LIMNOLOGICAL MONITORING OF FALLEN LEAF LAKE, CA; TRANSLATION OF SEASONAL, VERTICAL, AND EXTRABASINAL CONTRIBUTIONS TO THE SEDIMENTARY RECORD

A monitoring program at Fallen Leaf Lake was established in the spring of 2009 to provide baseline limnological data for interpreting a Holocene paleoclimate signal from diatom-rich cores. Fallen Leaf Lake is a small (5.2 km²) and deep near-oligotrophic lake that fills the Glen Alpine glacial valley at the south end of the Lake Tahoe basin, and shows high potential for recording paleohydrological variability in the surrounding watershed. It is a dilute transparent lake, with secchi depths ranging from 8-15m, and develops a deep chlorophyll maximum (DCM) at 40m that is dominated by diatoms. We have observed a 5-6 month period of summer stratification, with the epilimnion reaching a maximum depth of 17.5m in August. Strong partitioning of species occurs between the epilimnion and hypolimnion, and Cyclotella bodanica and Dinobryon are the dominant summer epilimnial species. A strong seasonal succession was observed, where the early spring pre-stratification was dominated by Asterionella formosa, Aulacoseira subarctica, Urosolenia eriensis, and Fragilaria tenera. In the late spring, U. eriensis and F. tenera concentrations drop off, possibly tied with drops in N and P derived from snowpack. A. formosa and T. fenestrata persist throughout the summer, being dominant components of the CDM. In late summer, a large blooms of C. rossii become the successor species in the DCM, and persist until deep mixing in the late fall. Species observed as dead cells transported into the lake during spring runoff, include Discostella stelligera, Aulacoseira humilis, A. lirata, and a variety of periphyton. Diatoms in surface sediments and down core show large variations in % Aulacoseira. The abundance of D. stelligera, low mantled Aulacoseira species, and % periphyton also vary down core and may be more related to inputs from higher in the watershed than to changes in lake primary productivity. One significant distinction in the modern flora with fossil flora is the high percentage of araphid pennates. A. formosa, very abundant in the modern system, has been tied to anthropogenic eutrophication in other western alpine lake systems, and therefore may not be a good analog for past conditions. It is hoped that these monitoring data will be useful in discerning past changes in stratification, inflow conditions, and trophic status.