Two abyssal sites in the Southern Ocean influenced by different organic matter inputs: Environmental characterization and preliminary observations on the benthic foraminifera

The abundance and diversity of the deep-sea benthos are intimately linked to inputs of organic matter from the euphotic zone. However, it is often difficult to isolate the influence of surface productivity on benthic ecosystems from other environmental factors. To this end, two abyssal sites (∼4200 m water depth) located under contrasting productivity regimes around the Crozet Plateau, in the Indian Ocean sector of the Southern Ocean, were sampled during the austral summer of 2004/2005. One site (M5), east of the Crozet Isles, was located beneath an area where there was an enduring seasonal phytoplankton bloom. The second site (M6) was located in an oligotrophic high-nutrient low-chlorophyll (HNLC) region to the south of the islands. Organic fluxes to the seafloor at these sites are thought to reflect the overlying productivities, so that the benthic environment to the east of the islands was more eutrophic than at the southerly station. All other environmental variables were similar at the two sites, which are located just 460 km apart.The concentrations of chlorophyll-a and total organic carbon in the surficial sediments were significantly greater at the relatively eutrophic site, east of the islands (M5), than at the southerly site (M6). Total nitrogen, however, was similar at both sites. Significantly higher phytopigment concentrations were observed in the surficial sediments at the eutrophic site; in particular, the concentration of chlorophyll-a was 3 times greater than at the southern site, although the freshness of the labile component, as measured by chlorophyll-a to pheophorbide ratio, was not different between sites. These results confirm that fluxes of organic matter to the seafloor were higher at the site located beneath the bloom region. This was reflected in the abundance and diversity of live (stained) and dead benthic foraminifera (>125 μm), which were greater at the eutrophic site. The species composition of the dead foraminiferal assemblages were similar at both sites, however, and were dominated by Nuttallides umbonifera, Pullenia bulloides, and Melonis pompiloides. An exception was the “phytodetritus species” Epistominella exigua, which was more abundant at the eutrophic site, indicating a larger seasonal component to the export under the bloom region. Differences in the organic matter input regimes at the two sites appear to influence the abundance and diversity, but not the overall species composition, of the foraminiferal assemblages.