Regional and latitudinal variation in the diversity, dominance and abundance of microphagous microgastropods and other benthos in intertidal beds of dwarf eelgrass, Nanozostera spp.

Abstract

The smaller macroscopic members of the epifauna and shallowly-burrowing infauna of comparable intertidal beds of dwarf eelgrass and associated areas of non-vegetated sediment were investigated with uniform methodology in the cool-temperate English southern North Sea (Nanozostera noltii), warm-temperate southern coast of the Western Cape, South Africa (N. capensis) and in subtropical southern Queensland, Australia (N. muelleri capricorni), together with equivalent seagrass sites in tropical Sulawesi, Indonesia, and Seychelles, Western Indian Ocean. Epifaunal microphagous microgastropods dominated both the eelgrass and non-vegetated cool- and warm-temperate sites with >80% of macrofaunal individuals, but decreased markedly in density and dominance with decreasing latitude, down to near zero in the tropics; microgastropod species diversity in the Nanozostera increased with decreasing latitude, whilst their species richness per core sample was highest in the warm temperate zone. Other co-existing—largely infaunal—taxa (mainly annelid worms, bivalve molluscs and crabs), however, showed less marked latitudinal variation in density and no relationship of taxon diversity with latitude. With few exceptional cases, microgastropod density, dominance, species richness and diversity were greater in the eelgrass beds than in adjacent non-vegetated sediments, as were the densities and taxon diversities of the associated faunal groups, although within the beds themselves there were no significant correlations between seagrass density and the density or diversity of either the microgastropods or their associated fauna. The extent to which the presence or absence of seagrass influenced the underlying community composition of the benthic fauna varied between localities. These results are consonant with an increasing effect of predation in low latitudes on small epifauna.