ALBERT

Description: In Eckernförde Bay (western Baltic Sea) pockmark structures are induced by groundwater seeping out of the sediment. On 3 occasions in winter and spring 1993-94 we investigated the influence of groundwater on the reduction of salinity, on porewater chemistry, and on bacterial activities (methane oxidation and sulphate reduction). In 2 out of 3 sampling campaigns groundwater discharge could be detected. The concentration gradients of Cl- and SO4= are moved towards the sediment surface by the vertical advection of groundwater during seep times. Without groundwater discharge the porewater chemistry resembled the control site. Compared to the control site, the methane oxidation and sulphate reduction rates were elevated at the pockmark site, reaching maximum values of 49 and 269 µmol l-1 d-1 respectively. The groundwater venting from the pockmark had an end member composition of 80 mM Na+, 1.0 mM Ca++ and was depleted in Mg++. Due to mixing of these major cations along the groundwater/seawater interface, no CaCO3 precipitation was found around the pockmark site.

Description: Gas hydrates occur at the sediment surface on the southern summit of Hydrate Ridge, Cascadia convergent margin. The hydrates are found in mounds several meters in diameter and up to 2 m high, and are covered by sediment and mats of the filamentous sulfur-oxidizing bacteria Beggiatoa. The mounds are surrounded by vesicomyid clams (Calyptogena pacifica, C. kilmeri), which in turn are encircled by solemyid bivalves (Acharax sp.). The zonation pattern of 3 species (Calyptogena spp. and Acharax sp., which harbor chemoautotrophic bacteria in their gills, and the chemoautotrophic Beggiatoa), is also reflected in a change in the entire community structure. Beggiatoa, Calyptogena spp. and Acharax sp. are shown to be characteristic species for the different communities. The Beggiatoa community directly overlaying the gas hydrates consists of seep endemic species in high densities: gastropods (Provanna laevis, P. lomana, Pyropelta corymba, Hyalogyrina sp. nov.), bivalves (Nuculana sp. nov.) and polychaetes (Ampharetidae, Polynoidae, Dorvilleidae). Based on pooled samples, the rarefaction curves show a decrease in species diversity in the Beggiatoa and Calyptogena communities. The hydrogen sulfide gradients in the porewater of sediments below the different communities dominated by either Beggiatoa, Calyptogena spp. or Acharax sp. vary by 3 orders of magnitude. The diffusive sulfide flux based on the measured sulfide concentration gradients is highest in Beggiatoa sp. communities (23 ± 13 mol m-2 yr-1), slightly less in Calyptogena communities (6.6 ± 2.4 mol m-2 yr-1), and low in Acharax communities (0.05 ± 0.05 mol m-2 yr-1). The difference in the sulfide environment is a factor influencing the distribution patterns of the chemoautotrophy-dependant and heterotrophic species at the deep-sea sediments containing gas hydrate.