Marine Sediments major role in cycling of organic matter Thus, there is considerable interaction between these groups of organisms Many factors are thought to influence the spatial distribution of benthic sedimentary organisms, including sediment type, productivity, temperature, salinity, pressure, depth, oxygen, sediment stability, air exposure, current speeds and wave action, and biological interaction with other species A more complete treatment of benthic organisms is given in Gray and Elliott (2009) Marine Sedimentary Habitats Oceans range in depth from intertidal habitats to ocean trenches at 10,000 m depth (Figure 3) Light can penetrate to 1000 m in the clearest oceanic waters, but in coastal waters penetration may be limited to from 200 m to only a few meters Like most ecosystems, marine sedimentary communities are fueled by photosynthesis, and because many marine sediments occur below photosynthetic depths they rely on phytoplankton sinking from surface waters above and macrophyte detritus transported from nearshore environments Not surprisingly, water depth is a major variable in categorizing marine sedimentary habitats Intertidal Habitats The shallowest sedimentary habitats are those that occur at the land–sea interface They include habitat with emergent 107 vegetation such as mangroves and salt marshes as well as sandflats and mudflats without emergent vegetation that may or may not include diatom mats (Table 1) Mangroves and salt marshes occur intertidally, and as such they experience wide ranges of salinity and temperature They are also extremely productive, with abundant organic matter in the form of decaying vegetation The productivity of these systems promotes extremely high bacterial respiration and sediments are often hypoxic or even anoxic except at the sediment–water interface The organic matter produced by vascular plants is also relatively refractory and difficult for most species to digest In combination with oxygen limitation and tidal exposure, this refractory material contributes to the low diversity of organisms residing in the sediments Unvegetated sandflats and mudflats tend to be much lower in productivity unless they are immediately adjacent to vegetated areas, and their diversities are slightly elevated Nonetheless, intertidal areas experience high variation in environmental variables and are characterized by a low-diversity fauna consisting of organisms able to cope with this variability (see Levin et al., 2001) Subtidal Habitats At depths below low tide, species diversity increases relative to that of intertidal systems In the shallowest subtidal habitats, vegetation in the form of seagrass and kelp may occur, and some shallow sediments may be covered in diatom mats However, all these plant forms are confined to depths of tens of meters or less, and most subtidal sedimentary habitats rely Benthic environments Littoral Shelf (0−200 m) …sublittoral Slope (200−3000 m)…bathyal Depth (km) Rise (3000−4000 m) Abyssal plain (4000−6000 m) Trenches = Hadal (> 6000 m) 300 900 Distance from shore (km) Figure Schematic diagram of the depth zones of the oceans Dark area indicates the ocean bottom The distances and depths given are only examples of a range or values observed throughout the world The horizontal axis is greatly exaggerated in size relative to the vertical axis See text for more detailed explanation of depth zones