Microbial food web structure affects bottom-up effects and elemental stoichiometry in periphyton assemblages

Heterotrophic organisms constitute an important part of the overall biomass of periphyton assemblages, but the role of these components for the propagation of resource enrichment and top-down control has rarely been addressed systematically. We report results from six laboratory experiments differing in their trophic structure by the inclusion of different functional groups of microbial benthic organisms (i.e., algae, bacteria, flagellates, ciliates, and rotifers). We manipulated the supply of organic carbon, phosphate, light, and the presence of the respective top consumer in full factorial designs. Increased supply of dissolved organic carbon (DOC) increased total periphyton biomass in almost all experiments, whereas increased phosphorus (P) supply increased total biomass only if algae were present. The effects of DOC and P on the ratio of heterotrophic to autotrophic abundance depended on trophic structure, where additional resources enhanced the autotrophic component when the basal heterotrophs were limited by low organic carbon (C) or by consumer pressure. Light had a comparably minor influence on total biomass and relative abundances. Purely heterotrophic biofilms had higher C:P ratios than autotrophic assemblages. Increased P supply decreased periphyton C:P throughout all experiments except for the experiment with the highest trophic complexity, which yielded low C:P ratios across all treatment levels. Our experimental microcosms with artificially constructed periphyton communities revealed the importance of the benthic microbial food-web structure for both top-down and bottom-up propagating effects of consumers and resources, respectively.