Soft wet mud with a fine sand fraction, on the mid and lower shore of sheltered estuaries, usually with an anoxic layer present within the first 5 cm of the sediment. The infauna is relatively poor, dominated by the polychaetes Nephtys hombergii, Streblospio shrubsolii, and Aphelochaeta marioni. The oligochaete Tubificoides benedii is also characterising for this biotope, and Hediste diversicolor may be common. Situation: This unit may occur on the same shores as the A2.31 biotopes, A2.3222 or A2.3221. Higher up on the shore, and/or further towards the head of the estuary, A2.3223 may occur, changing to A2.323 at the upper extreme of the estuary. Temporal variation: Enteromorpha spp. and Ulva lactuca may form mats on the surface of the mud during the summer months, particularly in areas of nutrient enrichment.