Spatial patterns of benthic diversity: Is there a latitudinal gradient along the Norwegian continental shelf?

Abstract

1. We examined data on soft-sediment macrobenthos (organisms retained on a 1-mm sieve) from a transect of c. 1960 km along the Norwegian continental shelf (56-71°N), covering a range of water depths (65-434 m) and varying sediment properties. 2. A total of 809 species was recorded from 101 sites. Of these, 36% were restricted to one or two sites, and 29% were represented by one or two individuals. No species spanned the entire transect. Polychaetes were the dominant taxonomic group, followed by crustaceans, molluscs and echinoderms. 3. Alpha diversity (sample species richness) was highly variable (35-148 species) but showed no evidence of a relationship to latitude or other environmental variables. 4. Beta diversity was measured as Whittaker's βW, the number of shared species, complementarity (biotic distinctness) and Bray-Curtis similarity, and there was no evidence of a latitudinal trend on the shelf. Beta diversity increased with the level of environmental variability, and was highest in the southern-central area, followed by the most northern area. Change in environmental variables had a stronger effect on beta diversity than spatial distance between sites. 5. Gamma diversity was computed by pooling samples over large areas. There was no convincing evidence of a latitudinal cline in gamma diversity, but gamma diversity increased with the level of environmental heterogeneity. Mean alpha diversity and gamma diversity were not significantly correlated. Whereas mean complementarity and mean Bray-Curtis similarity were related to gamma diversity, βW was not.