Effects of snail grazers and light on the benthic microbial food web in periphyton communities

Periphyton harbours a complex microbial food web with different trophic levels, but little is known about trophic interactions within this food web and its response to factors that potentially control the entire community. We used natural periphyton communities from Lake Erken (Sweden) in 2 experiments manipulating grazer presence. In addition to an ungrazed control, we included natural densities of either of 2 snail species (Theodoxus fluviatilis and Bithynia tentaculata) in order to test how generalist consumers affect different components of the periphyton. In the second experiment, we additionally manipulated light supply to alter the heterotroph-autotroph ratio in the periphyton and thus potentially the trophic interactions. In both experiments, grazer presence decreased algal biomass immediately and shifted the community composition from dominance of large filamentous forms to dominance of prostrate and motile unicellular species. Biomass of bacteria, heterotrophic nanoflagellates, ciliates and meiofauna also decreased with grazing, but with differentiated temporal dynamics and effect strength. In the ungrazed control, first heterotrophic protists and later meiofaunal biomass increased, indicating strong bottom-up and top-down propagation of trophic interactions in the microbial food web. High light increased the biomass of mixotrophic ciliates and nanoautotrophs but not total algal biomass. Grazer presence decreased algal richness in the second experiment, but increased evenness of the algal community under high light conditions. We conclude that grazer presence puts different pressure on the components of the periphyton, which are further linked by direct or indirect internal trophic processes.