METEOROLOGY AFFECTS ALGAL COMMUNITIES IN UPPER LAKE ST. CROIX, WISCONSIN

Pevan T., Large Lakes Observatory, University of Minnesota Duluth, and Upper Lake St. Croix Lake Association
Hecky R., Large Lakes Observatory, University of Minnesota Duluth, and Upper Lake St. Croix Lake Association
Guildford S., Large Lakes Observatory, University of Minnesota Duluth, and Upper Lake St. Croix Lake Association
Heim J., Large Lakes Observatory, University of Minnesota Duluth, and Upper Lake St. Croix Lake Association

Upper Lake St. Croix, the headwaters for the St. Croix River, is a eutrophic lake that has experienced several large algal blooms in recent years. The eutrophication of temperate lakes leads to increases in algal biomass and changes in community structure. Cyanobacteria (blue-green algae) often become the dominant group in enriched lakes and may create surface scums along with taste and odor problems. Some cyanobacteria species are also potentially toxic to fish, livestock, and even humans. Factors that can lead to cyanobacteria blooms include the interaction of physical, chemical, and biological parameters acting on the lake. Mechanisms that allow cyanobacteria to dominate in freshwater include N:P ratio competition, differential light requirements, CO₂ competition, buoyancy, high temperature tolerance, avoidance by herbivores, superior cellular nutrient storage, ammonium-N exploitation, and trace element competition (Downing, 2001). The goal of this ongoing study is to identify what mechanisms are the most important factors associated with cyanobacteria dynamics and overall water quality in Upper Lake St. Croix. To identify these factors, different in situ and in vivo assessments were made during the summer and fall of 2008-2009 and the summer of 2010. Meteorological conditions varied widely for the three summers with 2008 and 2010 being exceptionally warm while 2010 had unusually high rainfall. The three years exhibited quite different values for total chlorophyll, phytoplankton community structure, Secchi disk depth and nutrient concentrations. While it is still too early to determine which factors play the largest role on cyanobacteria occurrence and abundance, the data demonstrates that weather may play a larger role than previously believed.

Suggestions for Reading

Downing, J., Watson, S. and McCauley, E. 2001. Predicting Cyanobacteria dominance in lakes. Can. J. Fish. Aquat. Sci. 58(10): 1905-1908.

Sorrano, P. Factors affecting the timing of surface scums and epilimnetic blooms of blue-green algae in a eutrophic lake. University of Wisconsin Madison, Center for Limnology. 1997.