Distribution and Diversity of Polychaeta in the Southern Irish Sea

The distribution of the polychaete assemblages in southern Irish Sea and its approaches (7–130 m depth) was investigated using cluster analysis and non-metric multidimensional scaling (MDS). Three main station groupings were determined from the 51 quantitatively assessed stations and their distinguishing species identified. The possible factors influencing these assemblages were studied using canonical correspondence analysis (CANOCO ordination program) and the recently developed BIO-ENV procedure of the PRIMER (Plymouth Routines in Multivariate Ecological Research) software package. Both techniques indicated sediment particle size and depth as the variables best "explaining" the species distributions. Species diversity was found to be highest in the offshore gravelly sediments and lowest in the inshore sands and muddy sands. There was nevertheless evidence for a depth-related increase in diversity and evenness among the softer sediments. The high species richness values (as determined by the Hurlbert rarefaction methodology) of the more uniformly diverse gravels compared well with estimates from soft deep-sea areas. These findings lend support to the theory that benthic diversity in the deep-sea is higher than on the shelf—at least for sandy and muddy sediments. Conversely, the richness of the gravels contradicts the belief that high benthic diversities are only to be found in the deep-sea and tropical shallow waters.