Influence of water column dynamics on sulfide oxidation and other major biogeochemical processes in the chemocline of Mariager Fjord (Denmark)

Major electron donors (H2S, NH4+, Mn2+, Fe2+) and accepters (O-2, NO3-, Mn( IV), Fe(III)), process rates ((SO42-)-S-35 reduction, dark (CO2)-C-14 fixat ion) and vertical fluxes were investigated to quantify the dominant biogeoc hemical processes at the chemocline of a shallow brackish fjord. Under stea dy-stare conditions, the upward fluxes of reductants and downward fluxes of oxidants in the water column were balanced. However, changes in the hydrog raphical conditions caused a transient nonsteady-state at the chemocline an d had a great impact on process rates and the distribution of chemical spec ies. Maxima of S-0 (17.8 mu mol l(-1)), thiosulfate (5.2 mu mol l(-1)) and sulfite (1.1 mu mol l(-1)) occurred at the chemocline, but were hardly dete ctable in the sulfidic deep water. The distribution of S-0 suggested that t he high concentration of S-0 was (a) more likely due to a low turnover than a high formation rare and (b) was only transient, caused by chemocline per turbations. Kinetic calculations of chemical sulfide oxidation based on act ual conditions in the chemocline revealed that under steady-state condition s with a narrow chemocline and low reactant concentrations, biological sulf ide oxidation may account for more than 88% of the total sulfide oxidation. Under nonsteady-state conditions, where oxic and sulfidic water masses wer e recently mixed, resulting in an expanded chemocline, the proportion of ch emical sulfide oxidation increased. The sulfide oxidation rate determined b y incubation experiments was 0.216 mu mol l(-1) min(-1), one of the highest reported for stratified basins and about 15 times faster than the initial rate for chemical oxidation. The conclusion of primarily biological sulfide oxidation was consistent with the observation of high rates of dark (CO2)- C-14 fixation (10.4 mmol m(-2) day(-1)) in the lower part of the chemocline . However, rates of dark (CO2)-C-14 fixation were too high to be explained only by lithoautotrophic processes. CO2 fixation by growing populations of heterotrophic microorganisms may have additionally contributed to the obser ved rates. (C) 2001 Elsevier Science B.V. All rights reserved.