Factors affecting the spatial and temporal variability of nodularia blooms in Farmington Bay, Great Salt Lake, Utah

Farmington Bay of Great Salt Lake has been studied extensively over the last two decades and observations indicate high levels of both nutrients and the toxic cyanobacteria Nodularia spumigena. During 2012-2013 we measured the physical, chemical, and biological characteristics of surface water in a series of five longitudinal transects at nine stations along the estuary’s salinity gradient. The mean chlorophyll level on the five dates was 113 ?g/L, indicating that the bay was hypereutrophic, but there was considerable spatial-temporal variation, with values at individual stations and dates ranging from 5-247 ?g/L. Our observations in June of 2013 in the southern part of the bay showed Nodularia cell concentration and the hepatoxin nodularin peaked at 393,026 cells per mL and 69 µg/L respectively, where salinity was 1.5% or 15 g/L. These levels exceed the World Health Organization’s “moderate risk level” standards for recreational waters by over 200%. This cyanotoxin peak corresponded with high levels of total nitrogen (TN) and total phosphorus (TP), which peaked at 7.3 mg/L and 0.58 mg/L and had means across the bay of 5.6 mg/L and 0.59 mg/L, respectively. A complex interaction between zooplankton grazing of phytoplankton and corixid predation on the grazers may have produced some top-down controls on the clearing of the water column by zooplankton. Our study indicates that the dynamics of phytoplankton growth and nutrient interaction are complex, but we are moving towards a better understanding of the seasonal and temporal variability in this hypereutrophic bay of Great Salt Lake