A comparison of zooplankton communities in saline lakewater with variable anion composition

Although salinity and aquatic biodiversity are inversely related in lake water, the relationship between types of salts and zooplankton communities is poorly understood. In this study, zooplankton species were related to environmental variables from 12 lakes: three saline lakes with water where the dominant anions were SO₄ and CO₃, four saline lakes with Cl-dominated water, and five dilute, subsaline (0.5–3 gl^?1 total dissolved solids) lakes of variable anion composition. Although this study comprised only 12 lakes, distinct differences in zooplankton communities were observed among the two groups of chemically defined saline lakes. Canonical correspondence analysis identified total alkalinity, sulphate, chloride, calcium, sodium, potassium, and total phosphorus as all contributing to the first two ordination axes (λ₁ = 0.97 and λ₂ = 0.62, P<0.05). The rotifer Brachionus plicatilis and the harpactacoid copepod Cletocamptus sp. prevailed lakes with Cl-dominated water. In contrast, the calanoid copepods Leptodiaptomus sicilis and Diaptomus nevadensis were dominant in the SO₄/CO₃-dominated lake water with elevated potassium (79–128 mg l^?1) and total phosphorus concentrations (1322-2915 μg l^?1). The contrasting zooplankton species distribution among these two saline lake types is likely explained by variable selective pressure on zooplankton and their predators from differing physiological tolerances to salt stress and specific ions. While inland saline lakes with Cl as the dominant anion are relatively rare in Canada and SO₄/CO₃ are the common features, our study provided an opportunity to compare zooplankton communities across the two groups of lakes.     

Hierarchical population structure and genetic diversity of lake trout (Salvelinus namaycush) in a dendritic system in Northern Labrador

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Parallel and non‐parallel morphological divergence among foraging specialists in European whitefish (Coregonus lavaretus)

Parallel phenotypic evolution occurs when independent populations evolve similar traits in response to similar selective regimes. However, populations inhabiting similar environments also frequently show some phenotypic differences that result from non‐parallel evolution. In this study, we quantified the relative importance of parallel evolution to similar foraging regimes and non‐parallel lake‐specific effects on morphological variation in European whitefish (Coregonus lavaretus). We found evidence for both lake‐specific morphological characteristics and parallel morphological divergence between whitefish specializing in feeding on profundal and littoral resources in three separate lakes. Foraging specialists expressed similar phenotypes in different lakes in both overall body shape and selected measured morphological traits. The morphology of the two whitefish specialists resembled that predicted from other fish species, supporting the conclusion of an adaptive significance of the observed morphological characteristics. Our results indicate that divergent natural selection resulting from foraging specialization is driving and/or maintaining the observed parallel morphological divergence. Whitefish in this study may represent an early stage of divergence towards the evolution of specialized morphs.     

Chinese and Mongolian saline lakes: a limnological overview

More than half of China's lakes are saline (viz. have salinities > 3 g L⁻¹). Most salt lakes are in northwestern China (Tibet, Qinghai, Sinkiang, Inner Mongolia). Most Mongolian salt lakes are in the west of that country. Tectonic movements have been of the greatest importance in lake origins, but aeolian activity and deflation have also played a role. Many salt lakes in Qinghai-Tibet lie at altitudes > 4 000 m.a.s.l.; Aiding Hu (Sinkiang) lies at −154 m.a.s.l. Again, many lakes are large in area and deep. Small, shallow lakes are also common. Dimictic thermal patterns prevail in deep lakes, polymictic patterns in shallow ones. The highest salinity recorded is 555 g L⁻¹. The salinity of Qinghai Lake, the largest Chinese salt lake, is 14 g L⁻¹, but mean lake salinity on the northern Tibetan plateau is about an order of magnitude greater. Lop Nor has a salinity of ∼ 5 g L⁻¹. Dominant ions are Na and Cl; Mg, Ca, SO₄ and HCO₃ + CO₃ are important in certain lakes. Most major ions originate by weathering and leaching from rocks. pH values are generally high (often > 9.0). There are no bird or fish species confined to salt lakes, though many are associated with lakes of low or moderate salinity. Artemia occurs widely inland and in coastal salt pans, but is the only major macroinvertebrate of highly saline lakes. In lakes of only low to moderate salinity, invertebrate communities comprise widespread halotolerant freshwater forms and halophiles, some regionally endemic. Submerged and emergent macrophytes occur in lakes of low salinity, but phytoplankton species are more halotolerant. Ctenocladus circinatus, a green alga, is known from a Tibetan salt lake with a salinity of 200 g L⁻¹. There is a dearth of basic limnological information on Chinese and Mongolian salt lakes. More work in particular is needed on a variety of geographically widespread lakes to (a) document seasonal physico-chemical events, and (b) compile comprehensive biological inventories of taxa present. Chinese salt lakes are significant sites for palaeoclimatic research, for conservation purposes, and for the resolution of several important biological questions (especially of an ecological and biogeographical sort). They also have important economic values. Unfortunately, the natural existence of many appears to be threatened by decreased inflows, largely the result of human impact on catchments.     

