Spatial patterns of diversity in the sea: bryozoan species richness in the North Atlantic

1. We have examined large-scale geographical patterns in species richness for continental shelf bryozoan assemblages in the North Atlantic. Bryozoans are common and often abundant benthic organisms, but they have not previously been examined at this scale of resolution. 2. Assemblage species richness was estimated by sample species richness. This was highest at intermediate depths (10–75 m) at all latitudes where there were sufficient data, but there was no statistically significant variation with depth for the overall data set (all latitudes pooled). Mean assemblage species richness showed no significant variation with latitude, although the highest individual values were generally from lower latitudes. There is thus as yet no convincing evidence for a latitudinal cline in the alpha diversity of North Atlantic bryozoans. 3. Pooling of data into bins of 10 degrees of latitude, or into biogeographic provinces, to estimate regional species richness revealed significant undersampling. Two independent techniques to correct for this undersampling revealed a latitudinal cline in the regional species richness of North Atlantic bryozoans, with a peak around 10–30°N, and a steady decrease in richness north to 80°N. 4. Two measures of beta diversity (Whittaker and Jaccard) revealed relatively high turnover, presumably related to habitat heterogeneity within regional bins or to significant environmental variation across bins. There was a tendency for beta diversity to be higher at lower latitudes, as would be expected from a combination of a latitudinal cline in regional diversity with a mean assemblage species richness invariant with latitude. Null models were used to clarify the expected relationship between the two measures of beta diversity, and these indicated a strong influence of species-abundance structure in the North Atlantic bryozoan data.