Cold Seeps
A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs. Cold seeps are distinct from hydrothermal vents: the former's emissions are of the same temperature as the surrounding seawater, whereas the latter's emissions are super-heated. Cold seeps constitute a biome supporting several endemic species. Cold seeps occur over fissures on the seafloor caused by tectonic activity. Oil and methane "seep" out of those fissures, are diffused by sediment, and emerge over an area several hundred meters wide. Methane (CH4) is the main component of what we commonly refer to as natural gas. In addition to being an important energy source for humans, methane also forms the basis of a cold seep ecosystem. Cold seep biota below 200 m typically exhibit much greater systematic specialization and reliance on chemoautotrophy than those from shelf depths. Deep-sea seeps sediments are highly heterogeneous. They sustain different geochemical and microbial processes that are reflected in a complex mosaic of habitats inhabited by a mixture of specialist (heterotrophic and symbiotic-associated) and background fauna.
During the initial stage, when methane is relatively abundant, dense mussel beds also form near the cold seep. Mostly composed of species in the genus Bathymodiolus, these mussels do not directly consume food. Instead, they are nourished by symbiotic bacteria that also produce energy from methane, similar to their relatives that form mats. Chemosynthetic bivalves are prominent constituents of the fauna of cold seeps and are represented in that setting by five families: Solemyidae, Lucinidae, Vesicomyidae, Thyasiridae and Mytilidae. This microbial activity produces calcium carbonate (CaCO3), which is deposited on the seafloor and forms a layer of rock. During a period lasting up to several decades, these rock formations attract siboglinid tubeworms, which settle and grow along with the mussels. Like the mussels, tubeworms rely on chemosynthetic bacteria (in this case, a type that needs hydrogen sulfide instead of methane) for survival. True to any symbiotic relationship, a tubeworm also provides for their bacteria by appropriating hydrogen sulfide from the environment. The sulfide not only comes from the water, but is also mined from the sediment through an extensive "root" system a tubeworm "bush" establishes in the hard, carbonate substrate. A tubeworm bush can contain hundreds of individual worms, which can grow a meter or more above the sediment.
Unlike hydrothermal vents, which are volatile and ephemeral environments, cold seeps emit at a slow and dependable rate. Likely owing to the differing temperatures and stability, cold seep organisms are much longer-lived than those inhabiting hydrothermal vents. Indeed, recent research has revealed seep tubeworms to be the longest living non-colonial invertebrates known, with a minimum lifespan of between 170 and 250 years. Cold seeps do not last indefinitely. As the rate of gas seepage, slowly decrease, the shorter-lived, methane-hungry mussels (or more precisely, their methane-hungry bacterial symbionts) start to die off. At this stage, tubeworms become the dominant organism in a seep community. As long as there is some sulfide in the sediment, the sulfide-mining tubeworms can persist. Individuals of one tubeworm species Lamellibrachia luymesi have been estimated to live for over 250 years in such conditions. Cold seeps were discovered in 1983 by Dr. Charles Paull on the Florida Escarpment in the Gulf of Mexico at a depth of 3,200 meters (10,500 ft). Since then, seeps have been discovered in other parts of the world's oceans. They have also been grouped into several biogeography provinces Gulf of Mexico, Atlantic, Mediterranean, East Pacific, and West Pacific and under an ice shelf in Antarctica. Those localities include the Monterey Canyon just off Monterey Bay, California, the Sea of Japan, off the Pacific coast of Costa Rica, in the Atlantic off of Africa and in waters off the coast of Alaska. The deepest seep community known is found in the Japan Trench at a depth of 7,326 m (24,035 ft). With the recent discovery of a methane seep in the Southern Ocean, methane seeps are now known in all oceans. Cold seeps are patchily distributed and they occur most frequently near ocean margins from intertidal to hadal depths. Methane seeps are common along continental margins in areas of high primary productivity and tectonic activity, where crustal deformation and compaction drive emissions of methane rich fluid.
Major threats cold seeps ecosystems and their communities face today are seafloor litter, chemical contaminants and climate change. Seafloor litter alters the habitat, by providing hard substrate where none was available before or by overlying the sediment, inhibiting gas exchange, and interfering with organisms on the bottom of the sea. Studies of marine litter in the Mediterranean include surveys of seabed debris on the continental shelf, slope, and bathyal plain. In most studies, plastic items accounted for much of the debris, sometimes as much as 90% or more of the total, owing to their ubiquitous use and poor degradability.
Weapons and bombs have also been discarded at sea and their dumping in open waters contributes to seafloor contamination. Another major threat to the benthic fauna is the presence of lost fishing gear, such as nets and longlines, which continue to ghost fishing and can damage fragile ecosystems such as cold-water corals.
Chemical contaminants such as persistent organic pollutants, toxic metals (e.g., Hg, Cd, Pb, and Ni), radioactive compounds, pesticides, herbicides, and pharmaceuticals are also accumulating in deep-sea sediments. Topography (e.g. presence of canyons) and hydrograph (e.g., cascading events) play a major role in the transportation and accumulation of these chemicals from the coast and shelf to the deep basins, affecting the local fauna. Recent studies have detected the presence of significant levels of dioxins in the commercial shrimp Aristeus antennatus and significant levels of persistent organic pollutants in mesopelagic and bathypelagic cephalopods.
Climate driven processes and climate change will have an impact on the frequency and intensity of cascading, with unknown effects on the benthic fauna. Another potential effect of climate change is related to energy transport from surface waters to the seafloor. Primary production will change in the surface layers according to sun exposure, water temperature, major stratification of water masses, for example and this will affect the food chain down to the deep seafloor, which will be subject to differences in quantity, quality, and timing of organic matter input. As commercial fisheries move into deeper waters, all of these impacts will affect the communities and populations of organisms in cold seeps and the deep sea in general.