DISSIMILATORY SULFATE REDUCTION AND METHANE PRODUCTION IN GOTLAND-DEEP SEDIMENTS (BALTIC SEA) DURING A TRANSITION PERIOD FROM OXIC TO ANOXIC BOTTOM WATER (1993-1996)

During a transition period from oxic to anoxic conditions in the botto m water, rates of sulfate reduction and methane production, methane fl uxes, as well as concentration profiles of sulfate, sulfide and methan e were measured in sediments at a central site of the Gotland Deep (St n AL 93, 241 m depth), which is regarded as representative for the dee pest part of this basin. During this period from 1993 to 1996 oxic con ditions in the bottom water prevailed from spring 1994 until summer 19 95 with oxygen concentrations decreasing progressively with time. In t he sediments methane production occurred primarily in layers below 1 m depth and flux rates of methane to the sediment surface were characte rized by a steep concentration gradient from approx. 5 mM at 4 m depth to values close to 30 mu M at the surface, determined by diffusion pr ocesses and anaerobic oxidation of methane. Both processes were indepe ndent of changes at the sediment surface. Differences in the flux rate s of methane between the deeper part with a mean value of 259 mu mol m (-2) d(-1) and the upper layers with a mean of 47.7 mu mol m(-2) d(-1) indicate that a considerable proportion of the methane is oxidized wi thin the anoxic horizon of the sediment (71 to 86% in the layer from 4 0 to 70 cm). Low rates of methane production found within the top 20 c m of the sediment during periods of oxic bottom water increased after depletion of oxygen and resulted in a clear maximum of the methane con centration in the top 2 cm. Sulfate concentrations declined exponentia lly from values of 11.5 mM in June 1994 and 8.5 mM in October 1995 at the sediment surface to values of 2.5 mM at 20 cm depth and of less th an 0.5 mM at 50 to 60 cm depth. High sulfate reduction rates (150 to 2 50 nmol cm(-3) d(-1)) in the upper part of the sediment (8 to 13 cm) c oincided with maxima of sulfide concentrations. During the time period of this investigation an increase of maximum sulfide concentrations i n the sediment from 1 to 10 mM was measured together with decreasing o xygen concentrations in the deep water. At the same time sulfate reduc tion established a small but distinct maximum at the top layer of the sediment (0 to 2 cm). The relative importance of sulfate reduction and methanogenesis in the carbon budget of the Gotland Deep sediments is calculated on the basis of the actual measurements.