Epilithic biofilms play a key role in marine ecosystems. They also provide a tractable system to investigate the relative roles of environmental stressors, bottom-up physicochemical factors and top-down biological control in regulating communities. Patterns of photosynthetic microbial biomass were recorded over a four-year period at several tidal levels on shores in the Isle of Man. Photosynthetic biomass and the abundance of diatoms were consistently greater during winter than summer. Biomass was negatively correlated with insolation stress and air temperature, but was not correlated with grazing intensity, dissolved nutrients, sea temperature, or planktonic chlorophyll. Field experiments confirmed that reducing insolation stress led to substantial increases in photosynthetic biomass, predominantly of diatoms and macroalgal germlings. Reducing grazing intensity also led to considerable increases in photosynthetic biomass, but reducing desiccation stress or increasing nutrient availability had no effect. Although grazing can regulate microalgal biomass, seasonal patterns of grazing activity were driven by temperature and were decoupled from photosynthetic biomass. Our study demonstrates the importance of physiological stresses for the direct and indirect regulation of the balance between primary producers and consumers. Based on these findings, we present a model that combines the roles of stressors together with bottom-up forcing and top-down regulation in controlling communities on wave-exposed shores.

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