CYCLOTELLOID TO ARAPHID-DOMINATED DIATOM COMMUNITY SHIFT IN CASTLE LAKE, SISKIYOU COUNTY, CALIFORNIA

Diatoms recovered from a 20-cm long sediment core from Castle Lake indicate a recent community shift in the diatom population from a cyclotelloid-dominated community to one with an increasing component of araphid pennates. Castle Lake is a meso-oligotrophic glacial lake located on the eastern slope of the Klammath Mountains near Mount Shasta in Northern California. Two-cm thick slices of the core were processed and 500 diatom valves per sample were counted. Overall, phytoplankton represent ~70-80% of the diatoms, and periphyton the remaining 20-30%. In the lower 14 cm of the core, the phytoplankton are dominated by the centric diatoms, Discostella stelligera-pseudostelligera group (>50% of total diatoms), and Cyclotella occelata-rossii-tripartita complex (9-18%). The dominant periphyton group in the lower 14 cm is the araphid diatom Staurosirella pinnata (13-22%). The top 6 cm show an increase in araphid phytoplankton, most notably Asterionella formosa and Fragilaria tenera. In the uppermost sample the cyclotelloid component is reduced to ~30%, down from 60-70% in the lower 14cm, and this sample also has a significant increase in Staurosirella pinnata (38%). These araphid species have been associated with eutrophication in other lake systems. A recent shift from a cyclotelloid-dominated to an A. formosa-dominated diatom community has been linked to atmospheric nitrogen deposition in subalpine lakes of the Beartooth Mountains of Montana and the Colorado Front Range, and may indicate a similar effect at Castle Lake. An age model is pending for the Castle Lake core in order to determine how recent this shift has occurred, however monitoring data from the 1970’s show that A. formosa and F. tenera were present, but in smaller numbers. Broad eutrophication effects have not been noted in Castle Lake; continuous monitoring of lake productivity however did show a short-lived increase in primary productivity in the 1980s after a whole lake fish manipulation, followed by a multi-decade recovery. Further testing will be required to determine if the current diatom assemblage and trophic status is more strongly linked to fish manipulations, climate forcing, or N deposition.