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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
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
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.
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
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
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
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.
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
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
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
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
190 The Mono Basin Ecosystem TABLE 6.5 Area of Hard Substrate (Tufa and Mudstone) Hard Substrate (acres) Lake Elevation to 10 ft below 30 ft below (ft) level level 6430 6420 6410 6400 6390 6380 6370a 6360 6350 6340 6330 3,095 1,964 2,315 2,919 4,280 8,253 3,191 1,611 853 556 451 7,374 7,198 9,514 15,452 15,724 13,055 5,655 3,020 1,860 1,336 958 aElevations above 6370 are underestimates of hard substrates. Hard substrates north and northeast of Negit Island are not indicated because the area could not be ac- cessed for the bathymetric survey. SOURCE: From data in Pelagos Corporation (1987~. 6370 ft (relative to the hard substrate area at the current level of 6380 ft) would certainly be detrimental to the brine fly populations. Aquatic Bird Populations The critical food resources for aquatic birds using Mono Lake are brine shrimp and brine flies. Eared grebes and California gulls feed primarily on brine shrimp, while red- necked phalaropes, and to a lesser extent Wilson's phala- ropes, specialize on brine flies. Although we do not know
Ecological Responses to Changes in Lake Level 191 the relationship between overall brine shrimp and brine fly abundance and their availability to birds in profitable con- centrations, it is certain that at some point increasing sa- linities in the lake would cause a sufficient drop in brine shrimp and brine fly abundance that birds would be affected. In Table 6.6 the possible impacts of changes in food supplies on grebes and phalaropes for various lake levels are predicted. Except insofar as they affect prey populations, changes in the salinity of Mono Lake are ex- pected to have little or no direct physiological effect on birds. In addition to adequate food supplies, California gulls breeding at Mono Lake also require nesting sites safe from disturbance and predation. For these sites, the gulls de- pend on islands in the lake, where they are safe from canids. Island area decreases with increasing lake levels above 6380 ft as islands are inundated, and is roughly con- stant for lake levels between 6380 and 6350 ft. Below 6350 ft there is a precipitous loss of island area (Table 6.7~. What is harder to predict are possible changes in the hab- itat quality of the available island area. The gulls have shown a willingness to change sites as land bridges form, and the committee assumes in this analysis that one area can be substituted for another with little effect. Likewise, the figures of island area reflect the present conditions, and do not take into account the erosion of the small, unconsolidated islets that result from fluctuating, particu- larly rising, water levels. To the extent that these islets are important nesting sites, their accelerated erosion could result in greater restriction of gull nesting habitat than is apparent from the overall figures of island area. It should be noted that, if the lake level fell to the level that would significantly decrease island area, most gulls would likely have already deserted Mono Lake because of lack of food. Mono Lake is of regional importance to populations of eared grebes, Wilson's phalaropes, and California gulls, and of local importance to red-necked phalaropes. The lake is one of the major Great Basin staging areas used by eared grebes and Wilson's phalaropes during the fall migration. Birds currently using Mono Lake as a staging area, or even as a migratory stopover (red-necked phalaropes), may be able to shift to alternative sites, such as the Salton Sea or
192 The Mono Basin Ecosystem TABLE 6.6 Predicted Response of Aquatic Birds to Chang- ing Lake Levels Lake Elevation (ft) 6370-6430 6360-6370 6350-6360 6330-6350 Response of Bird Populations Migrant grebes remain at Mono Lake, molting and fattening, until brine shrimp populations become depleted. Migrant Wilson's phalaropes remain at Mono Lake until they complete their molt and become fat; red-necked phalaropes fatten during migratory stopover. Gulls breed in present numbers with present levels of success. Migrant grebes leave Mono Lake earlier than they do at higher lake elevations. Wilson's phalaropes find it difficult to complete molt and premigrating fattening before departing; red-necked phalaropes' stopover shortened. Gulls may show some decreases in numbers or reproductive success due to food shortages. Duration of grebesi stopover probably no longer permits molting or fat deposition. Migrant phalaropes cannot use Mono Lake as a major stopover site. Breeding numbers of gulls drastically reduced and reproductive success lowered due to lack of food. Grebes and phalaropes do not use Mono Lake as a stopover site. Few gulls attempting to breed. Those present largely dependent on food resources distant from Mono Lake. San Francisco Bay, provided such appropriate habitats con- tinue to exist. However, the committee does not know whether these sites can sustain such a major influx of new individuals and the long-term implications of such a shift. The islands of Mono Lake provide the nesting sites of a significant proportion of the North American (world) popu- lation of California gulls. Loss or severe damage to any of the populations of eared grebes, California gulls, or phala- ropes at Mono Lake would not only change strikingly the scenic features and the ecology of the Mono Basin scenic area, it may also have a potential impact on Great Basin and western North American populations of these species.
