Patterns of benthic fauna and benthic respiration on the celtic continental margin in relation to the distribution of phytodetritus

Abstract

In-situ and shipboard measurements of sediment community oxygen consumption (SCOC), in conjunction with a study of, the distributions of macro and megafauna and phytopigments, were used to determine and, where possible, explain the distribution of labile particulate organic matter (POM) on the NE Atlantic continental slope (Goban Spur, SW Ireland). A specific issue concerned the existence of depocentres of labile POM on the slope caused by lateral transport. a phenomenon that has been found previously in ,the' NW Atlantic. The SCOC data from October 1993 and May 1994 showed a steady decrease with increasing water depth. SCOC values ranged from 5.4 mmol m-? d-' at the shelfbreak to 0.3 mmol m-' d-' at 4,500 m depth. No evidence was found for seasonal variation in SCOC. A clear seasonal signal was observed with regard to sediment phytopigments and phytopigment fluxes into sediment traps attached to the benthic lander. The upper-and mid-slope values of both parameters were much higher in May 1994 than in October 1993 and August 1995. This is consistent with the normal spring bloom pattern; but because of the degraded state of the May phytodetritus in the nearbottom water, reflected in the lack of a response in SCOC and the low chlorophyll-o concentrations, it was concluded that the material was not derived from the overlying photic zone, but instead transported from elsewhere in the benthic nepheloid layer (BNL). In August 1995, the lower slope (>3,000 m) had received a strong and fresh phytodetritus pulse (3 g C m-') forming a mucous layer on top of the sediment. Using phytopigments and sterols as molecular markers, it was shown that the pulse was derived from an offshore bloom with an important contribution by dinoflagellates. By contrast, no mucous layer was found on the upper slope stations in August 1995. Macrofauna biomass showed a distinct decline from the upper slope down to the lower slope conforming to the diminishing supply of labile POM. The total wet biomass of megafauna reached relatively high values at the lower slope (>3,500 m) owing to large motile sea cucumbers. The presence of these "vacuum-cleaner" sea cucumbers is considered indicative of the occurrence of phytodetritus pulses. In spite of their assumed adaptation to periodic pulses, the estimated contribution by the sea cucumbers to the total benthic mineralization is minor, When combining data from different yeardseasons we observed decoupling between the food supply to the lower slope and the upper and mid slopes. The major pulse to the former comes from an offshore summer bloom. The upper and mid slope appear to be fuelled by spring bloom material which is subsequently redistributed on the upper slope in a BNL. The quality of the seston in the BNL diminished in the offshore direction as indicated by the phytopigment concentrations.

The carbon cycle at the ocean margin in the northern Gulf of Biscay

“…On the other hand, Heip et al (2001) estimated the total carbon mineralization rates (open circles in Fig. 11), based on the sediment community oxygen consumption (SCOC) measurements (Duineveld et al, 1997). In contrast to the trap measurements, the depositional fluxes agree well with the decay curves proposed by Betzer et al (1984); Berger et al (1987).…”

“…Pore water profiles show that oxygen penetrates deeper into the sediments with increasing water depth and distance from the shelf. As indicated above, measurements of oxygen consumption (Duineveld et al, 1997;Lohse et al, 1998;Balzer et al, 1998) demonstrate that the organic carbon deposited at the sediment-water interface is almost entirely respired before burial. Therefore, only a very small fraction of the organic matter deposited, which is highly refractory, is preserved in the sediments as confirmed by the burial fluxes (Fig.…”

“…Nevertheless, several measurements or estimations of the organic matter respiration were performed in the Goban Spur area (Duineveld et al, 1997;Balzer et al, 1998;Lohse et al, 1998). They show that most of the organic matter deposited at the sediment-water interface is respired before burial.…”

“…13). Measurements of oxygen consumption (Duineveld et al, 1997;Heip et al, 2001) show that most of the respiration is due to bacterial activity and the role of macro-and meio-fauna in the mineralization of organic matter is minor except at the shallowest station (200-m depth).…”

Enzymatically hydrolysed protein and carbohydrate pools in deep-sea sediments:estimates of the potentially bioavailable fraction and methodological considerations

“…Quantity and quality of organic matter (OM) in marine sediments are recognised as major factors affecting metabolism, distribution and dynamics of benthic organisms, from bacteria to megafauna (Grant & Hargrave 1987, Graf 1989, Duineveld et al 1997. Food availability in the sediments has long been expressed as the bulk concentration of organic carbon, nitrogen (by gas chromatography) or total OM (by ignition loss, Fabiano et al 1995).…”