Enhanced microbial utilisation of dissolved amino acids indicates rapid modification of organic matter in the benthic boundary layer

Recent information on activity, abundance, and size of bacteria as well. as concentration and composition of particles in the benthic boundary layer (BBL, range of water depths 191 to 479 m) was compared to the distribution of these parameters in 2 layers of the water column, the chlorophyll maximum layer (range of water depths 13 to 25 m) and the intermediate water depth (IWC, range of water depths 136 to 486 m). Microbial activities and concentrations of various biochemical parameters displayed distinct variation patterns in the BBL, exhibiting higher values compared to their variations in the chlorophyll maximum layer and the IWC. In the BBL the microbial utilisation of C-14-amino acids revealed no correlation with concentrations of particulate organic carbon (POC), but was related to chlorophyll a (chi a) equivalents and the ratio of POC and chi a equivalents. The distribution of dissolved free amino acids (DFAA) suggests that not necessarily the concentration but alternatively the accessibility (concentration and transport rate) of DFAA to bacteria determines heterotrophic activity. Results indicate that the BBL represents a distinct environment with rapid modification of organic matter prior to its final incorporation into the sediment.