Caring for the Deep Sea

The deep sea is one of the most remote and expansive habitats on Earth, spanning depths of 200 m to beyond 10,000 m in the deepest trenches. The oceanic seafloor forms through a combination of seafloor spreading at mid-ocean ridges (mor) and sedimentation of materials from the sea surface over millions of years, as ocean plates move from their origin at mor to subduction zones. The generally uniform temperature and salinity, and absence of light, define an environment far less variable than in most shallow-water environments.1 The area of sediment-covered seafloor comprises more habitat than all others on Earth combined. Near the continental shelf, sediments often contain terrigenous material transported by rivers and coastal currents, whereas sediments in the abyssal plains (sometimes more than one kilometer thick) are derived from the shells of open-water organisms. The composition of sediments defines the fauna living on and within them. Because of the absence of light, most deep-sea organisms depend on sinking food material produced in surface waters (phytodetritus), fecal pellets, fish or zooplankton carcasses, or material transported laterally (pieces of kelp and land-based organic material such as wood). Exposed hard substratum occurs mainly in areas with relatively steep profiles, such as the walls of submarine canyons and the flanks of seamounts, as well as on newly produced seafloor near spreading centers. Strong currents typically characterize steep sloping environments, limiting accumulation of sediments, and exposing hard substratum. Submarine canyons incise the continental slope and can range over 1,000 m in depth. Because of their topography, canyons can act as conduits of sediment,