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Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
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OCR for page 179
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
OCR for page 180
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
OCR for page 181
Ecological Responses to Changes in Lake Level
.,. ~ . ; .............. ; ;.;
.. . . ....................... . . .. ..
.. .......... ..
Elevation 6360 ft
1
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10000 000 0 10000 20000
SCALE IN FEET
Elevation 6~0 ~
N
10000 5000 0 10000
SCA E IN FEE
181
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A................
.~ ... - - .
:--. ..~ of:
FIGURE 6.1 (continued) Shoreline of Mono Lake at lake
elevations of 6360 and 6340 ft above sea level.
OCR for page 182
182
The Mono Basin Ecosystem
.~ ~ i.... ~ .. ~ ~ ~ ~~
........... :.:.: :.::.:.: ~· 0 it:. 6400 ::::::::::::::::::: :::
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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
OCR for page 183
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
OCR for page 184
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
OCR for page 185
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 0°C) 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.
OCR for page 186
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
OCR for page 187
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
OCR for page 188
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
OCR for page 189
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
OCR for page 202
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OCR for page 203
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
OCR for page 204
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.
.
OCR for page 205
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.
OCR for page 206
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
OCR for page 207
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.
OCR for page 208
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
OCR for page 209
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
OCR for page 210
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
OCR for page 211
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.
OCR for page 212
Representative terms from entire chapter:
mono lake
