Microbial activities in a permanently stratified estuary. I. Primary production and sulfate reduction

Abstract

Physical, chemical and biological parameters have been measured in the water column of Saelenvann, a shallow estuary in western Norway, over a period of nearly 4 years. Due to the presence of a permanent pycnocline, a layer of anoxic water with high sulfide and ammonia concentrations persists under a layer of oxic water with normal photosynthetic and heterotrophic microbial activities. The magnitude of the sulfate reduction in the estuary was correlated with the magnitude of primary productivity. Increases in sulfate reduction following increased primary productivity occurred first in the free anoxic water, and only after an appreciable time lag in the upper sediment layers. The highest rates of sulfate reduction were found just below the interface with the oxic water and in the upper few centimeters of the sediment. It is estimated that twice as much sulfate is reduced in the free water mass as in the sediment. During the summer, the amount of organic material produced by primary productivity exceeds the amount of organic material mineralized by sulfate reduction, the inverse being the case during the winter.