Sediments hosting gas hydrate: oases for metazoan meiofauna

Abstract

The effect of methane seepage from sediments harbouring shallow gas hydrates on standing stocks and the distribution pattern of meiobenthic organisms, in particular Nematoda and Rotifera, was studied at about 800 m water depth at Hydrate Ridge, Cascadia subduction zone, off Oregon. The presence of shallow gas hydrates, buried only a few 10s of centimetres below the sediment surface, was indicated by extensive bacterial mats of chemosynthetic Beggiatoa sp. and clam fields of the bivalve mollusk Calyptogena spp. Mean abundances of meiobenthic organisms integrated over the upper 10 cm of the sediment were highest (1294 ind. 10 cm -2 ) at clam fields, closely followed by control sediments least affected by gas hydrates (1199 ind. 10 cm -2 ) and lowest in sediments covered with bacterial mats (762 ind. 10 cm ). The dominant taxa of meiobenthic organisms at the investigated sites were nematodes and, unexpectedly, Rotifera that are almost unknown from the deep marine habitat. In terms of abundance, rotifera dominated the meiobenthic community in gas-hydrate-influenced sediments, while control sediments and deeper basins adjoined to Hydrate Ridge were dominated by nematodes. Nematodes were concentrated in the sediment surface at all sites, whereas rotifers were almost evenly distributed at all depths, with a slight preference for deeper sediment horizons. The horizontal as well as vertical distribution of nematodes and rotifers is likely to be determined by competition or predation, and by the high adaptive capability of rotifers to highly sulphidic and anoxic conditions. Estimates of meiobenthic carbon turnover in relation to the bulk organic carbon supply indicate that, in contrast to other meiobenthic communities in cold seep environments, the meiobenthos in the studied gas-hydrate-containing sediments do not benefit from the excess availability of organic carbon via the chemoautotrophic food web. This may be because, for most meiobenthic organisms (other than rotifers), tolerance mechanisms are overwhelmed by the deleterious environmental conditions of reduced oxygen availability and extremely high sulphide fluxes.KEY WORDS: Meiofauna · Nematoda · Rotifera · Gas hydrate · Carbon turnover · Hydrate Ridge · Cascadia subduction zone Resale or republication not permitted without written consent of the publisher

Characteristics of meiofauna in extreme marine ecosystems: a review

“…An exceptional abundance of rotifers was found in anoxic and highly sulphidic sediments associated with shallow gas hydrates at the southern crest of Hydrate Ridge off Oregon (USA), NE Pacific (Sommer et al 2007). Yet, the occurrence of high abundances of rotifers in several marine habitats has been questioned in light of the likely contamination from freshwater (even distilled water) used during the extraction of meiofauna (Funch et al 1996;Guilini et al 2012).…”

“…Specifically, they were reported at deep-water hydrocarbon seeps in microbial mats of Beggiatoa, in fields of the clam Calyptogena and associated with tubeworm and mussel aggregations (Robinson et al 2004;Sommer et al 2007;Bright et al 2010). as well as at shallow-water methane seeps (Revkov and Sergeeva 2004) and deep and shallow mud volcanoes (Olu et al 1997;Zeppilli et al 2011;Lampadariou et al 2013).…”

Hydrate Zone Ecology

“…Chemosynthetic communities locate at water-sediment interfaces of these carbon-rich sites and enter into the methane-sulfate-sulfide-carbon dioxide balance (Boetius and Suess, 2004;Wegener et al, 2008;Sommer et al, 2007). Specific community species oxidize sulfides, where the sulfides come primarily from AOM, and this conversion helps replenish sulfates extracted by AOM from seawater in the near-surface sediments.…”

“…In this manner Beggiatoa convey nitrates 40 times faster than diffusion, storing the nitrates in large vacuoles to later access for sulfide oxidation and for metabolism energy (Boetius and Suess, 2004;Fossing et al, 1995). Fossing et al (1995) suggest the stoichiometry in Equations 9.10 and 9.11 as Beggiatoa utilize nitrates to oxidize sulfides: Trace concentrations of oxygen in bottom waters directly above mats penetrate only a few millimeters into the mats before being completely extracted by the Beggiatoa, leaving sediments directly below absent of oxygen and containing 18,000 mmol/l sulfides (Sommer et al, 2007;Boetius and Suess, 2004). Equally efficient is the oxidation of sulfides in the oxygendepleted sediments below (Fossing et al, 1995).…”

“…In the GOM, heavier hydrocarbons supplied by seeps may supplement the methane (Boetius et al, 2000;Van Dover et al, 2003;Sommer et al, 2007;Zhang et al, 2005). In the GOM, heavier hydrocarbons supplied by seeps may supplement the methane (Boetius et al, 2000;Van Dover et al, 2003;Sommer et al, 2007;Zhang et al, 2005).…”