BIOLOGICAL, CHEMICAL, AND PHYSICAL PARAMETERS THAT AFFECT CYANOBACTERIA IN UPPER ST. CROIX LAKE

Pevan, T., DeMars, J. and Guildford, S.
Large Lakes Observatory, University of Minnesota-Duluth

Upper St. Croix Lake, the headwaters for the St. Croix River, is a eutrophic lake that has experienced several large algal blooms in recent warm years. Noxious and occasionally toxic cyanobacteria (blue green algae) often dominate phosphorus-rich lakes. The goal of this ongoing study is to identify environmental factors associated with cyanobacteria dynamics and overall water quality of Upper St. Croix Lake. During the summer months of 2008 and 2009, phytoplankton biomass and composition, variable fluorescence, Secchi depth, total phosphorus and total nitrogen measurements were taken from near-shore and off-shore sites. Starting in 2009, an in situ chlorophyll fluorometer and CTD instrument were also used at the sites. Total chlorophyll peaked in both years during the month of August with a concentration of 15ug/L in 2008 and 9ug/L in 2009. Secchi depths decreased during the summer months for both years, and were lowest in August when chlorophyll was at its maximum. However, Secchi depths during August in 2008 were 1.5m while secchi values in August of 2009 were 3.0m. The lower secchi in 2008 was likely due to the higher total chlorophyll concentrations. In August 2008, blue green algae accounted for over 2/3 the total algal composition. However, in August 2009, blue green algae accounted for less than a 1/3 the total algal composition. The difference in blue green algae composition may be due to the high total phosphorus that peaked during August 2008, which had surface phosphorus concentrations around 40ug/L. Internal release of phosphorus from the sediments has been documented to be a major source of phosphorus during the summer growing season of Upper St. Croix Lake (Hoverson and McGinley, 2007). The August 2009 total phosphorus chemistry has not been completed at this time, but may differ significantly from the 2008 values. The water of Upper St. Croix Lake showed little to no temperature stratification during the summer of 2008 and 2009. Though the surface water was almost 2°C warmer in 2008 (23°C mean) than 2009 (21°C mean), which may have been another environmental factor that would explain the difference in proportion of cyanobacteria from 2008 to 2009. Variable fluorescence, which is a good indicator of algal cell health, displayed a similar pattern in both 2008 and 2009 with an increase in variable fluorescence during the summer with a peak in August. This suggests that the phytoplankton are not stressed during the summer growing season.

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

Hoverson, D. and McGinley, P. 2007. Internal Phosphorus Loading from Sediment in Upper St. Croix Lake, Douglas County, WI. University of Wisconsin Stevens Point, Center for Watershed Science and Education.