Sheltered estuarine shores of sandy mud, which may become firm and compacted if present in the upper shore where there is more time for drainage between high tides. An anoxic layer is usually present within the first 5 cm of the sediment. The infauna is very sparse, usually only the ragworm Hediste diversicolor and the amphipod Corophium volutator are present in any abundance. Occasionally, oligochaetes or the spire shell Hydrobia ulvae may be present. Corophium multisetosum may also be found. There may be organic pollution of the sediment. Situation: This unit may occur on the same shores as units A2.313, A2.312, NhomAph, and 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. or Ulva lactuca may form mats on the surface of the sediment during the summer months, particularly in areas of freshwater influence and/or where there is nutrient enrichment.