Print

Couture, Sam C.

Rubenstein School of Environment and Natural Resources

2006

M.S.

In the last 8-10 years, the summer phytoplankton community of Missisquoi Bay in Lake Champlain has become dominated by cyanobacteria. Within a similar time frame, a population of white perch (Morone americana) has become established in the bay. The purpose of this study was to determine whether or not food web inhibition of large cladocerans in the genus Daphnia may be related to the expansion of white perch into Missisquoi Bay, leading to conditions that favor cyanobacteria. Laboratory experiments in 200L aquaria were conducted using adult white perch and assemblages of zooplankton containing differing densities of Daphnia collected from Lake Champlain to determine which, if any, zooplankton taxa were depleted as a result of white perch grazing. Over a 16-hour experiment, white perch grazing significantly reduced the density of Daphnia in aquaria (three-way ANOVA, p<0.0005). No other zooplankton taxa were depleted by white perch grazing. There was no interaction between initial density of Daphnia and the effect of white perch grazing (three-way ANOVA, p=0.5445) suggesting that the initial density of Daphnia had no effect on the amount of Daphnia depletion. White perch and zooplankton samples were also collected in Missisquoi Bay on ten dates in the spring and summer of 2005 to determine the in situ diet and, zooplankton selectivity of white perch. White perch made up around 70% of all fish caught in the gill net in Missisquoi Bay. Stomach content analysis of 204 fish showed that cladocerans were one of the major prey items in the white perch diet when available. White perch exhibited the greatest stomach fullness on dates corresponding to the highest available Daphnia densities (>20 L⁻¹) and on dates when Daphnia made up more than 50% of the zooplankton assemblage. For all dates, some type of large cladoceran was positively selected for over other zooplankton groups (T-tests comparing Manly/Chesson selectivity values to neutral selectivity value). The results of this study show that white perch are feeding on large numbers of Daphnia in Missisquoi Bay, they are positively selecting for Daphnia over other zooplankton taxa, and their grazing can deplete Daphnia abundances. The combination of these results suggests that the establishment of a white perch population in Missisquoi Bay may have resulted in a food web inhibition of Daphnia, which could be linked to cyanobacteria dominance of the phytoplankton community through changes in grazing pressure and through related changes in nutrient cycling regimes.