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 biomass and activity in near-bottom waters was investigated at six stations in the 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 sampler or with a CTD-rosette during two cruises in May 1997 and February 1998. The vertical variability in suspended particle concentrations as well as in bacterial activity was higher than the regional and temporal variability. 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 bacterial leucine incorporation (1.81 pmol 1(-1) h(-1)) and the activity of different 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 concentrations 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, exceeded the vertical POC flux. Thus, we conclude that the enhanced microbial activity and biomass close to the seafloor is mainly supported by the resuspension of small phytodetrital particles and by the DOC flux. (C) 2000 Elsevier Science Ltd. All rights reserved.