Analysis of Phytoplankton Nutrient Limitation in Farmington Bay and the Great Salt Lake

Wayne Wurtsbaugh, Utah State University
Amy Marcarelli

This work made publicly available electronically on June 28, 2012.


The Great Salt Lake is bordered to the south and east by a growing metropolitan area that contributes high nutrients to Farmington Bay. This large bay is eutrophic, and there is concern that continued increases in effluents from the Salt Lake City area could extend to impact the much larger, and currently less productive, Gilbert Bay. This study focused on determining how nutrient supplies might limit, and therefore control, algal populations in Farmington Bay and Gilbert Bay at different salinities. We tested both short and long-term responses of algal growth using laboratory nutrient addition bioassays in the summer and fall of 2003. Because some phytoplankton can alleviate nitrogen deficiency by fixing atmospheric nitrogen, we also determined how nutrients and salinity influenced nitrogen fixation.

Two types of assays were used in the analysis. To determine what nutrients currently control algal growth in Farmington Bay, four, week-long Simple Bioassays were used to measure how chlorophyll 8, nitrogen fixation and algal biovolume responded to additions of nitrogen, phosphorus, or nitrogen+phosphorus. All four of these assays indicated that the algal population was stimulated by nitrogen and not by phosphorus. Additionally, nitrogen fixation rates by these N-limited populations were negligible. These results were consistent with earlier studies that showed nitrogen limitation of Great Salt Lake algae.

To understand how changes in nutrient loading and salinity might interact to control the algal population, particularly nitrogen-fixing cyanobacteria, two Factorial Bioassays were conducted. In these experiments salinities were varied from 1 % to 10%, and nitrogen or phosphorus was added. Algal inocula from water bodies of varying salinity were introduced at the start of the experiments. In both of these assays, the algal populations were initially stimulated by nitrogen and not by phosphorus, as observed in the Simple Bioassays. However, at lower salinities, nitrogen-fixing cyanobacterial communities developed after 2-3 weeks, allowing the communities to overcome nitrogen limitation and become phosphorus limited. The two experiments differed, however, because in the first experiment, nitrogen-fixing communities developed in salinities up to 7%, whereas in the second experiment significant nitrogen fixation occurred only in the 1 % salinity treatment. The upper limit for nitrogen fixation for the Great Salt Lake plankton community appears to be near 7%. Nutrients and salinity thus appear to interact to control whether nitrogen of phosphorus ultimately limits the abundance of phytoplankton in the lake. More experiments are needed to more precisely define the salinity range over which this interaction occurs, and to determine if these relationships hold under environmental conditions that more closely approximate those in the Great Salt Lake.