Quantifying the top-down and bottom-up effects of a non-native grazer in freshwaters

Quantifying the effects of grazing and nutrient remineralization by grazers on algal biomass and productivity is important to estimate the net effects of grazing species on ecosystem structure and function. These effects may be especially important in ecosystems threatened by the invasion of non-native grazing species, as grazers can regulate the quantity of algae through both top-down (grazing) and bottom-up (remineralization) processes. In this study, we coupled mesocosm and in situ experimental manipulations to quantify the impacts of a non-native, grazing fish (Loricariidae: Pterygoplichthys) on algal biomass and primary productivity in stream ecosystems. While loricariid grazing depressed algal biomass and rates of primary production in mesocosm and in situ experiments, loricariid-mediated nutrient remineralization enhanced algal biomass and primary production in mesocosms relative to control treatments without loricariids. In sum, even when nutrient limitation was alleviated, intensive grazing by high-densities of loricariids had a negative net-effect on algal biomass and primary productivity. Examined together, our results demonstrate the need to quantify both consumptive and remineralization effects of invasive grazers in order to develop a comprehensive understanding of how non-native organisms influence ecosystem structure and function.