OOS 49-1
Less water and more salt: Limitations on abundance and diversity and altered food webs in saline lakes of the western Great Basin
Diversions of stream flow to Great Basin terminal lakes have resulted in a history of falling lake levels and rising salinity, compounded now by prolonged drought. How these ecosystems respond is the question investigated in this research. At Walker Lake, Nevada, salinity has risen to a critical concentration and the benthic invertebrate community is undergoing a transition from one dominant assemblage to another, consistent with observations at other saline lakes of the region. Physiological salt tolerance bioassays correspond to observed declines in populations of midges (Cricotopus ornatus) and damselflies (Enallagma clausum) and the ascendance of a more salt tolerant ephydrid, the alkali fly Ephydra hians. As this shift in community structure occurs, so too will availability of the food resources that have been used by fish and birds, so cascading effects on food web top consumers that can be supported may occur. At Owens Lake, California, flood irrigation to control dust has benefited recovery of lost ecological values, while impoundment of the Chewaucan River at Abert Lake, Oregon, has caused near desiccation and collapse of the ecosystem. Comparative monitoring of benthic invertebrate abundance are coupled here to laboratory studies of responses to varied salinity to inform habitat requirements for conservation.
Results/Conclusions
Salinity tolerance bioassays show debilitation of survival, growth and foraging behavior among the dominant invertebrates found under current salt concentrations at Walker Lake. Experiments predict that further increase above 20-25 g/L will cause critical stress to population persistence. Population monitoring over the range of sublethal salinity stress is showing this to be true, as abundance varies in relation to salinity fluctuations in the past 5 years, with salt tolerant species now becoming prevalent. Varied salinities occur over a mosaic of flood-irrigated habitats at Owens Lake, giving rise to mixed benthic community composition as biotic and chemical gradients appear to control distribution and abundance. Abert Lake is becoming hypersaline because river inflow has been diverted, and even species adapted to high salinity are disappearing. Ecological values of terminal lakes can be preserved only by protecting river inflows, and rapid recovery is possible given resumed inflow or habitat restoration. Regional drought is compounding diversion of rivers from these lakes. As the productive invertebrate assemblages of saline lakes transition to different species, food web form and function will alter forage availability to surviving native fish and the types of water birds that use these lakes for feeding, breeding or migratory stopovers.