Microbial activities in a permanently stratified estuary. II. Microbial activities at the oxic-anoxic interface

Bacterial populations and activities at the oxic-anoxic interface of Saelenvann, a permanently stratified estuary in western Norway, have been studied. Concentrations of particulate material and direct counts of bacteria were higher at the interface than in the surrounding water. Dark assimilation of CO₂ was maximal at the interface, indicating high rates of chemoautotrophic productivity. Nitrification could possibly account for 10 to 50% of the measured dark CO₂ fixation. Nitrate is rapidly consumed by dentrifying organisms in the anoxic water and by photosynthetic organisms in the oxic water. A characteristic population of photosynthetic bacteria similar to Chlorobium phaeovibrioides flourishes at the interface, imparting a reddishbrown discoloration to the water.