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

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 biornass 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. It is therefore suggested that 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. Much recent ecological research on rocky shores and shallow subtidal regions (especially kelp beds) has involved extensive work on energy flow in an attempt to illuminate functional relations within the communities present. The general approach in such studies has been to identify the most abundant species present and to derive detailed energy budgets for them (e.g. Greenwood 1980, Shafir & Field 1980, Griffiths 1981, Koop & Field 1981). Wulff & Field (1983) have shown that energetic pathways in a kelp-bed community change under upwelling and downwelling conditions. In the present study we examine the trophic structure of intertidal communities which experience different physical conditions. We present data on biornass of the flora and fauna of rocky shores, subject to a range of conditions including different degrees of exposure to wave action, @ Inter-Research/Printed in F. R. Germany different temperature regimes and different kinds of rocky substrata. The influence of these factors on trophic structure and thus on fundamental patterns of energy flow through the communities is analysed. Physical factors appear to establish basic patterns of energy flow at atrophic level so that different trophic compartments predominate under different conditions (McQuaid 1980). Thus the balance between component trophic compartments may be altered and will determine whether a particular intertidal community will, overall, import or export energy. Species composition of these trophic compartments is, however, controlled by biogeographic considerations and the influence of chance recruitment of pelagic larvae. Wave action is often considered to have an important influence on intertidal communities and the importance of degree of wave exposure to community structure has long been recognised (Lewis 1964, Stephenson & Stephenson 1972). Comparisons are frequently Mar. Ecol. Prog. Ser 22: 153-161, 1985 made between exposed and sheltered shores (e.g. Seapy & Littler 1978, MacLachlan et al. 1981) and biological exposure scales have been developed to define wave action on the basis of the biota (Ballantine 1961, Lewis 1964) but such work has, in the past, been of a qualitative nature. We now compare exposed and sheltered rocky shores on a quantitative basis and examine the influence of wave exposure on the trophic structure of intertidal communities as a whole, including implications for energy flow between the intertidal and adjacent ecosystems.