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A laboratory device was used to simulate the environmental conditions in a seston layer of about 1 mm thickness, similar to those in aggregates of the open water of the Elbe estuary, Germany. During 98 h of oxic, suboxic and anoxic incubation, microbial degradation was investigated by measuring both changes in the chemlcal compos~tlon of the particulate organic carbon (POC) and heterotrophic microbial activities. Mineralization of the POC at 15°C occurred at mean rates of 16 under oxic, 26 under suboxic, and 0.24 pm01 cm-3 d-' under anoxic conditions. Different respiration processes accounted for ratios of carbon degradation equal to 1 7 for oxygen:nitrate, 430 for nitrate:manganese, and 3.5 for Mn(IV):Fe(lII) Dunng o x ~ c and suboxic incubation, particulate combined amino acids (PCAA) were preferred as an organic substrate, and PCAA:C ratios decreased from about 0.12 to 0.02. A total of 85% of the PCAA wds degraded within 56 h including PCAA and dissolution of silica from the cell wall material of diatoms; the remaining amlno acids were assumed to be those from active bacteria. Chlorophyll a (chl a ) was more resistant to degradation than total POC below a threshold value of 60 p M 02, which resulted in an increase of chl a.C ratlos up to 37. A decrease in the availability of particulate substrates for heterotrophic decomposition was expressed as an accumulation of glycine and serine and a depletion in aspartic and glutamatic a c ~ d in the PCAA. During anoxic incubation PCAA decreased by only 18% and remained unchanged in composition. Along the longitudinal profile of the freshwater section of the Elbe estuary, gradual decomposition of the seston material during summer downstream transport was followed by changes in composition similar to those observed in the oxic laboratory experiments. The results indicate that phytoplankton debris that is not available as a substrate for microorganisms in sediments and small suspended particles is rapidly degraded within seston aggregates.

Utilization of phytoplankton in seston aggregates from the Elbe estuary, Germany, during early degradation processes