A species-poor community found in mud or slightly sandy mud in low salinity conditions, typically at the head of estuaries. The infauna is dominated by the ragworm Hediste diversicolor which is typically superabundant. Oligochaetes, including tubificids and Heterochaeta costata, can be abundant, as well as spionids. The peppery furrow shell Scrobicularia plana may be present in low abundances. The mud is often very soft and fluid, with a 'wet' surface appearance, or it may be compacted and form steep banks in the upper parts of macro-tidal estuaries and along saltmarsh creeks. Situation: There are three oligochaete dominated upper estuarine mud biotopes. A2.323 is the most extreme upper estuarine biotope, occurring at the head of estuaries where there is a very strong freshwater influence. Further towards the mid estuary, this biotope may occur at the top of the shore, with A2.3223 further down. Unit A2.321 occurs furthest towards the mid estuary, or on the lower shore with A2.3223 and A2.323 higher up. 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.