Ecological Responses to Changes in Lake Level 193 TABLE 6.7 Predicted Changes in California Gull Nesting Habitat with Changes in Lake Elevation (baseline of 6380 feet) Lake Elevation (ft) 6430 6420 6410 6400 6390 6380 (current level) 6370 6360 6350 6340 6330 Response of Gull Nesting Habitat 1406 acres of emergent islands, no land bridges. 1439 acres of emergent islands, no land bridges. 1514 acres of emergent islands, no land bridges, most Paoha islets submerged. 1703 acres of emergent islands, no land bridges, many Paoha islets submerged, some loss of gull breeding habitat. 2016 acres of emergent islands, no land bridges, some Paoha islets submerged, some loss of gull breeding habitat. 2655 acres of emergent islands, no land bridges, most Paoha islets emergent, 40,000 to 50,000 gulls nesting. 2272 acres of emergent islands, some land bridges. Negit Island joined to shore, resulting in major loss of some currently used gull breeding area. 2490 acres of emergent islands. Paoha Island still an island. 2616 acres of emergent islands. As above. 195 acres of emergent islands. Paoha Island joined to shore. Very little nesting area left, although depending on its character it may still be sufficient to support a large gull colony. 163 acres of emergent islands. As above. SOURCE: Island area measurements are from Pelagos Corporation (1987). Shoreline Environment The shoreline around Mono Lake would be inundated or exposed if lake level were to rise above or fall below the current level. The area of shoreline that would be
194 The Mono Basin Ecosystem affected at different levels is shown in Table 6.~. Con- sequences of these changes for the abiotic components of the shoreline environment--nearshore groundwater, tufa formations, and air quality--and the biotic components of the shoreline environment--the snowy plover habitat and the shoreline vegetation--are discussed below. Nearshore Groundwater If the lake level were to rise or fall, locations of springs and marshes at the lake margin would generally rise or fall in elevation accordingly. Marsh geometries and spring locations would depend on locations of tufa, joints, and high permeability zones, as well as on surface topog- raphy. Large shoreline springs are located in the County Park area where alluvial fan deposits are recharged by Mill, Wilson, and Dechambeau creeks. Here and along the west side of the lake to the Old Marina, marshlands would mi- grate to lower elevations if lake level fell. The areal ex- tent of the seepage zones may increase if the lake eleva- tion were to fall below approximately 6380 ft because of the decreasing slope of the exposed lake bed. Similarly, at Navy Beach, where spring waters appear to be a mixture of shallow groundwater and fault-controlled flows, spring lo- cations would migrate if lake levels changed. With the exception of Simon's Spring, zones of seepage on the north and east sides of the lake are located away from the lakeshore, apparently occurring at the outcrop of shallow, water-bearing sediments. Unless the lake level were to rise to the elevation of these zones, seepage would not be affected by changing lake levels. But at Simon's Spring, groundwater flow is influenced by the presence of a fault that extends across the area into the lake. A com- parison of past and recent aerial photographs indicates that the surface area of the marshlands has increased as lake levels have dropped and exposed more of the fault zone. If the lake level changes in the future, the shape of the marshlands would be altered. The marshlands would mi- grate with the shoreline and might continue to enlarge if lake level fell and might become smaller if lake level rose.
