IMPACTS OF DAM REMOVAL ON CYANOBACTERIA BLOOMS AND ZOOPLANKTON COMMUNITY IN HYPEREUTROPHIC UPPER KLAMATH LAKE

Abstract

The Chiloquin Dam was removed from the Sprague River in 2008 to restore the natural hydrology of Upper Klamath Lake (UKL), allow fish access to historical spawning grounds, and alleviate the effects of long-term eutrophication. This provided a natural opportunity to investigate the effects of restoration efforts on eutrophication. Using a 10-year dataset including 5 years before and 5 years after dam removal, we developed a consensus modeling approach to investigate factors contributing to the cyanobacteria blooms, observe changes in the ecosystem since the recovery efforts, and to assess the performances of three model types: logistic regression, random forest, and classification tree. Our results showed that cyanobacteria cell counts decreased significantly after dam removal and that pH, temperature and total phosphorus were important for predicting bloom presence both before and after dam removal. Classification trees revealed that reductions in both average total phosphorus concentrations and average water temperatures exhibited synergistic control over bloom presence. The dam removal also resulted in shifts in zooplankton populations. Copepods increased post-dam removal, but chydorids decreased. Previous studies showed that the chydorid species endemic to UKL preferentially consume filamentous cyanobacteria, whereas copepods prefer algae. The changes in these two populations could indicate an ecological shift in the lake post-dam and our results suggest that eutrophication has been reduced.