Differential effects of Daphnia genotype composition on spatial environmental heterogeneity in experimental metacommunities

Spatial environmental heterogeneity is an important driver of aquatic biodiversity. Ecological and evolutionary theory often consider spatial heterogeneity as being driven by exogenous factors, yet heterogeneity can also be generated and modified by organisms. Here we used a mesocosm experiment to investigate if consumers influence the build-up of spatial heterogeneity. We expected that consumer effects on heterogeneity would depend on consumer composition and differ among response variables. We constructed metacommunities consisting of three mesocosms and manipulated the presence and composition of consumers, using four treatment levels: (1) no consumers, (2) two genotypes of Daphnia galeata, (3) D. galeata and Daphnia longispina, and (4) D. galeata and a hybrid of D. galeata x D. longispina. We then continuously increased heterogeneity among the three patches of each metacommunity by adding nutrients and dissolved organic carbon (DOC), respectively, to two of the three mesocosms. We found that consumers affected the build-up of heterogeneity, but the direction and magnitude of this effect differed among consumer compositions. Metacommunities with only D. galeata had increased heterogeneity in phytoplankton biomass, whereas metacommunities with D. longispina or the hybrid had low phytoplankton heterogeneity. The differential effects of Daphnia taxa on phytoplankton heterogeneity cascaded down to the abiotic environment and resulted in taxon-specific effects on heterogeneity in light extinction, dissolved inorganic nitrogen, and total inorganic carbon. Our results imply that changes in consumer species (e.g., due to environmental change or invasion) might affect not only the local environment but could also impact heterogeneity among environments, with important consequences for aquatic biodiversity.