Ecological Responses to Charges in Lake Level 195 TABLE 6.S Areas of Lake Bed Exposed at Different Eleva- tions with Incremental Changes Between Lake Elevations (baseline at 6430 ft) Exposed Lake Bed Incremental Change Lake Elevation (ft) Square miles Acres Hectares Square miles Acres Hectares 6430 0 0 0 . 4.9 3, 1 28 1 ,266 6420 4.9 3, 1 28 1 ,266 3.0 1,968 797 64 1 0 7.9 5,096 2,063 37 2,320 939 6400 1 1.6 7,416 3,002 4.5 2,929 1, 1 87 6390 16.1 10,346 4,189 6.8 6380 22.9 1 4,647 5,930 6370 35.9 22,969 9,299 6360 4 1.4 26,495 1 0,727 6350 45.4 29,060 1 1,765 6340 49.9 3 1 ,970 1 2,943 6330 54.2 34,7 1 7 1 i,o55 4,302 1 ~74 1 1 3.0 8,322 3,369 5.5 3,526 1,428 4.0 2,565 1,038 4.5 4.3 1,178 2,747 1, 1 1 2 SOURCE: Data from Pelagos Corporation (1987). Most of the shoreline north of Simon's Spring to the lakeshore southeast of Bridgeport Creek is bordered by salt flats, which have become progressively wider as lake levels have lowered. This region is characterized by low ground surface gradients and a very shallow water table. The shallow groundwater has high total dissolved solids near the lake, indicating relatively little flow toward the lake. These salt flats would probably continue to become wider if
196 The Mono Basin Ecosystem lake level were to drop and narrower if lake level were to rise. Tufa Formations If the lake level were to rise above the current level, tufa groves would be at least partially submerged. Many of the taller lithoid towers, however, would be visible even though their bases would be underwater. As discussed in chapter 5 and summarized in Table 6.9, elevations of tufa groves range from approximately 6368 to 6430 ft. If the lake rose to approximately 6404 ft. visitors would be able to view but would not have access to the South Tufa and Lee Vining Tufa areas. This situation would occur at lake levels of 6415 ft at the County Park and Bridgeport Creek tufa groves and at approximately 6425 ft at the Old Marina. Tufa towers occur above 6430 ft at Simon's Spring and Warm Springs and would consequently not be affected by lake elevations considered in this report. Wave action against the base of the lithoid tufa forma- tions could damage some of the towers, though information is not available to estimate the extent of the damage. Wide fluctuations in lake level would be more likely to cause damage to a larger number of tufa towers than would a more constant lake level. The sand tufa, more delicate than the lithoid towers, are more likely to be significantly damaged if inundated. The sand tufa occur at elevations ranging from approxi- mately 6390 to 6432 ft. Consequently, if the lake level were to rise above 6390 ft. some of the sand tufa would be destroyed. All of the formations would be destroyed if the lake level were to reach 6432 ft. If the lake level fell below its current level, tufa for- mations that are currently submerged would be exposed. The exposure would increase the areas of tufa groves available to visitors, and could also increase the number of formations subject to vandalism. Tufa groves at South Tufa, Lee Vining Tufa, and County Park areas would be completely exposed if lake level fell to approximately 636S, 636S, and 6370 ft. respectively.
