The role of top down interactions in cyanobacteria bloom development in Missisquoi Bay, Lake Champlain
Cyanobacteria blooms in shallow eutrophic lakes have been linked to numerous causes. Seasonality, high nutrient levels, and the interacting roles of zooplankton grazing can be important triggers for the growth of inedible and potentially toxic cyanobacteria. The spread of invasive planktivorous fish into lakes can lead to declines in large zooplankton, declines in overall zooplankton biomass and changes in trophic structure in large lakes. Two non-native planktivorous fish species, alewife (Alosa psuedoharengus) and white perch (Morone americana), now co-exist in Lake Champlain. We compared the diet composition of these species in Missisquoi Bay, a shallow, eutrophic bay. The objectives of the diet study were to (1) compare the diet composition of white perch before and after alewife invasion (2) compare the ambient zooplankton community with historical conditions, and (3) evaluate the ecological impacts of coexistence of both fish species, particularly their selectivity among zooplankton from the ambient community. Fish diet composition and ambient zooplankton were sampled between two and four times per month from April to September. Using the results from that study, we established treatments in mesocosms with enhanced or reduced quantities of large zooplankton for 10-day trials during three time periods (May, July and August) to evaluate the response of phytoplankton.
White perch were omnivorous with zooplanktivory that coincided with periods of Daphnia availability, while alewife diet was predominantly comprised of zooplankton. Manly Chesson's Selectivity Index suggests resource partitioning when these fish co-occur in this system. Large zooplankton were consumed more selectively by white perch than by alewife, and overall biomass of these taxa has declined significantly (t-test, p = 0.04) since 2005. The results of the mesocosm study indicated significant seasonal differences in phytoplankton community composition among the months. In May, large zooplankton biomass was associated with higher abundances of small-cell cyanobacteria and grazing-resistant taxa. In July, there were no treatment effects because a dense cyanobacteria bloom was underway. In August, the presence of large zooplankton led to a higher abundance of edible phytoplankton. Overall, there was a strong seasonal effect; the heaviest predation pressure by white perch is in early summer, when zooplankton grazing is most important to control the growth of cyanobacteria blooms. Declines in large zooplankton biomass may have profound implications for the trophic dynamics of Missisquoi Bay, Lake Champlain.