Sediments

Most of the sea floor is dominated by soft sediments such as muds and sands. They form the basis for a number of ecosystems dominated by burrowing animals and, in shallow and intertidal waters, by seagrasses, mangrove forests and salt-marshes. In some areas these sediments are highly dynamic, held in suspension or picked up and moved from place to place by tidal and seasonal currents.

Against this background considerable amounts of sediments are being added to coastal waters world-wide as a result of human activities and in some cases these are creating problems for natural ecosystems. Sediments may be released from soils following deforestation or from poor agricultural practises and typically enter the sea through the watershed, though there can be direct runoff in coastal areas and from urban areas. In some areas very fine sediments may be carried by the wind, and these may travel considerable distances, as in the case with dust from North Africa being carried to South America and the Caribbean Sea. While such sediments may include high levels of nutrients or toxins, they also have a physical role, and very high levels of sediment deposition can be a problem for coastal navigation, and can have a smothering effect on certain coastal communities such as rocky communities and coral reefs.

Although not so widespread geographically, the loss of coastal sediments from some river-mouths is also a problem. The supply of sediments may be important for the maintenance of beaches and offshore habitats. This sediment supply may be interrupted by structures which obstruct sediment flow, such as dams on rivers, or coastal engineering works, this can lead to rapid erosion and loss of beaches and coastline.