BENTHIC RESPIRATION AND STANDING STOCK ON 2 CONTRASTING CONTINENTAL MARGINS IN THE WESTERN INDIAN-OCEAN - THE YEMEN-SOMALI UPWELLING REGIONAND THE MARGIN OFF KENYA

During the Netherlands Indian Ocean Project (NIOP, 1992-1993) sediment community oxygen consumption (SCOC) was measured on two continental m argins in the Indian Ocean with different productivity: the productive upwelling region off Yemen-Somalia and the supposedly less productive Kenyan margin, which lacks upwelling. The two margins also differ in terms of river input (Kenya) and the more severe oxygen minimum in the Arabian Sea. Simultaneously with SCOC, distributions of benthic bioma ss and phytodetritus were studied. Our expectation was that benthic pr ocesses in the upwelling margin of the Arabian Sea would be relatively enhanced as a result of the higher productivity. On the Kenyan margin , SCOC (range 1-36 mmol m(-2) d(-1)) showed a clear decrease with incr easing water depth, and little temporal variation was detected between June and December. Highest SCOC values of this study were recorded at 50 m depth off Kenya, with a maximum of 36 mmol m(-2) d(-1) in the no rthernmost part. On the margin off Yemen-Somalia, SCOC was on average lower and showed little downslope variation, 1.8-5.7 mmol m(-2) d(-1), notably during upwelling, when the zone between 70 and 1700 m was cov ered with low O-2 water (10-50 mu M). After cessation of upwelling, SC OC at 60 m depth off Yemen increased from 5.7 to 17.6 mmol m(-2) d(-1) concurrently with an increase of the near-bottom O-2 concentration (f rom 11 to 153 mu M), suggesting a close coupling between SCOC and O-2 concentration. This was demonstrated in shipboard cores in which the O -2 concentration in the overlying water was raised after the cores wer e first incubated under in situ conditions (17 mu M O-2). This induced an immediate and pronounced increase of SCOC: Conversely, at deeper s tations permanently within the oxygen minimum zone (OMZ), SCOC showed little variation between monsoon periods. Hence, organic carbon degrad ation in sediments on a large part of the Yemen slope appears hampered by the oxygen deficiency of the overlying water. Macrofauna biomass a nd the pooled biomass of smaller organisms, estimated by the nucleic a cid content of the sediment, had comparable ranges in the two areas in spite of more severe suboxic conditions in the Arabian Sea. At the Ke nyan shelf, benthic fauna (macro-and meiofauna) largely followed the s patial pattern of SCOC, i.e. high values on the northern shelf-upper s lope and a downslope decrease. On the Yemen-Somali margin the macrofau na distribution was more erratic. Nucleic acids displayed no clear dow nslope trend on either margin owing to depressed values in the OMZ, pe rhaps because of adverse effects of low O-2 on Small organisms (meiofa una and microbes). Phytodetritus distributions were different on the t wo margins. Whereas pigment levels decreased downslope along the Kenya margin, the upper slope off Yemen (800 m) had a distinct accumulation of mainly refractory carotenoid pigments, suggesting preservation und er low O-2. Because the accumulations of C-org and pigments on the Yem en slope overlap only partly, we infer a selective deposition and pres ervation of labile particles on the upper slope, whereas refractory ma terial undergoes further transport downslope. (C) 1997 Published by El sevier Science Ltd.