Microbial activity and particulate matter in the benthic nepheloid layer (BNL) of the deep Arabian Sea

The distribution of suspended particulate matter as well as bacterial bioma ss and activity in near-bottom waters was investigated at six stations in t he deep Arabian Sea (2000-3500 m water depth), Water samples were obtained from heights between 0.1-1000 m above bottom (m a.b.) with a bottom water s ampler or with a CTD-rosette during two cruises in May 1997 and February 19 98. The vertical variability in suspended particle concentrations as well a s in bacterial activity was higher than the regional and temporal variabili ty. Compared to the deep water column (250-1000 m a.b.; 0.1 x 10(8) cells 1 (-1)), an increase in bacterial numbers was observed from 40-100 m a.b. (0. 24 x 10(8) cells 1(-1)) towards the seafloor (0.1-0.6 m a.b.; 0.53 x 10(8) cells 1(-1)). Suspended particulate matter (0.42 mg 1(-1)) as well as bacte rial leucine incorporation (1.81 pmol 1(-1) h(-1)) and the activity of diff erent enzymes were highest in the near-bottom water ( < 1 m) and decreased with increasing height above bottom (250-1000 m a.b.: 0.2 mg 1(-1) and 0.69 pmol 1(-1) h(-1), respectively). The suspended particles in the BNL had a high chlorophyll a to POC ratio and were of higher organic carbon concentra tions than the sinking particles or the particulate matter at the sediment surface. The carbon demand of the bacterial community in the BNL (20 mg C m (-2) d(-1)), which was estimated by leucine incorporation experiments, exce eded the vertical POC flux. Thus, we conclude that the enhanced microbial a ctivity and biomass close to the seafloor is mainly supported by the resusp ension of small phytodetrital particles and by the DOC flux. (C) 2000 Elsev ier Science Ltd. All rights reserved.