Physical stress and biological control regulate the producer-consumer balance in intertidal biofilms

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Description/Abstract

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.