Evidence of concurrent local adaptation and high phenotypic plasticity in a polar microeukaryote

Here we investigated whether there is evidence of local adaptation in strains of an ancestrally marine dinoflagellate to the lacustrine environment they now inhabit (optimal genotypes) and/or if they have evolved phenotypic plasticity (a range of phenotypes). Eleven strains of Polarella glacialis were isolated and cultured from three different environments: the polar seas, a hyposaline and a hypersaline Antarctic lake. Local adaptation was tested by comparing growth rates of lacustrine and marine strains at their own and reciprocal site conditions. To determine phenotypic plasticity, we measured the reaction norm for salinity. We found evidence of both, limited local adaptation and higher phenotypic plasticity in lacustrine strains when compared with marine ancestors. At extreme high salinities, local lake strains outperformed other strains, and at extreme low salinities, strains from the hyposaline lake outperformed all other strains. The data suggest that lake populations may have evolved higher phenotypic plasticity in the lake habitats compared with the sea, presumably due to the high temporal variability in salinity in the lacustrine systems. Moreover, the interval of salinity tolerance differed between strains from the hyposaline and hypersaline lakes, indicating local adaptation promoted by different salinity.     

Genomic dynamics of brown trout populations released to a novel environment

Population translocations occur for a variety of reasons, from displacement due to climate change to human‐induced transfers. Such actions have adverse effects on genetic variation and understanding their microevolutionary consequences requires monitoring. Here, we return to an experimental release of brown trout (Salmo trutta) in order to monitor the genomic effects of population translocations. In 1979, fish from each of two genetically (F _ST = 0.16) and ecologically separate populations were simultaneously released, at one point in time, to a lake system previously void of brown trout. Here, whole‐genome sequencing of pooled DNA (Pool‐seq) is used to characterize diversity within and divergence between the introduced populations and fish inhabiting two lakes downstream of the release sites, sampled 30 years later (c. 5 generations). Present results suggest that while extensive hybridization has occurred, the two introduced populations are unequally represented in the lakes downstream of the release sites. One population, which is ecologically resident in its original habitat, mainly contributes to the lake closest to the release site. The other population, migratory in its natal habitat, is genetically more represented in the lake further downstream. Genomic regions putatively under directional selection in the new habitat are identified, where allele frequencies in both established populations are more similar to the introduced population stemming from a resident population than the migratory one. Results suggest that the microevolutionary consequences of population translocations, for example, hybridization and adaptation, can be rapid and that Pool‐seq can be used as an initial tool to monitor genome‐wide effects.     

A plague of waterfleas (<Emphasis Type="Italic">Bythotrephes</Emphasis>): impacts on microcrustacean community structure,seasonal biomass,and secondary production in a large inland-lake complex

