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 ∼ 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 were controlled. Nitrification occurred in the layer only at oxygen concentrations > 1.5–2 mg liter −1 . Heterotrophic processes followed zero‐order kinetics with respect to oxygen; thus, anoxic conditions were reached within the seston layer at depths of 800–1,400 µ m. Oxygen consumption rates in the heterotrophic layer differed as much as 10‐fold, and values were highest at the oxic‐suboxic interface in the layer due to production of dissolved organic C by fermentation. Nitrification accounted for 100% of the total O 2 consumption in the layer during the first 20 h. Thereafter, an increase in the oxygen flux into the layer by factors as high as 2–9 was due to initiation of heterotrophic processes. About 120 h after settling, the aggregated seston material became refractory to aerobic microbial degradation. The observed short‐term succession of microbial autotrophic and heterotrophic O 2 consumption in aggregated seston material might control environmental conditions both at the sediment‐water interface and in flocs in the open water.