Dissimilatory sulfate reduction and methane production in Gotland Deep sediments (Baltic Sea) during a transition period from oxic to anoxic bottom water (1993-1996)

Abstract

During a transition period from oxic to anoxic conditions in the bottom water, rates of sulfate reduction and methane production, methane fluxes, as well as concentration profiles of sulfate, sulfide and methane were measured in sediments at a central site of the Gotland Deep (Stn AL 93. 241 m depth), which is regarded as representative for the deepest part of this basin. During this period from 1993 to 1996 oxic conditions in the bottom water prevailed from spring 1994 until summer 1995 with oxygen concentrations decreasing progressively with time. In the sediments methane production occurred primarily in layers below 1 m depth and flux rates of methane to the sediment surface were characterized by a steep concentrat~on gradient from approx. 5 mM at 4 m depth to values close to 30 pM at the surface, determined by diffusion processes and anaerob~c oxidation of methane. Both processes were independent of changes at the sediment surface Differences in the flux rates of methane between the deeper part with a mean value of 259 pm01 m-' d-' and the upper layers with a mean of 47.7 pm01 m-2 d-' indicate that a considerable proportion of the methane is oxidized within the anoxic horizon of the sediment (71 to 86% in the layer from 40 to 70 cm). Low rates of methane production found within the top 20 cm of the sediment during periods of oxic bottom water increased after depletion of oxygen and resulted in a clear maximum of the methane concentration in the top 2 cm. Sulfate concentrations declined exponentially 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 than 0.5 mM at 50 to 60 cm depth. High sulfate reduction rates (150 to 250 nmol cm-3 d-') in the upper part of the sediment (8 to 13 cm) coincided with maxima of sulfide concentrations. During the time period of this investigation an increase of maximum sulfide concentrations in the sediment from 1 to 10 mM was measured together with decreasing oxygen concentrations in the deep water. At the same time sulfate reduction established a small but distinct maximum at the top layer of the sediment (0 to 2 cm). The relative ~mportance of sulfate reduction and methanogenesis in the carbon budget of the Gotland Deep sediments is calculated on the basis of the actual measurements.

Methanogenic microbial communities in sediment from the coastal area of puck bay (Southern Baltic)

“…That situation has been documented in the Baltic ecosystem (Edlund 2007). The presence of methane in the Baltic was detected in benthic sediment and the near-bottom water (Jørgensen et al 1990;Piker et al 1998;Wilkens and Richardson 1998;Martens et al 1999;Schlüter et al 2004;Mathys et al 2005). The emission of this greenhouse gas from the coastal water ecosystem in the Baltic Sea into the atmosphere has also been confirmed by earlier studies (Heyer and Berger 2000;Liikanen et al 2009).…”

Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

“…2). Highest methane concentrations were detected close to the seafloor (504 nM at 230 m water depth) supporting an origin from methanogenesis in the sediment (Piker et al, 1998). Indeed, low δ 13 C CH 4 values (−82.4 ‰ to −75.2 ‰, Fig.…”

“…Under anoxic conditions, the final step of decomposition of organic matter leads to the generation of methane within the sediment. In the Baltic Sea, pore-water as well as acoustic investigations demonstrated that methane is abundant in high concentrations within the sediment and that in some regions methane is also released as free or dissolved gas into the water column (Dando et al, 1994;Piker et al, 1998;Thießen et al, 2006). Extensive water column investigations in the Baltic Sea identified the strongest methane enrichment within the stagnant anoxic water bodies of the deep basins (Gotland Deep and Landsort Deep; max.…”

Methane flux from sediment into near-bottom water in the coastal area of the Puck Bay (Southern Baltic)

“…Methane escapes into the gaseous phase from the sampled water when solubility conditions change and the sample is subjected to mechanical degassing via shaking (Schmaljohann 1996, Piker et al 1998, Sommer et al 2006. The near-bottom water samples were collected from chambers into the chromatographic vials by using a syringe, and immediately closed with the rubber stoppers.…”

“…Moreover, the harbor sediments sampled in the area of Howaldtswerke (Kiel, Germany) contained methane (Schmaljohan 1996). Methane emission from sediment into near-bottom water was detected in the Gotland Deep, the coastal zone of the islands Rügen and Hiddensee, and in the estuaries located in the Bothnian Bay (Piker et al 1998, Heyer and Berger 2000, Liikanen et al 2009). …”