Interactions between local oxygen deficiencies and heterotrophic microbial processes in the elbe estuary

Abstract

Between 1991 and 1994 an interdisciplinary research group studied the factors which control the formation and position of local oxygen depletion areas within the longitudinal freshwater profile of the Elbe Estuary. The investigations were based on the assumption that microbial oxygen consumption was important and that the quantities of the organic and inorganic substances which are metabolized by heterotrophic processes differ significantly along a longitudinal profile. To test this assumption, during different times of the year longitudinal concentration profiles were obtained for parameters characterizing the physico-chemical conditions, and the abundances of bacteria and phytoplankton were counted. Results indicated that the oxygen regime in the warmer seasons was controlled by microbial O 2 consumption, coupled to the degradation of freshly transported organic substances from upstream which mainly consisted of phytoplankton. Laboratory studies on the types and rates of heterotrophic processes in seston revealed that after formation of aggregates the oxygen consumption increased up to 1000 fold. Two independent processes could be identified to control this increase within the seston aggregates: the decay of fresh phytoplankton and the formation of low molecular weight DOC under low oxygen concentrations. Combining the results from both laboratory and field studies, we conlude that a decrease in the oxygen concentration in the Elbe Estuary caused an increase in heterotrophic processes in the seston material which again produced a further decrease of the oxygen concentration in the open water. This autocatalytic effect might also be important in cycling of organic matter and nutrients from particulate suspended matter in other aquatic systems.