ENHANCED MICROBIAL UTILIZATION 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 wel l. as concentration and composition of particles in the benthic bounda ry 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). Mi crobial activities and concentrations of various biochemical parameter s displayed distinct variation patterns in the BBL, exhibiting higher values compared to their variations in the chlorophyll maximum layer a nd the IWC. In the BBL the microbial utilisation of C-14-amino acids r evealed no correlation with concentrations of particulate organic carb on (POC), but was related to chlorophyll a (chi a) equivalents and the ratio of POC and chi a equivalents. The distribution of dissolved fre e amino acids (DFAA) suggests that not necessarily the concentration b ut alternatively the accessibility (concentration and transport rate) of DFAA to bacteria determines heterotrophic activity. Results indicat e that the BBL represents a distinct environment with rapid modificati on of organic matter prior to its final incorporation into the sedimen t.