Sediment is present in the marine environment as both suspended and deposited particles and comprises both organic and inorganic components. The extent to which particles are suspended or deposited is a function of their density and the hydrodynamics of the water column.

Suspended sediment influences the effects of some non-toxic substances such as nutrients by reducing light penetration and preventing uptake by primary producers. This allows nutrient concentrations to increase while eutrophication effects are not apparent. Some toxic substances have an affinity for sediments, in particular organic particles, and the presence of suspended sediments can reduce water column concentrations here, but the toxic substance may still be biologically available, e.g. to sediment feeders/dwellers.

Sediment type is critical, not only because of its role in determining the faunal community, but also because of its role in adsorbing pollutants. In general terms, fine mud/silt/clay sediments with high organic content retain more contaminants than relatively coarse sandy sediments. The smaller the particle size, the more likely sediments are to retain contaminants which may subsequently be released back into the water column.

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