DECOMPOSITION OF ORGANIC-MATTER IN AGGREGATED SESTON FROM THE ELBE ESTUARY - REDOX DEPENDENCY AND PRODUCTION OF LOW-MOLECULAR-WEIGHT DOC COMPOUNDS

Early microbial degradation processes and accumulation of carbonyl com pounds and carboxylic acids in seston aggregates under oxic and suboxi c environmental conditions were studied. In laboratory experiments, ae robic decomposition of seston material increased after aggregation and maximum rates were reached after about 70 h. Seston material that bec ame refractory to aerobic degradation was mineralized anaerobically wi th nitrate as the terminal electron acceptor at a rate amounting to ab out 50% of the maximum rate during the previous aerobic incubation. Du ring both early aerobic and anaerobic degradation of seston, a decreas e in particulate organic carbon (POC) and the C:N ratio was observed. A net production of glyoxylate and glycolaldehyde occurred under anaer obic heterotrophic conditions and concentrations up to 4 and 30 mu g l (-1), respectively, were detected in the field when dissolved oxygen c oncentrations were below 3 mg l(-1). Glycolaldehyde was identified as an indicator of anaerobic degradation processes which occurred in the open water of the Elbe estuary for suspended seston aggregates less th an or equal to 2 mm in diameter. Results indicate a strong coupling be tween aerobic and anaerobic processes in the aggregates whereby anaero bic net production of DOC could significantly enhance O-2 consumption.