Spatial patterns of soft-sediment benthic diversity in subtropical Hong Kong waters

It has long been assumed that, just as on land, a decline in diversity from the tropics to the poles is a feature of the marine environment, yet little is known about patterns of diversity in tropical/subtropical areas. Soft-sediment macrobenthos data from the entire Hong Kong waters (22degrees N, 113-114degrees E) were used to examine faunal patterns in a subtropical area. Grab samples were taken from a depth range of 5 to 47 m at 101 sites on a spatial sampling scale of 1100 km(2). A total of 386 species belonging to 12 phyla was recorded in 50.5 m(2) of sediment collected from the 1100 km2 sampling area. Species richness (alpha diversity) of the samples was low (mean 35 species) but highly variable (10 to 78 species). Relationships between community structure and environmental variables were weak. The number of rare species was high: 38% of the species were represented by 1 or 2 individuals, 45% were restricted to 1 or 2 sites, and no species spanned the whole sampling area. Polychaetes displayed the highest species richness, followed by molluscs and crustaceans. Molluscs were more restricted in their distribution than the other dominant groups, whereas polychaetes had the highest proportion of widespread species. Beta diversity, measured as Whittaker's beta(W) and Bray-Curtis similarity, was high. Beta diversity varied among the dominant groups and was highest for molluscs, followed by crustaceans and polychaetes. The faunal patterns changed with spatial scale. The study shows that patterns of diversity depend considerably on which element of biodiversity is measured.