Faunal communities of variable salinity Atlantic littoral mud

Description (English)

Upper estuarine sandy mud and mud shores, in areas with significant freshwater influence. Littoral mud typically forms mudflats, though dry compacted mud can form steep and even vertical structures, particularly at the top of the shore adjacent to saltmarshes. Little oxygen penetrates these cohesive sediments, and an anoxic layer is often present within millimetres of the sediment surface. The upper estuarine mud communities support few infaunal species and are principally characterised by a restricted range of polychaetes and oligochaetes.
Situation: There are three oligochaete dominated upper estuarine mud biotopes. Of these three, MA6-226 occurs the furthest towards the mid estuary, and possibly lower on the shore than the other two. MA6-228 is the most extreme upper estuarine biotope, occurring at the head of estuaries where there is no strong river flow and hence conditions are very sheltered, and there is a very strong freshwater influence. Further towards the mid estuary, this biotope may occur at the top of the shore, with MA6-2273 and MA6-226 further down the shore.
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.
Mid estuarine shores of fine sediment, mostly in the silt and clay fraction (particle size less than 0.063 mm in diameter), though sandy mud may contain up to 40% sand (mostly very fine and fine sand). Littoral mud typically forms extensive mudflats, though dry compacted mud can form steep and even vertical structures, particularly at the top of the shore adjacent to saltmarshes. Little oxygen penetrates these cohesive sediments, and an anoxic layer is often present within millimetres of the sediment surface. Most mid estuarine muddy shores are subject to some freshwater influence, though at some locations more or less fully marine conditions may prevail. Mid estuarine muds support rich communities characterised by polychaetes, bivalves and oligochaetes.
Situation: Principally along mid estuarine shores. The mid estuarine communities may also be present in sheltered inlets, straits and embayments which are not part of major estuarine systems, though usually there is some freshwater influence.
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 or where there is significant freshwater influence.

Source: EUNIS habitat classification