The spiny cladoceran (Bythotrephes longimanus) is an invasive, predaceous zooplankter that is expanding from Great Lakes coastal waters into inland lakes within a northern latitudinal band. In a large, Boundary Water lake complex (largely within Voyageurs National Park), we use two comparisons, a 2-year spatial and a 12-year temporal, to quantify seasonal impacts on food webs and biomass, plus a preliminary calculation of secondary production decline. Bythotrephes alters the seasonal biomass pattern by severely depressing microcrustaceans during summer and early fall, when the predator is most abundant. Cladoceran and cyclopoid copepods suffer the most serious population declines, although the resistant cladoceran Holopedium is favored in spatial comparisons. Microcrustacean biomass is reduced 40–60 % and secondary production declines by about 67 %. The microcrustacean community shifts towards calanoid copepods. The decline in secondary production is due both to summer biomass loss and to the longer generation times of calanoid copepods (slower turnover). The Bythotrephes “top-down” perturbation appears to hold across small, intermediate, and large-sized lakes (i.e. appears scale-independent), and is pronounced when Bythotrephes densities reach 20–40 individuals L^?1. Induction tests with small cladocerans (Bosmina) suggest that certain native prey populations do not sense the exotic predator and are “blind-sided”. Failure of prey to deploy defenses could explain the disproportionate community impacts in New World versus Old World lakes.     

Local adaptation in populations of a Brachionus group plicatilis cryptic species inhabiting three deep crater lakes in Central Mexico

1. Salinity is a strong selective force for many aquatic organisms, affecting both ecological and evolutionary processes. Most of our knowledge on the effects of salinity on rotifers in the Brachionus plicatilis species complex is based mainly on populations from waterbodies that experience broad environmental changes both seasonally and annually. We tested the hypothesis that, despite the supposedly high potential for gene flow among rotifers inhabiting neighbouring environments, constant salinity has promoted local adaptation, genetic population divergence and even cryptic speciation in B. plicatilis complex populations from three deep maar lakes of distinct salinities [1.1, 6.5 and 9.0 g L^?1 total dissolved solids (TDS)] in Central Mexico. 2. To look for local adaptation, we performed common garden experiments to test the effect of different salinities on population density and intrinsic growth rate (r). Then, we evaluated the genetic divergence by sequencing the cytochrome c oxidase subunit I (COI) gene and performed reproductive trials to assess the potential gene flow among the three populations and with other closely related B. plicatilis complex species. 3. We confirmed that the rotifer populations have phenotypic plasticity in tolerance of salinity, but only rotifers from the least saline lake are adapted to low salinity. Among the populations, sequence divergence at COI was very low (just a single haplotype was found), suggesting a persistent founder effect from a relatively recent single colonisation event and a subsequent dispersal from one lake to the others, and a very restricted immigration rate. In the phylogenetic analysis, rotifers from this area of Mexico clustered in the same clade with the middle‐sized species Brachionus ibericus and B. sp. ‘Almenara’. Mexican rotifers showed successful recognition, copulation and formation of hybrids among them, but interpopulation breeding with the Spanish B. ibericus and B. sp. ‘Almenara’ was unsuccessful. 4. We conclude that the B. plicatilis complex populations from these three lakes belong to a new biological species not yet described (presently named B. sp. ‘Mexico’). To our knowledge, this is the first report of local adaptation of a natural B. plicatilis complex population living in freshwater conditions (1.1 g L^?1 TDS).     

Selective predation on an introduced zooplankter ( Bythotrephes cederstroemi ) by lake herring ( Coregonus artedii ) in Harp Lake,Ontario

1. Bythotrephes cederstroemi (Crustacea: Onychopoda: Cercopagidae) invaded Harp Lake, Ontario in 1993, since when the zooplankton community has shifted from dominance of small-bodied to large-bodied species. During 1995 diets of adult lake herring (Coregonus artedii), Harp Lake’s primary planktivorous fish, were examined to determine the extent to which this conspicuous zooplankter has become integrated into the lake’s foodweb and whether fish condition has been affected in consequence. 2. Bythotrephes and Daphnia galeata mendotae were strongly preferred prey, whilst Holopedium gibberum and calanoid and cyclopoid copepods were negatively selected by lake herring. Predation on Bythotrephes and Holopedium was not size-selective, although D. galeata mendotae and calanoid copepods (Leptodiaptomus minutus and L. sicilis) consumed by herring were significantly larger than co-occurring conspecifics in the lake. 3. Caudal spines of Bythotrephes may form boluses in lake herring stomachs. However, the number of caudal spines in fish digestive tracts did not differ significantly from the number of Bythotrephes’ mandible pairs, indicating that the former were not differentially retained. 4. Lake herring weight-at-length relationships in lakes in Muskoka, Ontario, invaded by Bythotrephes did not differ from those in adjacent non-invaded lakes, indicating that Bythotrephes invasion of lakes apparently has not affected condition of lake herring.     

