Physical processes dominate in shaping invertebrate assemblages in reef-associated sediments of an exposed Hawaiian coast

The invertebrate assemblages in sediments bordering exposed fringing reefs at Hanalei Bay, Kauai, Hawaii, were examined during July to September 1994. Densities of invertebrate animals larger than 0.5 mm in sediments of the b ay ranged from counts of 10 260 m(-2) in the fine carbonate sands of the ce ntral bay to 870 m(-2) in the habitat dominated by terrigenous silt near th e reef edge close to the Hanalei river mouth. Similar sediment types suppor ted broadly similar infaunal communities. Within the primarily carbonate se diments, mean grain size and wave exposure appear to have an important infl uence on the community. Taxonomic richness, number of individuals, and dive rsity showed significant negative relationships with exposure to wave energ y las estimated by sand ripple wavelength). The number of individuals was a lso significantly correlated with mean grain size. Overall, polychaetes and small crustaceans were numerically dominant among the major taxonomic grou ps investigated. Macrophagous and microphagous polychaetes had significant, but opposite, associations with grain size. In addition, microphagous poly chaetes were significantly negatively correlated with wave exposure. No hab itat variable measured could explain the variation in percent composition o f crustaceans or echinoderms in the sedimentary habitats. The percentage of gastropods in the community was significantly negatively correlated with g rain size, grain-size standard deviation and exposure, and positively with percent organic carbon. Bivalves were significantly positively associated w ith depth and grain size. These strong relationships imply that, in Hanalei Bay, physical processes are especially important in influencing assemblage structure, and that community structure and composition vary continuously along environmental gradients.