Deepwater reducing systems off Chile: biology and trophic interactions at three environmentally contrasting conditions
Keywords: methane seeps, hydrothermal vents, bathyal ecology
Characteristic benthic chemosynthetic communities develop at sites where emissions of chemically reduced fluids (e.g., methane, sulfide) occur. At these systems, i.e. methane, hydrocarbon and brine seeps, dead whalefalls, wood falls, hypoxic sulfidic settings and hydrothermal vents, carbon is fixed chemosynthetically by prokaryotes. Some of the involved bacteria live in symbiosis with the endemic fauna (e.g. clams and tubeworms), and some others, including also archaea, are free-living. An abundant assemblage of heterotrophic species is attracted to these deep sea oases of biological activity. The southern Pacific Ocean remains one of the least explored regions of the world regarding these habitats. Given the tectonic and oceanographic settings along the Chilean margin, the occurrence of seep sites are expected to be widespread, and the occurrence of hydrothermal vents is also expected at a particular site, the Chile Triple Junction (CTJ). The CTJ is the only area in the world where an active spreading ridge is being subducted underneath a margin rich in methane hydrates, with the potential presence of hybrid systems (e.g. hot seeps). The only chemosynthetic system that has received significant attention is the Concepción Methane Seep Area (CMSA), at ~850 m water depth NW off Concepción (~36°S). Recent investigation of this si te, funded mostly by FONDECYT project #1061217, resulted in the discovery of many new megafaunal species, adding substantial knowledge to the ecology and functioning of deep-sea ecosystems off Chile. Most recently, during explorations of the upper slope of central Chile, a new seep site was discovered at relatively shallow water depths (~350 m) off El Quisco (~33°S; El Quisco Seep Site, EQSS hereafter). This new site constitutes a type of habitat that has not been often studied worldwide, i.e. a methane seep within the influence of an Oxygen Minimum Zone (OMZ). These three areas will be visited and sampled for seep and vent fauna with trawl and video-guided multicorer during March 2010 on a collaborative cruise aboard R/V Roger Revelle. Additional sampling at each place includes CTD casts and deployments of “state-of-the-art” mapping and visualization gear, including the autonomous underwater vehicle SENTRY. This cruise will thus offer the unique opportunity of studying the functioning of a suite of chemosynthetic habitats experiencing different environmental characteristics (e.g. affected by bottom water oxygen deficiency, different latitudes and depths, different driving forces of the chemically enriched fluid fluxes). We still do not know whether the less explored environments (i.e. EQSS and the CTJ) are similar to the CMSA fauna, but we predict that, given the differences in environmental settings, distinct assemblages will be found. Preliminary results for the CMSA indicate that it constitutes a hotspot of faunal aggregation, including much prized commercial species like the Patagonian Tootfish (Dissostichus eleginoides). At EQSS high megafaunal abundance and diversity has been observed, including commercially valuable resources (e.g., the shrimp Heterocarpus reedi and the yellow squat lobster Cervimunida jhoni). The CTJ fauna and habitats basically still remain a mystery to the worldwide scientific community. This cruise is a collaborative initiative between national and international institutions, in which ship time is provided by the Scripps Institution of Oceanography (SIO). The use of the mapping, visualization and sampling gear will be co-funded by the US National Science Foundation (NSF) and the National Ocean and Atmosphere Administration (NOAA), and general support is also provided by the Census of Marine Life program. The present project seeks funds only for the post-cruise analysis in Chile of sediments and biological samples that will be gathered. We thus aim to: i) characterize the benthic community structure (meio-, macro- and megafauna) of the three sites, ii) gather stable isotope (C and N) data, including smaller size fractions (meio- and macrofauna) in order to trace the fate of chemosynthetic C in the local food webs by using stable isotope analysis, and iii) characterize the different physicochemical environments where chemosynthetic communities thrive. This research project will contribute substantially to generate a first scheme of the structure of seep communities from the upper slope (EQSS), mid-slope (Concepción) to lower slope/trench (CTJ) of the SE Pacific Ocean, and under different geologic settings. Seep organisms are often indicators of the presence of extensive methane deposits (both in the form of hydrates at the deeper sites, or free gas pockets at the shallower ones). Moreover, seep-sites often constitute hot spots for non-seep benthic megafauna. Assessing the distribution, structure and biogeography of these communities is of critical importance for a developing country in a future scenario of increasing demand of energy and food, with a consequent increased pressure over marine ecosystems and their associated geological and biological resources.
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