An experimental analysis of species sorting and mass effects in freshwater bacterioplankton

While metacommunities of bacterioplankton are generally considered to be structured primarily by local environmental conditions (through species sorting), additional ecological processes such as dispersal limitation, neutral dynamics or mass effects can influence community structure. Under the mass effects paradigm, continuous or large influxes of organisms, not self-maintaining in the target environment, affect community structure. We used communities of freshwater bacterioplankton to quantify the outcome of mass effects and species sorting in a laboratory experiment in which we exchanged cells between two communities from contrasting ponds (eutrophic and mesotrophic) at a rate of 0.025, 0.25, 2.5 and 12.5% of the cells present per day. When cells were exchanged only once on day 1 of the experiment, the reciprocally exchanged communities in all dispersal treatments remained as divergent, after 20days, as the unexchanged control communities, reflecting strong species sorting and no strong influences of dispersal limitation and neutral dynamics on the observed bacterioplankton. Under daily cell exchange, however, community similarity between the reciprocally exchanged communities increased significantly with increasing dispersal rate, indicating an increasing influence of mass effects relative to species sorting. The responses differed at the level of community composition and ecosystem processes, and depended on pond identity. At the community level, a daily exchange of 2.5% was necessary to increase community similarity compared with the unexchanged controls, while at the ecosystem process level (Biolog GN2 metabolic profiles), a daily exchange rate of 0.25% was sufficient. Mass effects can evidently overcome species sorting in freshwater bacterioplankton, but only under relatively high dispersal rates that are unlikely to occur in nature among habitats without a direct hydrological connection. Mass effects in the freshwater bacterioplankton acted differently at the community composition and ecosystem process level.