Soft mud with a fine sand fraction, in variable salinity conditions, typically close to the head of estuaries. The infauna is dominated by the polychaete worm Streblospio shrubsolii, the polychaete Nephtys hombergii, oligochaetes of the genus Tubificoides, and the Baltic tellin Macoma balthica. The ragworm Hediste diversicolor and the spire shell Hydrobia ulvae are often common or abundant. Situation: This unit occurs in mid estuary conditions, usually on the low shore. Units A2.323 and A2.3223 may occur higher up the shore, as well as further towards the upper 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.