SALINITY CONTROLS PLANKTON RESPONSE TO NUTRIENT ENRICHMENT IN FARMINGTON BAY, GREAT SALT LAKE, UTAH

Farmington Bay is a nutrient-enriched, highly eutrophic 260 km² embayment of the Great Salt Lake. The highly variable salinity of the bay influences what species of plankton can survive there. At high salinities diatoms and green algae dominate, but at low salinities, nitrogen-fixing cyanobacteria can dominate. Consequently, nutrient limitation of phytoplankton may shift as salinity levels change. To examine how salinity and nutrient supply interact to control algal community composition and nutrient limitation, we conducted a series of bioassay experiments with plankton from both hypersaline Gilbert Bay and variable salinity Farmington Bay. Overall, the experiments indicated that plankton from Gilbert Bay were clearly nitrogen limited, and chlorophyll levels in nitrogen-enriched treatments increased about 500% above control treatments in nearly all salinity treatments. Nitrogen-fixing cyanobacteria were never observed in Gilbert Bay treatments. However, in the lower-salinity Farmington Bay water, positive chlorophyll responses as high as 500% were observed with phosphorus enrichment when nitrogen-fixing cyanobacteria were present. Nitrogen-fixation rates were as high as 8 μg N₂ L^-1 hr^-1 in Farmington Bay control and phosphorus treatments when salinities were less than 50 – 70 g L^-1. At salinities greater than 70 g L^-1 or with nitrogen additions, nitrogen fixation ceased. When nitrogen-fixing cyanobacteria were absent, the plankton community was routinely nitrogen-limited regardless of salinity. The results of these experiments indicate that if nutrients are to be controlled to reduce eutrophication in Farmington Bay, the expected salinity levels would also need to be incorporated into the decision process. Therefore, the issues of salinity and nutrient supply are inseparable when making water quality decisions for Great Salt Lake.