Environmental controls on biofilm succession drive invertebrate grazing pressure

Epilithic biofilms are influenced by both abiotic (e.g., light, flow) and biotic forces (e.g., invertebrate grazing). These factors have been extensively studied independently; however, there is less understanding of how abiotic conditions mediate grazer feeding preference during biofilm succession. Biofilms were grown on unglazed porcelain tiles under ambient, modified flow, dark, and flow/dark treatment conditions in a third order stream. The influence of abiotic factors on biofilm succession was characterized at 7, 14, 21, and 28 days. Biofilms grown under each treatment and at each stage of growth were then subjected to invertebrate grazing (snails, mayflies, and snails/mayflies) in the lab. Primary production, biomass and turnover were all significantly lower when grown under dark treatment conditions, while modified flow increased primary production and biomass at 21d. In the grazer feeding experiment, biofilm communities grown under ambient light conditions at later stages of growth were preferred by both grazer taxa. Mayflies significantly grazed modified flow biofilms 21d old, while snails significantly preferred communities grown under natural light conditions at 21 and 28d. However, when the invertebrate taxa grazed together, a distinct tiered pattern emerged. Biofilms under modified flow at 21d were grazed the most, with biofilms receiving natural light levels at 14/28d and 21d ambient biofilms grazed at intermediate levels. The preferred biofilm may be an intermediate community transitioning to a climax community. These results provide a mechanistic understanding of how both abiotic and biotic conditions mediate grazing activity during biofilm succession.