Temporal Changes and Altitudinal Distribution of Aerobic Anoxygenic Phototrophs in Mountain Lakes

Aerobic anoxygenic phototrophs (AAPs) are bacteriochlorophyll a-containing microorganisms that use organic substrates for growth but can supplement their energy requirements with light. They have been reported from various marine and limnic environments; however, their ecology remains largely unknown. Here infrared epifluorescence microscopy was used to monitor temporal changes in AAPs in the alpine lake Gossenköllesee, located in the Tyrolean Alps, Austria. AAP abundance was low (10³ cells ml⁻¹) until mid-July and reached a maximum of ∼1.3 × 10⁵ cells ml⁻¹ (29% of all prokaryotes) in mid-September. We compared the studied lake with other mountain lakes located across an altitudinal gradient (913 to 2,799 m above sea level). The concentration of dissolved organic carbon and water transparency seem to be the main factors influencing AAP abundance during the seasonal cycle as well as across the altitudinal gradient. While the AAP populations inhabiting the alpine lakes were composed of intensely pigmented large rods (5 to 12 μm), the lakes below the tree line were inhabited by a variety of smaller morphotypes. Analysis of pufM diversity revealed that AAPs in Gossenköllesee were almost exclusively Sphingomonadales species, which indicates that AAP communities inhabiting alpine lakes are relatively homogeneous compared to those in low-altitude lakes.     

Diversity of Aphanizomenonflos-aquae (Cyanobacterium) Populations along a Baltic Sea Salinity Gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Genetic divergence of chars of the genus Salvelinus from Kronotsky Lake (Kamchatka Peninsula)

Chars from the genus Salvelinus, inhabiting lakes and lake-river systems, belong to morphologically and ecologically different forms whose taxonomic status is under dispute. In the present work, we have examined genetic variation and divergence in various chars from the Kronotsky lake basin: the lacustrine chars (white, nose, and long-head) and Dolly Varden char Salvelinus malma. The study was conducted using analysis of allozyme and microsatellite loci, myogens, RAPD, and restriction analysis of two mtDNA segments. The estimates of heterozygoisty at allozyme and microsatellite loci were similar to the corresponding parameters in populations of northern Dolly Varden and Arctic char. Heterozygote deficit was recorded in both samples of individual forms, and in the combined sample of all chars from Kronotsky Lake. For both markers, appreciable genetic differentiation among the samples of different char forms was found, which was comparable to that among the spatially isolated populations of northern Dolly Varden. This result indicates reproductive isolation among the char forms examined. However, this isolation is not complete, because no fixed differences between the forms by any of the genetic systems analyzed was found. The genetic differentiation among different forms of lacustrine chars, which corresponds to the interpopulation rather than interspecies level, is thought to be explained by their comparatively recent divergence.     

Long- term effects of previous experience determine nutrient discrimination abilities in birds

Background

The evolution of reproductive traits, such as hybrid incompatibility (postzygotic isolation) and species recognition (prezygotic isolation), have shown their key role in speciation. Theoretical modeling has recently predicted that close linkage between genes controlling pre- and postzygotic reproductive isolation could accelerate the conditions for speciation. Postzygotic isolation could develop during the sympatric speciation process contributing to the divergence of populations. Using hybrid fitness as a measure of postzygotic reproductive isolation, we empirically studied population divergence in perch (Perca fluviatilis L.) from two genetically divergent populations within a lake.

Results

During spawning time of perch we artificially created parental offspring and F₁ hybrids of the two populations and studied fertilization rate and hatching success under laboratory conditions. The combined fitness measure (product of fertilization rate and hatching success) of F₁ hybrids was significantly reduced compared to offspring from within population crosses.

Conclusion

Our results suggest intrinsic genetic incompatibility between the two populations and indicate that population divergence between two populations of perch inhabiting the same lake may indeed be promoted by postzygotic isolation.     

