SPATIAL VARIATION OF BENTHIC MICROBIAL-PRODUCTION AND HYDROLYTIC ENZYMATIC-ACTIVITY DOWN THE CONTINENTAL-SLOPE OF THE CELTIC SEA

Microbial activity was determined in the upper 10 cm of sediments and in bottom water of the European continental shelf margin in the Celtic Sea (Goban Spur: 48 degrees to 50 degrees N, 10 degrees to 14 degrees W; 135 to 1680 m deep). Assays used were the measurement of bacterial abundance (determined by epifluorescence microscopy), bacterial produ ction (by thymidine incorporation) and hydrolytic enzymatic activity ( by cleavage of fluorogenic model substrates). The highest activity was always measured in the top horizon (upper 1 cm layer) of the sediment , while in the water overlying the sediment about 1 to 5% of the sedim ent's surface value was found. The highest hydrolytic activity was mos tly achieved by protease followed by esterase, chitinase, beta-glucosi dase, and alpha-glucosidase. Most tested activities in the sediment de clined with increasing depth down the continental slope, but the steep ness of decline was different for the various substrates. While protea se and chitinase showed nearly no depth relation, there was a strong r esponse recorded for the 2 glycosidases. This leads to shifting ratios between the activities, indicating that not only microbial growth and general decomposition activity but also the type of reactions involve d in the decomposition processes change with depth. Our observations m ay reflect differences in the microbial population structure as well a s availability and biodegradability of specific compounds at the sea f loor, as a result of successive losses of easily degradable substances from particulate organic matter on its way from the epipelagic zone t o the sediment.