Marine sediments

Marine sediments ranging from sand to fine muds all consist of loose particles and interstitial spaces variously filled with water, air, detritus and organisms. These have traditionally been referred to differently in terms of their size. The microfauna which includes bacteria and protists are extremely important in interstitial sulphur chemistry and the oxygenation of the sediment. Microfaunal primary productivity by some provides an autochthonous food source in shallow areas while heterotrophy is vital in incorporating allochthonous material into the food chain. Meiofauna, animals that live within the sediments and range in size from approximately 0.1 mm to 1 mm, is a major component of benthic ecosystems, particularly in the deep sea. Four phyla are regarded as being exclusively meiofaunal ? the Gnathostomulida, Kinorhyncha, Loricifera, Gastrotricha and Tardigrada though there only a few marine species of the latter. The smallest known polychaetes, species of Diurodrilus, are members of temperate sand meiofauna. In general nematodes are the most numerous component of the meiofauna, with harpacticoid copepods and foraminiferans also important, but their distribution is very patchy. High concentrations frequently occur around the burrows of deposit feeding molluscs and polychaetes. This abundance is associated with high bacterial numbers and raised nutrient fluxes illustrating a key ecological role of these organisms, the trophic linking of micro- and macrofauna. At much larger scales the patchiness of meiofaunal biomass and productivity is closely related to seasonal surface productivity and the downward flux of organic matter, showing an expected seasonal correlation with both light and nutrient (in upwelling areas) availability.

Distinctive groups of larger organisms exist in close association with the sediments of the deep sea: sea cucumbers, crinoids, polychaete worms, sea spiders, isopods and amphipods are most abundant, while molluscs and sea stars are less common than in shallow areas. Some taxa are only found here ? pogonophorans occur mostly below 3000m and 305 known species are restricted to the sediments of deep ocean trenches (>6000m). Small-scale variations in food supply such as the appearance of a large animal carcass may increase spatial structure and provide opportunity for other species to colonize. Variation in the physical environment is created by the burrowing activities of these organisms. A landscape of hills and valleys is created by the burrowing and faecal mounds of echiurid and polychaete worms. Strong bottom currents may result information of ripple marks and accumulate fine-grained particles of detritus.