Unique sympatric quartet of limnetic,benthic, profundal and piscivorous brown trout populations resolved by 3D sampling and focused molecular marker selection

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Do genetic drift and accumulation of deleterious mutations preclude adaptation? Empirical investigation using RADseq in a northern lacustrine fish

Understanding genomic signatures of divergent selection underlying long‐term adaptation in populations located in heterogeneous environments is a key goal in evolutionary biology. In this study, we investigated neutral, adaptive and deleterious genetic variation using 7,192 SNPs in 31 Lake Trout (Salvelinus namaycush) populations (n = 673) from Québec, Canada. Average genetic diversity was low, weakly shared among lakes, and positively correlated with lake size, indicating a major role for genetic drift subsequent to lake isolation. Putatively deleterious mutations were on average at lower frequencies than the other SNPs, and their abundance relative to the entire polymorphism in each population was positively correlated with inbreeding, suggesting that the effectiveness of purifying selection was negatively correlated with inbreeding, as predicted from theory. Despite evidence for pronounced genetic drift and inbreeding, several outlier loci were associated with temperature and found in or close to genes with biologically relevant functions notably related to heat stress and immune responses. Outcomes of gene–temperature associations were influenced by the inclusion of the most inbred populations, in which allele frequencies deviated the most from model predictions. This result illustrates challenge in identifying gene–environment associations in cases of high genetic drift and restricted gene flow and suggests limited adaptation in populations experiencing higher inbreeding. We discuss the relevance of these findings for the conservation and management, notably regarding stocking and genetic rescue, of Lake Trout populations and other species inhabiting highly fragmented habitats.     

Phylogeography of the Sponge Suberites diversicolor in Indonesia: Insights into the Evolution of Marine Lake Populations

The existence of multiple independently derived populations in landlocked marine lakes provides an opportunity for fundamental research into the role of isolation in population divergence and speciation in marine taxa. Marine lakes are landlocked water bodies that maintain a marine character through narrow submarine connections to the sea and could be regarded as the marine equivalents of terrestrial islands. The sponge Suberites diversicolor (Porifera: Demospongiae: Suberitidae) is typical of marine lake habitats in the Indo-Australian Archipelago. Four molecular markers (two mitochondrial and two nuclear) were employed to study genetic structure of populations within and between marine lakes in Indonesia and three coastal locations in Indonesia, Singapore and Australia. Within populations of S. diversicolor two strongly divergent lineages (A & B) (COI: p = 0.4% and ITS: p = 7.3%) were found, that may constitute cryptic species. Lineage A only occurred in Kakaban lake (East Kalimantan), while lineage B was present in all sampled populations. Within lineage B, we found low levels of genetic diversity in lakes, though there was spatial genetic population structuring. The Australian population is genetically differentiated from the Indonesian populations. Within Indonesia we did not record an East-West barrier, which has frequently been reported for other marine invertebrates. Kakaban lake is the largest and most isolated marine lake in Indonesia and contains the highest genetic diversity with genetic variants not observed elsewhere. Kakaban lake may be an area where multiple putative refugia populations have come into secondary contact, resulting in high levels of genetic diversity and a high number of endemic species.     

Distribution and Diversity of Cyanobacteria and Eukaryotic Algae in Qinghai–Tibetan Lakes

Cyanobacteria and eukaryotic algae are important primary producers in a variety of environments, yet their distribution and response to environmental change in saline lakes are poorly understood. In this study, the community structure of cyanobacteria and eukaryotic algae in the water and surface sediments of six lakes and one river on the Qinghai–Tibetan Plateau were investigated with the 23S rRNA gene pyrosequencing approach. Our results showed that salinity was the major factor controlling the algal community composition in these aquatic water bodies and the community structures of water and surface sediment samples grouped according to salinity. In subsaline–mesosaline lakes (salinity: 0.5–50 g L^?1), Cyanobacteria (Cyanobium, Synechococcus) were highly abundant, while in hypersaline lakes (salinity: >50 g L^?1) eukaryotic algae including Chlorophyta (Chlorella, Dunaliella), Bacillariophyta (Fistulifera), Streptophyta (Chara), and Dinophyceae (Kryptoperidinium foliaceum) were the major members of the community. The relative abundance ratio of cyanobacteria to eukaryotic algae was significantly correlated with salinity. The algae detected in Qinghai–Tibetan lakes exhibited a broader salinity range than previously known, which may be a result of a gradual adaptation to the slow evolution of these lakes. In addition, the algal community structure was similar between water and surface sediment of the same lake, suggesting that sediment algal community was derived from water column.     

