Large-scale forcing of coastal communities.

Abstract

Two contrasting areas of pelagic primary productivity (PPP) were noted in western Scotland, the Clyde (high PPP) and the west coast (low PPP). It was hypothesised that increased PPP would have a direct bottom-up influence on intertidal community structure with the Clyde expected to be dominated by filter feeders with potentially greater larval recruitment, increased density, faster growth and larger maximum sizes. The study was divided into five sections examining community structure, growth rates, predation and grazing pressures, effects of wave exposure, and stable isotope analysis. Individual species tended to vary between sites within lochs rather than between the two regions. Growth rates of the predator Nucella were found to follow peaks in Semibalanus size, rather than Mytilus, with increased growth on the west coast. An increased density of the grazer, Littorina, at a site had an increased effect on their growth rate although dense localised patches within sites were observed where growth rates were lowest. Mussel size classes were found to have different growth rates, most probably due to differing factors such as predation, food availability, and reproduction. Predation and grazing effects differed between regions. Barnacle cover was shown to be affected by both Nucella and Littorina although the latter may have been an indirect affect due to the biofilm cover which was greater in the Clyde. Predation rates of mussels were found to be greater on the west coast which was most probably due to a change in diet from barnacles to mussels. Mussel shell length and biomass declined with increasing wave exposure throughout western Scotland with the potential for factors varying on small scales to be more important in structuring mussel populations. This was evident when testing for differences in stable isotopes of mussels which suggested site specific variation due to increased freshwater input. The results of this study showed that small scale, local factors are as important, if not more, as regional differences in structuring communities. PPP is important, but only for a subset of the community