MICROBIAL DIVERSITY AND ACTIVITY IN A DANISH FJORD WITH ANOXIC DEEP-WATER

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 hy drographical and biological features of the system are described. Base d on chemical gradient profiles and measurements of process rates, we found that the relative importance of sulphate reduction, denitrificat ion and methanogenesis in terms of anaerobic terminal mineralisation w as about 5:1:0.4. It is possible, however, that methanogenesis is unde restimated because an unknown fraction of the methane production escap ed by ebullition. It was estimated that 10-15% of the net primary prod uction is mineralised anaerobically. The mean residence time of methan e, sulphide and ammonia beneath the chemocline is within the range 1.6 -2.3 yrs. Chemolithotrophic production in the chemocline (sulphide oxi dation and nitrification) accounted for about 3% of the net primary pr oduction of the system. Methane was oxidised (anaerobically or aerobic ally) throughout a large part of the water column, but most escaped to the atmosphere. The fjord has an impoverished zooplankton. This may b e due to the sulphidic deep water which will be lethal to sedimenting eggs. The protozoan biota were studied quantitatively and qualitativel y. Eighty-one species of protozoa were identified in the water column; of these, 37 were ciliates and the remainder were flagellates or rhiz opods. Only one new species of ciliate was found. All flagellates from the aerobic zone could be assigned a generic name. In contrast, the m icroaerobic and especially the anaerobic parts of the water column rev ealed about ten undescribed flagellates (three of which are formally d escribed), suggesting that the smaller protists of anaerobic habitats are still poorly known. Three relatively distinct protistan assemblage s could be identified, being associated with the oxic water column, th e microaerobic zone around the chemocline, and the anoxic zone, respec tively.