Morphological and genetic divergence in Swedish postglacial stickleback (Pungitius pungitius) populations

Background

An important objective of evolutionary biology is to understand the processes that govern phenotypic variation in natural populations. We assessed patterns of morphological and genetic divergence among coastal and inland lake populations of nine-spined stickleback in northern Sweden. Coastal populations are either from the Baltic coast (n = 5) or from nearby coastal lakes (n = 3) that became isolated from the Baltic Sea (< 100 years before present, ybp). Inland populations are from freshwater lakes that became isolated from the Baltic approximately 10,000 ybp; either single species lakes without predators (n = 5), or lakes with a recent history of predation (n = 5) from stocking of salmonid predators (~50 ybp).

Results

Coastal populations showed little variation in 11 morphological traits and had longer spines per unit of body length than inland populations. Inland populations were larger, on average, and showed greater morphological variation than coastal populations. A principal component analysis (PCA) across all populations revealed two major morphological axes related to spine length (PC1, 47.7% variation) and body size (PC2, 32.9% variation). Analysis of PCA scores showed marked similarity in coastal (Baltic coast and coastal lake) populations. PCA scores indicate that inland populations with predators have higher within-group variance in spine length and lower within-group variance in body size than inland populations without predators. Estimates of within-group P _ST (a proxy for Q _ST) from PCA scores are similar to estimates of F _ST for coastal lake populations but P _ST > F _ST for Baltic coast populations. P _ST > F _ST for PC1 and PC2 for inland predator and inland no predator populations, with the exception that P _ST < F _ST for body size in inland populations lacking predators.

Conclusions

Baltic coast and coastal lake populations show little morphological and genetic variation within and between groups suggesting that these populations experience similar ecological conditions and that time since isolation of coastal lakes has been insufficient to demonstrate divergent morphology in coastal lake populations. Inland populations, on the other hand, showed much greater morphological and genetic variation characteristic of long periods of isolation. Inland populations from lakes without predators generally have larger body size, and smaller spine length relative to body size, suggesting systematic reduction in spine length. In contrast, inland populations with predators exhibit a wider range of spine lengths relative to body size suggesting that this trait is responding to local predation pressure differently among these populations. Taken together the results suggest that predation plays a role in shaping morphological variation among isolated inland populations. However, we cannot rule out that a causal relationship between predation versus other genetic and environmental influences on phenotypic variation not measured in this study exists, and this warrants further investigation.     

EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of many Great Basin lakes has resulted from the diversion of tributary streams for agricultural or municipal uses. At Mono Lake, an alkaline salt lake in eastern California, salinities have risen from 50 to 100 g·L^?1 in just 50 years. Experimental mesocosms were established to simulate some of the potential ecological effects that could have accompanied this change. The influence of salinity on diatom diversity, taxonomic structure, and primary production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0.5 m deep, with open tops covered by 1 mm mesh net. Five treatments (50, 75, 100, 125, and 150 g·L^?1) with four replicates per treatment were used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g·m^?2) and diversity (from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities ≥75 g·L^?1. Photosynthetic oxygen production also was significantly lower at salinities ≥75 g·L^?1. Diatom indicator taxa for these shifts included Denticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Stephanodiscus oregonicus increasing in relative abundance in higher salinity treatments, accompanied by decreases in Achnanthes minutissima, Cymbella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting dominance at moderate salinity levels (75 to 125 g·L^?1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophyte, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occurred only in salinity treatments from 50 to 100 g·L^?1. Diversion of tributary stream flow and resulting salinity increases in this lake threaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision of the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to around 75 g·L^?1 and is expected to increase the diversity and productivity of the benthic algae of this ecosystem.