Benthic 02-fluxes of sublittoral sediments were measured in situ using continuous recording by polarographlc 02-sensors. Three stations (7, 15, and 22 m deep) were investigated at ca 2 mo intervals in the Northern Adriatic Sea over a 19 mo period High chl a contents of the sediments were found in spring (March. April) and fall (September, October) while sediment bacterial numbers fluctuated in a n inconsistent pattern. Porewater dissolved organic carbon (DOC) exhibited strong seasonal variation

more »

... with highest concentrations in September (20 mg C I-'); subsequently porewater D O C declined to ca 5 m g C I-' (March-April). In general, porewater DOC increased slightly with depth down to the 10-15 cm horizon. Benthic respiration was found to b e temperature dependent; below 10°C about 6 m g C m-' h-' were respired while at 20°C the rate was l? mg C m-' h-' Microphytobenthos gross primary production (GPP) was detected even at the deepest station and remained fairly constant from March to September (ca 100 mg C m-2 d-'1. Total community metabolism calculated over 24 h revealed net community production in spring for the shallowest station only. Highest net system consumption rates were obtained in September for all 3 stations ranging from 220 to 520 mg C m-' (24 h)-' caused by increased heterotrophic activity a s indicated by high night-time respiration rates. Calculated 02-consumption of the sediment and the water column below the thermocline indicates that subthermocline water column resp~ration was the principal cause for near-bottom hypoxia rather than sediment oxygen demand if the thermocline was at least 2 m above bottom. At a thermocline depth of 1.5 m above bottom, sediment 02-uptake and subthermocline water column respiration were equally important. ' Present address: Scripps Institution of Oceanography, Institute of Marine Resources, A-018,