Pevan, Thomas William2013-02-042013-02-042012-12https://hdl.handle.net/11299/143875University of Minnesota M.S. thesis. December 2012. Major: Water Resources science. Advisors: Stephanie J. Guildford, Robert E. Hecky. 1 computer file (PDF); vii, 80 pages.Upper Saint Croix Lake is a small, shallow, eutrophic lake that has experienced anthropogenic eutrophication. Nutrients, and in particular phosphorus availability, have long been recognized as a factor influencing the water quality of lakes. If phosphorus is excessive, cyanobacteria are often favored, leading to significant negative implications for the overall water quality and biodiversity of the lake, recreational enjoyment, and human and animal health. Nutrient concentrations, environmental factors, and phytoplankton efficiency were assessed to determine the role of seasonality and associated factors in the development of cyanobacteria blooms, and to examine the influence of year to year variability on the seasonal dynamics of the phytoplankton community. Light measurements, water temperature, and water samples were collected during the summers of 2008 (only year with a significant cyanobacteria bloom), 2009, and 2010, along with some winter sampling in 2010. Chemical analyses included chlorophyll a, particulate phosphorus, soluble reactive phosphorus, total and total dissolved phosphorus, total and total dissolved nitrogen, ammonia, nitrate, and soluble reactive silica in addition to active fluorometry (Phyto-PAM and Fluoroprobe). Results indicate that cyanobacteria in Upper Saint Croix Lake are controlled ultimately by phosphorus, and to a lesser degree, nitrogen and water temperature. Phosphorus concentrations, are in turn, controlled by environmental factors (precipitation, outflow, and inflow) that manipulate the dilution and flushing rate of the phosphorus present in the lake.en-USCyanobacteria bloomsEnvironmental factorsEutrophicationPhosphorusThesis or Dissertation