Spatial and temporal variations in sediment accumulation in an algal turf and their impact on associated fauna

The relationships between the fauna inhabiting an intertidal algal turf, Osmundea pinnatifida, and the accumulated sediment were studied in the autumn and summer. The investigation was carried out at two levels on sheltered, moderately exposed and very exposed shores on the temperate rocky coast of the Isle of Man, British Isles. Twenty-four species of invertebrates were found associated with the turfs, and their abundance and diversity varied with season, degree of wave exposure, shore level and the amount and particle size of the sediment trapped within the turfs. Multivariate analysis indicated that most organisms were most strongly influenced by sediment accumulation and temporal changes in the turf plants. Sediment provides a heterogeneous habitat for psammophylic organisms, supplies materials for tube-building species, and is a food source for detritivores. However, it also has adverse effects; its seasonal movement resulted in an unstable community. Sediment grain size was also important; certain grades appeared to be correlated with the abundance or sparsity of some species. "Season" was, however, the overriding factor influencing this microcommunity, since the frond complexity and the productivity of the turf plant, as well as the supply and movement of sediment, vary seasonally.