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.KEY WORDS: Gas hydrate · Community structure · Biomass · Diversity · Sulfide · Chemoautotrophy · Cold seep Resale or republication not permitted without written consent of the publisher

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We thank the officers, crew, and shipboard scientific party for excellent support during RV SONNE cruises SO143 and SO148. Tina Treude helped with sampling on board, Julia Polansky is acknowledged for assistance with FISH, Sabine Schäfer for total cell counts, Tina Lösekann and Heiko Löbner for sulfate reduction rates, Doris Setzkorn for thymidine incorporation, and Dirk Rickert for sulfate and porosity data. We thank Beth Orcutt and the two reviewers for their extremely helpful comments on the manuscript. This study was part of the programs MUMM (Mikrobielle UMsatzraten von Methan in gashydrathaltigen Sedimenten, 03G0554A) and TECFLUX I and II (TECtonically induced FLUXes, 03G0148A) supported by the Bundesministerium für Bildung und Forschung (BMBF, Germany). Further support was provided from the Max-Planck Society, Germany. This is publication GEOTECH-17 of the program GEOTECHNOLOGIEN of the BMBF and the Deutsche Forschungsgemeinschaft (Germany).

Linke, P. and Lutze, G.F., 1993. Microhabitat preferences of benthic foraminifera--a static concept or a dynamic adaptation to optimize food acquisition? In: M.R. Langer (Editor), Foraminiferal Microhabitats. Mar. Micropaleontol., 20: 215-234.In situ observations of microhabitat preferences of living benthic foraminifera are presented from sediments of the Norwegian-Greenland Sea, the upwelling area off northwestern Africa and the shallow-water Kiel Bight (Baltic Sea).Certain foraminiferai species (e.g. Cibicidoides wuellerstorfi and Rupertina stabilis) can be regarded as strictly epibenthic species, colonizing elevated habitats that are strongly affected by bottom water hydrodynamics. Large epibenthic foraminifera (e.g. Rhabdammina abyssorum and Hyperammina crassatina) colonize the sediment surface in areas where strong bottom currents occur and might have by virtue of their own size an impact on the small-scale circulation patterns of the bottom water. Motile species changing from epifaunal to infaunal habitats (e.g. Pyrgo rotalaria, Melonis barleeanum, Elphidium excavatum clavatum, Elphidium incertum, Ammotium cassis and Sphaeroidina bulloides ) are regarded here as highly adaptable to changes in food availability and/or changing environmental conditions. This flexible behaviour is regarded as a dynamic adaptation to optimize food acquisition, rather than a static concept leading to habitat classification of these ubiquitous rhizopods.

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