Ecological Responses to Changes in Lake Level TABLE 6.9 Formations Lake Elevation (ft) 6430 6420 6410 6400 6390 6380 6370 6360-6330 197 Predicted Effects of Lake Elevation on Tufa Effects on Tufa Formations Old Marina tufa are submerged. Tufa towers still visible. All sand tufa inundated. County Park and Bridgeport Creek tufa areas submerged. Tufa towers still visible. South Tufa and Lee Vining tufa areas submerged. Tufa towers still visible. Some additional sand tufa inundated. Some sand tufa inundated. Current level. Currently submerged tufa formations exposed at South Tufa, Lee Vining, and County Park. Some currently submerged tufa formations would be exposed with declining lake levels. NOTE: Locations of tufa groves are shown in Figures 5.8 and 5.9. Air Quality Falling lake level would expose more of the former lake bed and thus presumably provide more particulates and thus more aerosols during windstorms. Similarly, should the lake continue to rise, less of the former lake bed would be exposed and, presumably, dust storms would be less intense. A major factor in dust storms is the frequency and dur- ation of strong wind events. These vary from one year to another depending on storm types, frequency of frontal passages, and the direction and strength of polar and sub- tropical jet streams. Simply put, some years are much windier than others. Consequently, the concentration of dust depends not only on the area of former lake bed ex- posed but also on the hydrometeorological conditions. For example, strong wind episodes that occur when the basin is snow-covered will raise few aerosols.
198 The Mono Basin Ecosystem The character and moisture content of the lake bed material also affect the severity of dust storms. Saint- Amand et al. (1986) note that the physical and chemical morphology of alkali crystals creates fine dusts most sus- ceptible to airborne transport. The potential availability of aerosols depends on future mitigation efforts, including the use of sand fences, stabilization by vegetation, and other innovative projects. Snowy Plover Nesting Sites As summarized in Table 6.10, the alkali flats along the eastern shore of the lake that provide the nesting for the snowy plover would be gradually inundated if lake levels rose. At levels above approximately 6420 ft the population of snowy plovers would be reduced to a few pairs nesting in elevated dunes. For lake levels between 6380 and 6420 ft. the areal extent of the alkali flats would be incrementally reduced, but information is not available to predict precisely how these changes would incrementally affect the snowy plover populations. Though the population has benefited from decreases in lake level and exposure of the alkali flats, continued ex- pansion of the flats is not likely to expand the snowy plover habitat because distances from the nest to the shoreline feeding areas would be too great. More impor- tantly, access to feeding areas may be restricted with re- duction in the length of the lake's shoreline with decreases in lake level. The nesting population would probably approach its maximum level at a lake elevation of approxi- mately 6360 ft. Shoreline Vegetation The shoreline vegetation mosaic is directly related to the amounts of freshwater flow and the salinity of soil pore water. Areas that exemplify the influence of fresh- water availability can be found at Simon's Spring, Warm Springs, along the northwestern shore, and near Gull Bath. The influence of pore water salinity is most obvious along
Ecological Responses to Changes in Lake Level 199 TABLE 6.10 Predicted Response of Snowy Plovers to Changes in Lake Elevation Lake Elevation (ft) Response of Snowy Plovers 6430-64 1 0 64 1 0-6380 6380 6370-6360 6360 6360-6330 Nesting population reduced from current level to a few pair located in elevated dunes. Nesting population decreased from current level in proportion to area of exposed playa. Nesting population of approximately 350 pair. Nesting population increased in proportion to area of exposed lake bed. Nesting population probably approaches maximum level. No further increases in nesting population, possible decreases. the eastern and northeastern shores of the lake, where the wind-swept salts and sands accumulate on the exposed lake bed. Vegetation immediately associated with the springs and seeps is characteristically marshy. If the shoreline is steep, as on the western shore, the marsh vegetation may grow adjacent to the lake. However, when the lake level was lower in the late 1 970s (6378 ft and lower), there was barren shoreline below the marshes in these areas. It is not clear whether these areas were barren because plants did not have sufficient time to colonize the exposed lake bed or because the salinity of the exposed soils was too high for plant establishment. Spring or seepage areas near the lakeshore at the east- ern end of the lake form localized marshy areas. In some cases the marsh areas grade into barren, alkaline lake bed through a narrow band of saltgrass (Distichlis spicata) and associated herbaceous vegetation (e.g., Warm Springs). In other areas where freshwater flow is sufficient to maintain vegetation over larger areas, the marsh thins toward the lake or grades into saltgrass, which continues on to the shoreline (e.g., Simon's Spring). -
