MONO LAKE – MORE THAN A CENTURY OF RESEARCH AND HUMAN IMPACT

The natural history of Mono Lake has been a topic of interest since the later part of the 19^th century. Early studies provided an understanding of the biology, limnology, and geological processes taking place in the modern lake system. Evaluation of stratigraphic sections, tufa features, and strandlines over the past few decades have provided an increasingly detailed paleoenvironmental history of the lake basin and regional hydroclimate on a scale of hundreds to thousands of years. However, there are still lengthy gaps in the record, due to the disruptive effects of local volcanic activity, impenetrable tephra layers, and downslope sediment transport. While these records provide a history of lake levels associated with variations in precipitation and evaporation, they provide only a limited history of changes in lake water chemistry, sedimentation, and productivity. Recent coring efforts have resulted in the collection of shallow-water (2.8 m water depth) and deep-water (18 m water depth) cores, which when combined, provide a sediment and microfossil record spanning the last 16,000 years. The deep-water core provides evidence of a relatively stable environment, while sediments from the shallow-water core suggest a more complex record with multiple transgression-regression cycles, and deposition affected by downslope sediment transport, pycnoclines, and sediment reworking. Compositional changes in the diatom assemblage and taphonomic characteristics are used to evaluate changes in salinity and nutrient load.

Stephanodiscus, a freshwater, eutrophic planktic genus, comprises as much as 97% of the diatom assemblage in the deep-water core, with abundances higher (>85%) during late glacial and Younger Dryas highstands (<70%). Undated, likely contemporaneous diatom assemblages in the shallow-water core are dominated first by epiphytic species (Cocconeis spp.) followed by Stephanodiscus during the Younger Dryas and Early Holocene.

The overall record suggests that lake level was relatively high and stable compared to pre-diversion (1941) levels and stable during the latest Pleistocene and earliest Holocene but changes in diatom abundance, assemblage composition, and taphonomic characteristics indicate lower and less stable lake levels.