Mid and upper shore wave-sheltered muddy fine sand or sandy mud with narrow-leafed eel grass Zostera noltii at an abundance of frequent or above. It should be noted that the presence of Z. noltii as scattered fronds does not change what is otherwise a muddy sand biotope. Exactly what determines the distribution of Z. noltii is not entirely clear. It is often found in small lagoons and pools, remaining permanently submerged, and on sediment shores where the muddiness of the sediment retains water and stops the roots from drying out. An anoxic layer is usually present below 5 cm sediment depth. The infaunal community is characterised by the polychaetes Scoloplos armiger, Pygospio elegans and Arenicola marina, oligochaetes, the spire shell Hydrobia ulvae, and the bivalves Cerastoderma edule and Macoma balthica. The green algae Enteromorpha spp. may be present on the sediment surface. The characterising species lists below give an indication both of the epibiota and of the sediment infauna that may be present in intertidal seagrass beds. The biotope is described in more detail in the British National Vegetation Classification (see the chapter on saltmarsh communities in Rodwell, 2000). Situation: Z. noltii is most frequently found on lower estuary and sheltered coastal muddy sands, together with biotopes such as unit A2.242. Temporal variation: There may be seasonal variation in the area covered by intertidal seagrass beds, as plants die back during cold temperatures in winter. Intertidal seagrass beds may also be subject to heavy grazing by geese, which can reduce the extent of the plant cover significantly. The rhizomes of the plants will remain in place within the sediment in both situations.