200 The Mono Basin Ecosystem Two areas with vegetative cover atypical for the shore- line at Mono Lake are the alluvial fans where Lee Vining and Rush creeks enter the lake. The alluvial fans have formed terraces that drop abruptly into the lake. Because of their height above the lake and the supply of fresh groundwater from the Sierra, they support a shrub com- munity dominated by rabbitbrush or grassy vegetation. Sparse stands of willows and other riparian species are found in the river channels. The changes in shoreline vegetation that could be ex- pected with changes in lake level are summarized in Table 6.1 1. Generally, the changes in vegetation would be con- trolled by the changes in fresh water available from spring or seeps, as described in the above sections on nearshore groundwater. Upland Environment If the lake level were to be maintained at any given level, specific amounts of water would have to flow into the lake each year. Based on the modeling results described in chapter 2, necessary inflows required to main- tain given lake levels are reported as average flows at the diversion points (Table 6.12~. The consequences of chang- ing streamflows for the riparian vegetation and riparian habitats are discussed below. Riparian Vegetation Riparian vegetation is dependent on more soil moisture than is supplied by normal percolation of rain or snow fall. For this reason, riparian species typically occur along stream courses, on the lake edge, and where groundwater reaches the surface at springs and seeps. Reduction or elimination of the supplemental water source from surface runoff or from lakes and springs may stress riparian spe- cies so severely that they will be eliminated from the system. The riparian systems of streams that enter Mono Lake are discussed in chapter 5. Four of these streams have
Ecological Responses to Changes in Lake Level 201 TABLE 6.11 Predicted Shoreline Responses to Changes in Lake Elevation (baseline of 6380 ft) Lake Elevation (ft) 6430 6400 6390 6380 6370 6360 6350 6340-6330 Shoreline Responses Continued inundation of grassland flats and barren shore and possible local upward migration of marshes. Inundation of barren shores on northeast, east, and southeast, phreatophytic saltgrass flats and spring areas Inundation in nonvegetated areas on east of lake, reducing barren playas. Inundation of many phreatophytic grass areas. Inundation of marshes on west end steep shoreline. Possible upward migration of marshes on western shore. Present status--baseline. Eastern shoreline recedes by 4000-6000 surface ft and up to 8000 ft near Negit Island. Annual plants will invade black sands along north shore. Most exposed lake bed will remain barren. Warm Springs will continue to maintain a marsh well above shoreline. Simon's Spring as a source of fresh water may cause encroachment of saltgrass and other phreatophytic herbs along the fault that controls the spring. Tufa areas on south central and western shores may become freshwater spring areas and be invaded by marsh vegetation. Marshes on northwestern end of lake may gradually invade shore where fresh water percolates to surface. Upland vegetation may migrate downward. Steep shorelines near Lee Vining and Rush creek deltas and around islands will erode and may prevent vegetation establishment. Most exposed lake bed will remain barren, especially northeast, east, and south shores. Simon's Spring seepage may continue to allow localized encroachment of saltgrass onto exposed lake bed along fault. West and northwest shores may still have sufficient freshwater seepage to permit marsh establishment on exposed lake bed. Tufa areas may have sufficient springs to support herbaceous vegetation. Rush, Lee Vining, and other creeks will be deeply incised with upstream downcutting eliminating established vegetation and denuding banks; stream deltas will be expanded. Newly exposed lake bed probably will support no vegetation; it may be too saline. Exceptions to barren lake bed may be along Simon's Spring fault and tufa areas. Surface erosion from windblown particles will become more critical to vegetation establishment, especially along eastern end of lake. Newly exposed shoreline will be barren.
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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
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. .
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.
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
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.
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
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
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
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.