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.

Description: The effect of methane released from decomposing surficial gas hydrates (SGH) on standing stocks and activities of the small-sized benthic biota (SSBB; i.e. bacteria, fungi, protozoa, and meiobenthic organisms) was studied at about 790 m water depth, at the Hydrate Ridge, Cascadia subduction zone. Presence of SGH and elevated sulfide concentrations in the sediment were indicated by extensive bacterial mats of Beggiatoa sp. and clam fields of the bivalve mollusc Calyptogena sp. Vertical and horizontal distribution patterns of the SSBB biomass were derived from DNA and total adenylate (TA) sediment assays. Potential bacterial exoenzymatic hydrolytic activity was measured using fluorescein-di-acetate (FDA) as substrate. Estimates of chemoautotrophic production of particulate organic carbon (POC.) were determined by 14CO2 uptake incubations. Inventories of chl a and pheopigments were determined as parameters of surface water primary produced POC input. Average SSBB biomass in clam field sediments integrated over the upper 10 cm (765.2 gC m-2, SD 190.1) was 3.6 times higher than in the adjacent control sites (213 gC m-2, SD 125). Average SSBB biomass in bacterial mat sediments, which were almost devoid of eukaryotic organisms 〉 31 µm, was 209 gC m-2 (SD 65). Significant correlations between FDA, DNA and plant pigments imply that productivity of the SSBB at SGH sites is only partially uncoupled from the primary production of the surface water. Areal estimates of autotrophic Corg production at control sites, bacterial mat sites and in clam field sites were 5.7, 59.7 and 190.0 mgC m-2 d-1, respectively. Based on different models predicting vertical POC fluxes from surface water primary production and water depth, these autotrophic POC productions account for 5 to 17% (controls), 35 to 68% (bacterial mats), and 63 to 87% (clam fields) of the bulk POC (sum of allochthonous POC input through the water column and sedimentary autochthonous autotrophic POC production) provided at the various sites. At SGH sites inventories of chl a and pheopigments, integrated over the upper 10 cm of the sediment, were half of that found at the control sites. This might be due to enhanced degradation of phytodetritally associated organic matter. The resulting low molecular weight organic carbon compounds might stimulate and fuel sulfate reduction, which is conducted in a microbial consortium with anaerobic methane consuming archaea. This syntrophic consortium might represent a prominent interface between gas hydrate derived carbon and allochthonous Corg flow. We infer that degradation kinetics of SGH is affected by, e.g., seasonally varying input of allochthonous organic matter.