PARENT SESSION
OOS 2: Anthropogenic Disturbances to Western Alpine Lakes: Past, Present, and Future .
Organized by: JE Saros and CE Williamson
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room E 143.
Assessing effects of fish introduction and removal using diatoms in lake sediments.
Drake, Deanne1, Naiman, Robert1, 1 University of Washington School of Aquatic and Fishery Sciences, Seattle, WA, USA
ABSTRACT- Recent concern for the decline of native amphibians has prompted removal of exotic trout (stocked from ∼1890 to 1973) from lakes at Mt. Rainier National Park, Washington. Introduction of top predators is known to fundamentally alter the structure of food webs in many lakes (i.e. via trophic cascade); we used diatoms preserved in sediments to assess effects of the original perturbation (fish introduction) and restoration efforts (fish removal) in Mt. Rainier lakes. Diatoms were of particular interest because they dominate the microflora of these oligotrophic lakes, are frequently preserved in sediments, and are sensitive to many environmental variables. Sediment records from eight lakes (480-year series) were used to 1) describe lake-specific diatom assemblages and natural variation before fish introduction, 2) determine whether fish introduction and removal were associated with changes in diatom floras, and 3) determine whether restoration (fish removal) resulted in a return to pre-fish conditions. Diatom floras remained relatively similar between 315 and 90 years before present in each lake; we used this period to define lake-specific baseline conditions. Dissimilarity analysis (squared-chord distance) showed dramatic, sustained changes in diatom floras that occurred approximately 80-100 years ago (around the time of fish introduction) in four of five stocked lakes, and no significant change in two unstocked lakes over the last 315 years. Diatoms were not preserved in an eighth lake. Even 20-30 years (2-4 time steps) after fish removal, diatom assemblages had not returned to pre-fish condition, suggesting that fish introduction causes long-lasting ecosystem-level perturbations that cannot simply be reversed by the removal of fish.
Key words: lake restoration, fish stocking, paleolimnology, Mt. Rainier