Microbial diversity and activity in a Danish Fjord with anoxic deep water

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Abstract
Microbial diversity and activity were studied in a stratified basin of Mariager Fjord, Denmark in August 1994. The basin is about 30 m deep and the lower half of the water column is anoxic and sulphidic. The hydrographical and biological features of the system are described. Based on chemical gradient profiles and measurements of process rates, we found that the relative importance of sulphate reduction, denitrification and methanogenesis in terms of anaerobic terminal mineralisation was about 5:1:0.4. It is possible, however, that methanogenesis is underestimated because an unknown fraction of the methane production escaped by ebullition. It was estimated that 10–15 % of the net primary production is mineralised anaerobically. The mean residence time of methane, sulphide and ammonia beneath the chemocline is within the range 1.6–2.3 yrs. Chemolithotrophic production in the chemocline (sulphide oxidation and nitrification) accounted for about 3% of the net primary production of the system. Methane was oxidised (anaerobically or aerobically) throughout a large part of the water column, but most escaped to the atmosphere. The fjord has an impoverished zooplankton. This may be due to the sulphidic deep water which will be lethal to sedimenting eggs. The protozoan biota were studied quantitatively and qualitatively. Eighty-one species of protozoa were identified in the water column; of these, 37 were ciliates and the remainder were flagellates or rhizopods. Only one new species of ciliate was found. All flagellates from the aerobic zone could be assigned a generic name. In contrast, the microaerobic and especially the anaerobic parts of the water column revealed about ten undescribed flagellates (three of which are formally described), suggesting that the smaller protists of anaerobic habitats are still poorly known. Three relatively distinct protistan assemblages could be identified, being associated with the oxic water column, the microaerobic zone around the chemocline, and the anoxic zone, respectively.