ANTHROPOGENIC AND CLIMATIC INFLUENCES ON THE DIATOM FLORA WITHIN THE FALLEN LEAF LAKE WATERSHED, LAKE TAHOE BASIN, CALIFORNIA
AuthorJohnson, Briana Emily
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Sediment cores and water quality data from the Fallen Leaf Lake (FLL) watershed were analyzed in order to gauge the effects of climate, land use, and atmospheric nitrogen deposition over the past 1200 years. High resolution diatom and geochemical analyses were conducted on core 2E-1G-1 from FLL, a moderately impacted lake that is the lowest catchment in the watershed. The FLL core delineated 3 zones of interest that correspond to the Little Ice Age (LIA), a transitional zone of warming and anthropogenic influence following the LIA, and an upper anthropogenic zone. The LIA is characterized by increased abundances of the diatoms <italic>Stephanodiscus alpinus</italic> and <italic>Aulacoseira subarctica</italic>, and is calibrated with an age model derived from <super>14</super>C and <super>210</super>Pb dates to between 1385 and 1807 AD, and indicates a time of cooler, windier conditions. The transitional zone represents a ~140 year period of gradual warming following the LIA, and is characterized by increases in <italic>Cyclotella rossii</italic> and <italic>Discostella stelligera</italic> and decreases in <italic>Pseudostaurosira brevistriata</italic>. The FLL core transitional zone also shows sedimentologic and geochemical changes that include increases in elemental cobalt, zinc, and tin that may be attributed to an increase in coal burning and smelting activities in California and Nevada. Beginning ~1910 AD, increased building, land-use, and recreation around the lake caused an increase in sediment accumulation. The increased sediment accumulation rate can be partly attributed to terrestrial organic input into the lake, as both total organic carbon (TOC) and C:N ratios increase beginning ~1943 AD. Coincident with the sedimentologic shift is the appearance of mesotrophic diatoms responsive to nutrient enrichment, including: the <italic>Fragilaria tenera-nanana</italic> group, <italic>Tabellaria flocculosa</italic> strain <italic>IIIp</italic>, and <italic>Nitzschia gracilis</italic>. These mesotrophic diatoms characterize the anthropogenic zone, increase rapidly in abundance ~1950 AD, and may be linked to the increased development around FLL. Down core proxies for atmospheric nitrogen (N) deposition in the FLL watershed are, at best, weakly expressed and appear to be overshadowed by stronger signals. Although the nitrogen responsive diatom species <italic>Asterionella formosa</italic> is a dominant component in FLL today, it is not a newcomer and has been present in similar abundances for at least the last 1200 years. Nitrogen stimulation of <italic>A. formosa</italic> and other N-sensitive phytoplankton in FLL is attributed to the natural process of flushing N-rich water from the upper watershed during spring runoff. A negative shift in δ<super>15</super>N, another proxy of atmospheric N deposition, is weakly expressed in the FLL core and is overprinted by additional down-core variation. Evidence supporting atmospheric N deposition from Gilmore Lake (GL), a low impact site higher in the watershed, has also proved evasive. In GL, <italic>A. formosa</italic> was found in the water column, but not in surface sediments, indicating that its appearance is very recent, and not synchronous with the anthropogenic zone shift in FLL that commenced in the 1950s. Furthermore, N concentrations in spring runoff into GL were <1 ppb, indicating that, at least for the sampling period, N inputs from wet deposition were negligible. Together, these data show that the FLL record is sensitive to climatic cooling during the LIA and to anthropogenic activities commencing in the 1800s that increased throughout the latter half of the 20<super>th</super> century; however there is no strong coherent signal of anthropogenic N deposition.