Square Lake has historically been among the clearest lakes in the Minneapolis-St. Paul (Minnesota, U.S.A.) metropolitan area, but it has experienced declining water clarity due to increased levels of algal biomass over the past several decades. Predation by rainbow trout (annually stocked since the early 1980s by the Minnesota Department of Natural Resources (MDNR)) is hypothesized to have initiated cascading top-down effects that have caused the lake’s eutrophication trend. Specifically, predation by the stocked trout is proposed to have inhibited population growth of the large-bodied zooplankton grazer Daphnia pulicaria, thereby diminishing grazing pressure by D. pulicaria on planktonic algae, increasing algal biomass, and decreasing water clarity. To enable evaluation of the hypothesis that rainbow trout predation is responsible for the decline in the lake’s water clarity, a three-year moratorium on trout stocking by the MDNR was imposed. Here we compare water quality and zooplankton community monitoring data from two years prior to the moratorium (2010 & 2012), to monitoring data from the three moratorium years (2013-2015) to evaluate the effects of this food web manipulation.
The moratorium on rainbow trout stocking in Square Lake resulted in several significant changes in the lake’s zooplankton community and its water quality. These include: 1) an increase in the abundance and biomass of the large-bodied D. pulicaria, 2) an accompanying decrease in the abundance and biomass of a smaller-bodied Daphnia species (D. mendotae), 3) more pronounced springtime ‘clear-water phases’ during which D. pulicaria reached its highest densities, and 4) increased water clarity (secchi depth). In addition, surface water algal biomass (Chl a) was lower (though not to a statistically significant degree), and dissolved oxygen in deep water persisted at higher concentrations in the 2013-2015 moratorium years compared to 2010 & 2012. In summary, results of the rainbow trout stocking moratorium in Square Lake support the hypothesis that predation by trout on large-bodied herbivorous zooplankton is the primary cause for the pattern of eutrophication in Square Lake over the past several decades. Natural resource managers from the MDNR are using the conclusions from this study to determine the future fisheries management plan for Square Lake.