Abyssal polychaete assemblages along latitudinal gradients of productivity in the equatorial Pacific and North Atlantic Oceans
Patterns in polychaete abundance, body size and diversity were investigated at 12 sites between 4300 and 5000 m in the central Pacific and the north-east Atlantic. In the central Pacific, three of the sites (EqPac 0N, 2N and 5N) were observed to lie under high surface productivity regimes, and they were known to receive significant accumulations of food-rich phytodetrital material. The EqPac 9N, HOT 23N, DOMES A, ECHO 1 and PRA sites, which did not receive phytodetritus, were used as control sites with which to investigate the effect of this phytodetrital input. In the north Atlantic, one of the sites (PAP) was known to receive phytodetrital input, and one of the sites (MAP) had been subjected to a large-scale natural disturbance in the form of a turbidite emplacement. All specimens were identified to species level. Two families, the Pilargidae and Cirratulidae were selected for a more detailed alpha taxonomy assessment. New characters were developed for the identification of cirratulid thoracic fragments. Benthic polychaete abundance was correlated with surface productivity in both the Pacific and Atlantic Oceans. There was some evidence to suggest that there was a stronger benthic-pelagic link in the Pacific Ocean, where small changes in surface productivity generated larger changes in abundance than in the Atlantic Ocean. When data from previous studies are included, it is suggested that at levels of surface productivity above 200 gCm'2yr4, there is an upper limit to benthic polychaete abundance. Significant differences in body size between sites were found at species level, family level and for the entire polychaete taxon. At a species level, several abundant cosmopolitan deposit feeding species showed reduced body size in the food-rich phytodetrital sites. The only species to show increased body size in the food-rich sites were two predatory species. Polychaetes in the Atlantic Ocean responded more strongly in terms of body size reduction in phytodetrital sites than they did in the EqPac sites. Three hypotheses were put forward to explain these patterns: increased metabolic efficiency of large organisms in food-poor regions, seasonal recruitment pulses at phytodetrital sites and increased competition at phytodetrital sites. The former was favoured as the most likely explanation. Species diversity was shown to be highest in the phytodetrital sites in the central Pacific. It was hypothesised that this was the result of increased productivity, increased spatio-temporal heterogeneity and increased sediment heterogeneity at these sites. An increase in species diversity at phytodetrital Atlantic sites was not observed. The differences between north Atlantic and central Pacific sites were attributed to regional enrichment of local diversity in the Pacific. At the MAP turbidite site, alpha diversity was significantly lower than at other sites, and dominance was high, indicating the potential for large-scale natural disturbance in the abyss. A new spatio-temporal scale of disturbance was highlighted that may have evolutionary as well as ecological significance. Although levels of alpha diversity were generally shown to be high in the abyss compared to shallow water, the evidence did not suggest in favour of high beta diversity on scales of 1000 to 3000 km in either the central Pacific or north Atlantic. The low levels of beta diversity observed suggest that total species richness in the deep sea may not be as high as previously hypothesised, and that regional processes are likely to significantly impact local ecology in the deep-sea.