Ecological implications of surficial marine gas hydrates for the associated small-sized benthic biota at the Hydrate Ridge (Cascadia Convergent Margin, NE Pacific)

Abstract

The effect of methane released from decomposing surficial gas hydrates (SGH) on stand- ing 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 sub- duction 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. Verti- cal and horizontal distribution patterns of the SSBB biomass were derived from DNA and total adeny- late (TA) sediment assays. Potential bacterial exoenzymatic hydrolytic activity was measured using fluorescein-di-acetate (FDA) as substrate. Estimates of chemoautotrophic production of particulate or- ganic carbon (POC.) were determined by 14 CO2 uptake incubations. Inventories of chl a and pheopig- ments were determined as parameters of surface water primary produced POC input. Average SSBB bio- mass 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). Sig- nificant 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) pro- vided 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 degra- dation 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.