Expanses of clean or muddy fine sand and sandy mud in shallow water and on the lower shore (typically to about 5 m depth) can have dense stands of Zostera marina/angustifolia Note: the taxonomic status of Z. angustifolia is currently under consideration. In A5.5331 the community composition may be dominated by these Zostera species and therefore characterised by the associated biota. Other biota present can be closely related to that of areas of sediment not containing Zostera marina, for example, Laminaria saccharina, Chorda filum and infaunal species such as Ensis spp. and Echinocardium cordatum (e.g. Bamber 1993). From the available data it would appear that a number of sub-biotopes may be found within this biotope dependant on the nature of the substratum and it should be noted that sparse beds of Zostera marina may be more readily characterised by their infaunal community. For example, coarse marine sands with seagrass have associated communities similar to A5.133, A5.137 or A5.135 whilst muddy sands may have infaunal populations related to A5.241, A5.243 and A5.242. Muddy examples of this biotope may show similarities to A5.332, A5.343, A5.342 or A5.351. At present the data does not permit a detailed description of these sub-biotopes but it is likely that with further study the relationships between these assemblages will be clarified. Furthermore, whilst the Zostera biotope may be considered an epibiotic overlay of established sedimentary communities it is likely that the presence of Zostera will modify the underlying community to some extent. For example, beds of this biotope in the south-west of Britain may contain conspicuous and distinctive assemblages of Lusitanian fauna such as Laomedea angulata, Hippocampus spp. and Stauromedusae. In addition, it is known that seagrass beds play an important role in the trophic status of marine and estuarine waters, acting as an important conduit or sink for nutrients and consequently some examples of Zostera marina beds have markedly anoxic sediments associated with them.