Benthic community metabolism and microbial dynamics in the Gulf of Trieste (Northern Adriatic Sea)

Abstract

Benthic O2-fluxes of sublittoral sediments were measured in situ using continuous recording by polarographic O2-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 an inconsistent pattern. Porewater dissolved organic carbon (DOC) exhibited strong seasonal variation with highest concentrations in September (20 mg C 1-1); subsequently porewater DOC declined to ca 5 mg C 1-1 (March-April). In general, porewater DOC increased slightly with depth down to the 10-15 cm horizon. Benthic respiration was found to be temperature dependent; below 10.degree.C about 6 mg C m-2 h-1 were respired while at 20.degree.C h-1 were respired while at 20.degree.C the rate was 17 mg C m-2 h-1. 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-2 (24 h)-1 caused by increased heterotrophic activity as indicated by high night-time respiration rates. Calculated O2-consumption of the sediment and the water column below the thermocline indicates that subthermocline water column respiration was the prinicpal cause for near-bottom hypoxia rather than sediment oxygen demand if the thermocline was at least 2 m above bottom. At a thermocline depth 1.5 m above bottom, sediment O2-uptake and subthermocline water column respiration were equally important.