Pfannkuche¹, Olaf, Antje Boetius², Karin Lochte³, Stefan Sommer¹ and Ursula Witte²

¹GEOMAR, Research Center for Marine Geosciences, Wischhofstr. 1-3, 24148 Kiel, Germany, Tel: +49-(0)431 600 2113, Fax: +49-(0)431 600 2911, E-mail: opfannkuche@geomar.de and ²MPI, Max-Planck- Institute for Marine Microbiology, Bremen, and ³IFM, Institute for Marine Research, Kiel

BIGSET investigations of benthic boundary layer processes at JGOFS sites in the NE-Atlantic and Arabian Sea

BIGSET (Biogeochemical transports of energy and matter in the deep sea) is a collaborative programme focussing on the benthic boundary layer (BBL) The overall objective is to identify, quantify and model the principal processes within the abyssal BBL which intercede between the incoming material flux and its incorporation into the permanent sedimentary record.

BIGSET field studies were carried out at sites also investigated by JGOFS process studies: the Northeast Atlantic at 47°N /20°W (BIOTRANS) and the Arabian Sea. Our results greatly benefited from multi-year sediment trap deployments. We expected that the trophic gradient established along a spatially varying range of POC deposition in the bi-annual monsoon pattern (Arabian Sea) would leave its imprint on the biogeochemistry of the deep-sea benthos. A spatial gradient, similar to that of surface water productivity was found for a number of abyssal benthic turnover rates and various biochemical parameters. Overall high benthic remineralisation rates measured with landers are comparable to rates found on upper continental slopes at upwelling regions. High bacterial production rates and activities of extra-cellular enzymes give further evidence of unusually high turnover rates.

Benthic carbon consumption rates are higher than the POC flux determined by the sediment traps (1000mabsf) can sustain. There is evidence from these and a number of other measurements that lateral input from the continental shelf increases the amount of organic matter consumed and deposited at these sites.

Although pronounced seasonal fluctuations of particle fluxes occur in the Arabian Sea, we could detect only a few significant temporal changes of benthic processes. This is partly due to the high variability in the benthic data but also perhaps due to the timing of our investigations. Apparently our investigations did not always hit the peak particle pulses which can vary more than one month in comparison to the multi-year average. Unexpectedly, most parameters describing biomass, metabolic activity or degradation rates exhibited a stronger increase after the NE-monsoon indicating that the impact of the NE-monsoon on the carbon cycling in the Arabian Sea was so far greatly underestimated. Results from the NE-Atlantic give further insight to the extent of the pelago-benthic coupling in the abyssal open ocean in which the magnitude of POM influx determines the amplitude and duration of the benthic response. In situ enrichment experiments demonstrate the fast response of the microbial community to detritus enrichment and give evidence to significant temporal variation in benthic carbon turnover.