PHENOLOGY AND ENVIRONMENTAL DRIVERS OF ALGAE AND ZOOPLANKTON IN TWELVE HIGH NUTRIENT, SHALLOW LAKES (CARNELIAN-MARINE-ST. CROIX WATERSHED DISTRICT, WASHINGTON CO., MN)

Toben Lafrancois, St. Croix Watershed Research Station, Science Museum of Minnesota

Twelve small but culturally important lakes in the Carnelian Marine St. Croix Watershed district have been listed as impaired for high nutrients. Algal response to nutrient increases are a primary component of eutrophication, and zooplankton grazers can mediate algal blooms and improve water clarity. Because zooplankton are favorite food of some fish, plankton analysis addresses the mechanisms of two important water quality interests: fishing and water clarity. As part of the TMDL study of these lakes, algae and zooplankton were sampled monthly for five months (May to September, 2008). Relationships between water quality and plankton communities were assessed using multivariate ordination techniques. Algal communities showed a strong response to nutrients among lakes, and within-lake analyses showed that chlorophyll-a correlated strongly with nutrient concentrations. Algal blooms were not composed of the same species among lakes or within lakes over time, an important result given the potential of some species for toxic blooms (e.g. Microcystis spp). Zooplankton community composition changed over time within each lake, and in some cases dramatic community shifts clearly indicated predation by planktivores. These changes were not strongly related to differences in water quality parameters or changes in the algae community. In small, shallower lakes, this limited zooplankton relationship to nutrients and algae may relate to both natural variation and very high rates of fish stocking. Overall, changes over time for both algae and zooplankton were more informative than average community values or single sampling periods. The TMDL analysis was greatly enhanced by the plankton study, but questions remain that require further work (including paleo-zoology and experimental manipulation).