Friday 9 March 2012

Hydrothermaly heated active seeps from the continental shelf margin, south of Costa Rica.

Hydrothermal vents are areas in (usually the deep) ocean, where water percolates through hot, volcanic rocks, before escaping back into the water column, super-heated and laden with minerals. These have been discovered to host ecosystems based upon bacteria that gain their energy from chemicals within the seep water, the only ecosystems on Earth divorced from the light of the sun. Cold, or methane, seeps are areas where oil and/or natural gas (methane) bearing sediments are exposed, allowing methane, hydrocarbons and sulphides to enter the water column. Like the hydrothermal vents these seeps support biological communities based upon bacteria which feed on chemicals in the water - though since these are feeding on fossil fuels they are ultimately utilizing energy from the sun. When first discovered these two ecosystems were thought to be completely different, but more recent studies have shown a considerable overlap between the organisms living at the two types location.

In a paper published in the Proceedings of the Royal Society Series B (Biological Sciences) of 7 March 2012, a team of scientists led by Lisa Levin of the Center for Marine Biodiversity and Conservation at Scripps Institution of Oceanography, announce the discovery of a volcanically active methane seep, named the Jaco Scar, with a diverse biological community showing similarities to both hydrothermal vent and cold seep communities. The area hosts a large number of methane seeps, but this is the first hydrothermal vent recorded.

The location of the Jaco Scar.

The site was visited by the submersible Alvin, which found a dense growth of Lamellibrachia barhami tubeworms growing on a hydrothermal vent at a depth of 1790 m. The water around this vent was 3° C higher than the surrounding water, with low salinity and a raised methane content. The water was also enriched in Helium. A similar vent system was located 118 m to the west.

A 1.9 m Lamellibrachia barhami tubeworm 'bush' on the Jaco Scar vent. Levin et al. (2012), supplementary material.

The site also hosted colonies of a second type of tubeworm, Escarpia spicata, colonies of Vesicomyid clams (Archivesica gigas) clustered around the bases of the tubeworm colonies, and large numbers of Zoarcid fishes (Pachycara sp.). Large numbers of gastropods and galatheid crabs (Munidopsis sp.) were also present, a fields of pogonophorid worms occurred between the vents.

Zoarcid fish Pachycara sp. on a Lamellibrachia barhami bush. Levin et al. (2012).

Lamellibrachia barhami tubeworm colonies are common on the cold seeps of the Costa Rican coastal shelf. Archivesica gigas clams are common in the Guaymas Basin, an area of hydrothermal vents in the Gulf of California, where Lamellibrachia barhami tubeworm colonies are also common. Zoarcid fishes are the dominant form of fish at all hydrothermal vents, but are also known from cold seeps off the coast of Florida, and in the Mediterranean. This gives the vent community a mosaic community, made up of both cold seep and hydrothermal vent animals.

A bed of Vesicomyid clams (Archivesica gigas) at the Jacos Scar site. Levin et al. (2012).