Damming of streams inserts a lentic system (a reservoir) into a lotic system (the stream) thereby changing downstream hydrological, biogeochemical, and ecological attributes of streams. One way in which reservoirs alter ecological interactions is through the conversion of both primary and secondary productivity from benthic to planktonic organisms. Planktonic subsidies cause a shift in downstream community composition toward filter feeders. Filter feeders obtain resources as a function of the density of those resources in the environment. Measuring zooplankton density is a labor intensive process, so in this study we assess whether the density of zooplankton subsidies can be predicted by water quality variables, which would be a much more efficient way of quantifying zooplankton subsidy. During the 2019 growing season we monitored zooplankton subsidy from four polymictic reservoirs and assessed 22 water quality variables for their ability to predict subsidies. Phosphate concentration and conductivity were the best combined predictors in 2019. These variables were used in a 2018 pilot study following similar methodology. When both years were considered conductivity emerged as the best predictor of zooplankton density, ultimately predicting about a third of the variation in zooplankton density. We detected shifts in water quality conditions during 2019, but these shifts in water quality did not affect the density of zooplankton subsidies. Conductivity can be used as a screening tool to identify small polymictic reservoirs with higher densities of zooplankton subsidy.