Dynamics of Cu release during early aerobic degradation in aggregated seston from the Elbe estuary

Using a new laboratory device, the mobilization of Cu, Cd, and Pb was monitored continuously by analysing the water overlying and underlying a ≈ 1 mm seston layer using voltammetric methods. The mineralization of organic matter was quantified by the heterotrophic oxygen consumption. The CO2 formed was analysed for its 13C-content to characterize the organic C-pool used in degradation. Changes in the biochemical composition of the particulate material were determined at the beginning and end of an aerobic incubation period. Heterotrophic processes were initiated about 20 h after settling of the seston. The aggregated seston became refractory for aerobic microbial degradation after about 120 h of aerobic incubation. At this stage nitrification again balanced the total oxygen consumption. The mineralization of the organic matter was between 7.7 and 17.4% of the total and 62–75% of this consisted of dying algal biomass. A significant release of Cu from the seston layer was restricted to a period of about 40 h and was correlated to the period when a limitation of heterotrophic processes by organic substrates occurred. Then, Cu concentrations increased to between 8 and 28 nM in the water overlying the layer that equalled a mobilization rate of between 0.24 and 0.94 (nmol/cm2 d). Cd and Pb did not show a similar pattern and were detected at concentrations of always below 2 nM. Deeper parts of the layer functioned as a sink for Cu at oxygen concentrations of ≤ 25 μM and concentrations of Cu in the water underlying the seston layer remained below 1.5 nM. During periods of short-term release Cu in solution accounted for between 3 and 10% of the total particulate Cu. The results suggest that a significant amount of the Cu within the seston material was retained in the protein fraction of the algal matter and is released into the overlying water during its aerobic degradation.