. Mono Basin Geology and Hydrology. Los Angeles, Calif. Melack, J. M. 1985. Interactions of detrital particulates and plankton. Hydrobiologia 125:209-220. Pelagos Corporation. 1987. A Bathymetric and Geologic Survey at Mono Lake, California. Los Angeles ~~~ Diego, Calif. Saint-Amand, P., L. Reinking. 1 986. Valleys, California. Report prepared for Department ot water and Power. San A. Mathews, C. Gaines, and R. Dust Storms from Owens and Mono NWC TP6731. China Lake, Calif.: Naval Weapons Center. 79 pp. Vorster, P. 1985. A Water Balance Forecast Model for Mono Lake, California. Master's thesis, California State University, Hayward. Earth Resources Monograph No. 10. San Francisco, Calif.: U.S. Forest Service, Region 5.

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