Trophic structure of rocky intertidal communities response to wave action and implications for energy flow

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Abstract
The influence of physical factors on the trophic structure of rocky intertidal communities in the Cape of Good Hope was investigated by examining species composition and biomass on shores subject to different conditions of substratum, sea temperature and wave exposure. Biomass of herbivores on shores with an unstable substratum was particularly high while the sessile filter-feeders and algae were impoverished, but actual rock type did not affect trophic structure. Sea temperature influenced species composition but not trophic structure which was most strongly affected by the degree of wave exposure. Exposure influenced both vertical distribution of biomass and trophic composition of total biomass. Total biomass showed a simple decrease upshore on sheltered shores but the pattern was more complex with greater exposure. Filter-feeders, carnivores and omnivores all exhibited significantly higher biomass under exposed conditions. Consequently the balance between consumers and primary producers shifted, implying alterations in the net balance between importation and exportation of production between these communities and the inshore marine system. The addition of huge filter-feeder components to the intertidal system resulted in significantly higher total biomass under exposed conditions. This allows the importation of production from the water column to a community in which consumers exhibit considerably higher standing crops than primary producers. Benthic carnivores exhibited a positive correlation with filter-feeder biomass. Domination of exposed shores by sessile filter-feeders and of sheltered shores by algae and mobile herbivores may explain divergent views in the literature on the relative importance of competition and disturbance/predation to community structure on rocky shores.