MID-20TH CENTURY INFLUENCE OF ATMOSPHERIC RIVERS ON DIATOM ASSEMBLAGES IN WILDCAT LAKE, POINT REYES NATIONAL SEASHORE, CALIFORNIA

Wildcat Lake (38.0^o N, 122.8^o W, 62 masl) is located in a 12,000-10,000-year-old landslide complex near the southern end of Point Reyes National Seashore, California. A sediment record collected from the lake spanning the period 1890-2021 was used to evaluate the record of atmospheric rivers in central California. Diatom assemblages from the interval 1926-1953 were evaluated to test the effect of atmospheric rivers on primary productivity.

The lake during this time was slightly alkaline and mesotrophic to eutrophic. The diatom assemblage is dominated by the planktic taxa Stephanodiscus and Cyclotella, except for two intervals in which Nitzschia and Fragilaria dominate. Diversity is low to moderate (5-21 species/sample), and the assemblages are dominated by only a few species (five most abundant species account for 75-100% of the assemblage). Peaks of almost an order of magnitude greater in diatom abundance (frustules/g sediment) occur around 1930, 1932-1935, 1942, and 1947. The period ~1928-~1938 is dominated by brackish conditions changing rapidly to freshwater for the remainder of the record.

Although there is robust correspondence between the sediment erosion signal and strong atmospheric river years in the instrumental record, the connection to the diatom record is less clear. The clearest relationship occurs between diatom abundance and the erosion signal (detrended Si/Al, Ti/Al) occurs ~1928-~1938. Although total organic carbon values show little relationship to the peaks in diatom abundance, they are about 8% higher under freshwater conditions. These results indicate that the remainder of this sequence must be analyzed before drawing any conclusions as to the impact of atmospheric rivers on modern diatom assemblages and assessing the potential for using diatoms as a tool in identifying atmospheric rivers in past lacustrine systems.