We examined how a zooplankton assemblage of a eutrophic reservoir (Acton Lake) is responding to improvements in agricultural practices in the watershed. These improvements caused substantial declines in phosphorus and non-volatile suspended sediment (NVSS) inputs to this reservoir over the last 13 years. Mean annual phytoplankton biomass (chlorophyll) increased during this period, apparently due to decreased light limitation. Using a 12-year data set (1994-2006), we tested three hypotheses. 1. Zooplankton biomass will increase with time due to increases in chlorophyll and decreased NVSS concentrations. 2. Zooplankton community composition will shift from rotifer dominated to crustacean dominated. 3. Temporal changes in biomass and community composition will differ between spring and summer, because NVSS inputs are higher in spring while phytoplankton biomass is higher in summer.
Zooplankton dynamics were very different in 1996 compared to other years, because of very high NVSS inputs, low phytoplankton biomass and exceedingly low larval fish (gizzard shad) biomass (0.46 ind/m3 in 1996 vs. 3.5-14 in other years). Therefore, we conducted our analysis with all years and with1996 excluded. Mean zooplankton biomass (183 µg dry mass/L) was considerably higher in 1996 than other years (9.2-92.3 µg/L). The zooplankton community was dominated by crustaceans (mainly copepods) in 1996 (68 % of biomass), while rotifers dominated the community in all other years (40-89 %). Contrary to our predictions, we observed no significant changes in total zooplankton biomass over time in spring, due to the contrasting temporal patterns in rotifer and crustacean biomass. Spring rotifer biomass significantly decreased over the 13 years. Copepod biomass significantly increased in spring, but only when 1996 was excluded from the analysis. A multiple regression model including chlorophyll (negative effect, -) and larval gizzard shad (positive effect, +) explained 37% of the variation in rotifer biomass, while 71% of the variation in copepod biomass was explained by a model including chlorophyll (+) and NVSS (-). In the summer, significantly declining trends in total zooplankton were mainly driven by unusually high copepod biomass during 1996. No significant trend was detected when 1996 was excluded from the analysis. Multiple regression models including chlorophyll (-) and NVSS (+) explained 80% and 66% of the variation in total zooplankton and copepod trends respectively. Our results suggest that in spite of high turnover rates, zooplankton responses to changes in agricultural subsidies lagged behind those of primary producers.