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CROZEX

CROZEX stands for CROZet natural iron bloom and EXport experiment

 

Project leader: David Billett, National Oceanography Centre Southampton

About the project

The Indian sector of the Southern Ocean is one of the remotest locations in the world ocean, with the result that we know very little about the fauna of this region.   To this end, during the austral summers of 2004/5 and 2005/6, three research cruises were carried out to this area on the RRS Discovery, with scientists participating from British and Irish Research Centres and Universities. The abundance and diversity of the deep-sea benthos are closely linked to inputs of organic matter from surface waters.  However, it is often difficult to isolate the influence of surface productivity on benthic ecosystems from other environmental factors.  To further understand the relationship between fluxes of organic matter and processes at the seafloor, the “Benthic Crozex” programme is investigating the variation in the response of benthic ecosystems to the differing productivity regions at two abyssal sites (~4200 m water depth) located under contrasting productivity regimes around the Crozet Plateau.  One site, east of the Crozet Isles, was located beneath an area where there was an enduring seasonal phytoplankton bloom, while the second site was located in an oligotrophic High Nutrient Low Chlorophyll (HNLC) region to the south.  All other environmental variables were similar at the two sites. 

Cruises

Three cruises have been carried out to the Crozet region on the research vessel RRS Discovery, during the austral summers of 2004/2005 and 2005/2006.  The dedicated Benthic CROZET cruise (1.12.2005 – 14.1.2006) was particularly relevant to CeDAMar. On board were participating scientists from the Universities of Liverpool and Aberdeen, the National Oceanography Centre, Southampton, the British Antarctic Survey, the Natural History Museum and the University College Galway.

 

Prellminary Results

Although this study is ongoing, initial results indicate that the dominant megafaunal species are quite different at the two sites, which appears to be related to the different organic inputs.  Remarkably, the holothurians (which accounted for 90% of the megafaunal biomass) at the northern, eutrophic site showed a greater affinity to those in the NE Atlantic, 16,000 km distant, than to those found at the more southerly site, only 460 km away. 

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 three 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. 

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