CHANGES IN OXYGEN-CONSUMPTION AT THE SEDIMENT-WATER INTERFACE FORMED BY SETTLING SESTON FROM THE ELBE ESTUARY

A new laboratory device was used to study microbial processes within a similar to 1-mm layer of aggregated seston material during different seasons. The conditions below and above this sediment-water interface were simulated, and the concentrations of plant nutrients and oxygen w ere controlled. Nitrification occurred in the layer only at oxygen con centrations > 1.5-2 mg liter(-1). Heterotrophic processes followed zer o-order kinetics with respect to oxygen; thus, anoxic conditions were reached within the seston layer at depths of 800-1,400 mu m. Oxygen co nsumption rates in the heterotrophic layer differed as much as 10-fold , and values were highest at the oxic-suboxic interface in the layer d ue to production of dissolved organic C by fermentation. Nitrification accounted for 100% of the total O-2 consumption in the layer during t he first 20 h. Thereafter, an increase in the oxygen flux into the lay er by factors as high as 2-9 was due to initiation of heterotrophic pr ocesses. About 120 h after settling, the aggregated seston material be came refractory to aerobic microbial degradation. The observed short-t erm succession of microbial autotrophic and heterotrophic O-2 consumpt ion in aggregated seston material might control environmental conditio ns both at the sediment-water interface and in flocs in the open water .