Differences in photosynthetic marine biofilms between sheltered and moderately exposed rocky shores

Photosynthetic microbial biofilms are an important functional component of rocky intertidal habitats worldwide. The abundance of organisms within these films varies both seasonally and according to vertical emersion gradients. However, the effects of horizontal gradients of wave exposure, which have a key influence on macrobiotic assemblages, have received little attention. Here, we examine the relative importance of exposure to wave action, tidal elevation and season on intertidal microbial assemblages. The abundance of micro-organisms was quantified using counts of major functional groups and total photosynthetic biomass, assessed using extracted chlorophyll. These data were compared between 2 moderately exposed and 2 sheltered shores on replicated sampling dates during summer and winter at 3 tidal levels. Diatoms were approximately 6 times more abundant on moderately exposed shores than on sheltered shores. This pattern was consistent between sites and seasons. The percentage cover of cyanobacteria and total photosynthetic biomass was also typically greater on exposed shores. Seasonal variations were also apparent with a trend of greater photosynthetic microbial biomass during the winter than during the summer. Photosynthetic biomass and, to a lesser extent, the abundance of cyanobacteria were greater on the lower shore than on the upper shore. The possible causes of the differences in microbial assemblages between sheltered and exposed shores are discussed. We suggest that indirect effects of habitat amelioration from macroalgal canopy and grazer density, which also covary along the wave exposure gradient, may be more important than the direct effects of wave action itself.