GOLDSCHMIDT CONFERENCETOULOUSE 1998 Biogeochemical processes in the stratified water column of Mariager Fjord (Denmark) J. Zopfi B. Thamdrup B. B. Jorgensen N. B. Ramsing A. Teske T. G. Ferdelman MPI for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany Dept. of Microbial Ecology, Aarhus University, DK-8000 Aarhus, Denmark Woods Hole Oceanographic Institution, Woods Hole, 02543 MA, USA MPI for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany Mariager Fjord is a narrow, shallow fjord in north- eastern Denmark connected to the Kattegat by a long (>20 kin) channel of less than 10 m depth that limits water exchange between the sea and the central basin (28 m depth). The water column in the central basin is stabilized by a density gradient and exhibit a distinct redox stratification, where the 02 downward flux balances the overall upward H2S flux (Ramsing et al., 1996; Fenchel et al., 1995). In order to better understand the redox reactions taking place in the chemocline and the microorgan- isms involved, high resolution profiles of different electron donors (Fe2+, Mn 2+, NH 4§ H2S), acceptors (Fe3+, Mn 4§ NO~, 02) and sulphur intermediates were determined and combined with direct rate measurements (35S-sulphate-reduction in the sedi- ment, 14C-bicarbonate fixation rates in the water column). Additionally, traditional microbiological methods, e.g. enrichments and MPN counts, as well as modem molecular methods were applied. Results The chemocline was located between 13.25 and 13.5 m, and varied slightly from day to day. The gradients of oxygen and sulphide were very steep and showed only little overlap (15-50 cm). Below the chemocline sulphide increased monotonically with depth. In contrast to an earlier study, no clear evidence was found for anaerobic sulphide oxidation as concluded from the HzS profiles (Ramsing et al., 1996). In addition to sulphide and sulphate, elemental sulphur, thiosulphate and sulphite were determined. Whereas sulphite was not detected, a peak of thiosulphate (5.7 gM) was found just below the zone of O2/H2S coexistence. Below 15 m thiosulphate concentrations decreased to a background level of about 100 nM. Elemental sulphur was found from 12.75 m to 14.25 m with a maximum concentration of 19 p.M in the chemocline. Below 14.25 m the elemental sulphur concentration was below the detection limit of 1 p.M. Peaks of particulate manganese (3.5 p.M) and particulate iron (3.3 p.M) were found above the chemocline, whereby the manganese maximum was located above the iron maximum. Below these maxima dissolved reduced manganese and reduced iron increased and reached concentrations of 25.6 gM and 1.2 laM respectively. Ammonia concentrations decreased from 240 p.M at the bottom to 37 p.M in the chemocline. In the oxic layer ammonia decreased further to 3 gM. Discussion Although anoxygenic phototrophs were enriched from the chemocline and below, they do not seem to contribute significantly to sulphide oxidation as was concluded from the shape of the sulphide profile. That no pigments of phototrophic sulphide oxidizers and no associated elemental sulphur was found at these depth support this conclusion. Dark incubation experiments with 14C-bicarbonate showed a maximum of carbon fixation ( 2.3 p.mol 1-1 h -1) just below the chemocline, indicating a significant population of chemolithoautotrophs. The responsible organisms for this peak in COa-assimilation remain to be identified. Besides thiosulphate (2 p.M) and very low levels of nitrate (0.4 p.M), apparently no electron acceptors are present below the chemocline. This region could therefore form a niche for anaerobic 1709