Communities of Mediterranean mediolittoral mud estuarine

In Mediterranean estuaries tidal amplitude is very weak and tidal currents, which generate vertical mixing of the water, are negligible. This favors vertical stratification of salinity with a counter current of saline water beneath the less dense river water.(salt wedge estuaries). The small tidal range (20-40cms) also means that large expanses of mediolittoral soft sediments along estuaries are rare, especially when compared to more northern latitudes in Europe. 

This habitat is present on estuarine shores of soft substrates, generally under substantial freshwater influence, and may form a delta at the mouth of the estuary. It occurs in the mediolittoral and the upper part of the infralittoral where the sediment is fine sand, muddy sands and mud according to the course of the river bed. The banks are relatively stable, but the beds change with the violent winter flooding. The surface salinity is low (0.03 to 2.5 psu for the Rhône) whereas that of the deep layer, in contact with the benthic fauna, is much higher (16 to 21 psu for the Rhône). A marine salty patch typically lies underneath the fresh water of the river. Tides are weak and only cause minor changes in the water chemistry. The winds have a more marked influence on the position of the salty patch. When parts of the estuary or estuary lagoons are cut off, either naturally or by human action, the salinity of the water may increase considerably.

In the absence of the tide effect, the transition is rapid between the (freshwater) limnic environment and the marine environment. Thus there is no gradient in the distribution of the fauna, which occurs patchily. This habitat is characterized by communities of polychaetes, bivalves and oligochaetes. The species present are typically have short cycles of development that permit rapid colonization. The habitat is used as a feeding area by birds and by some fishes (grey mullet and eels). 

Indicators of quality:

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time. 

Indices developed to assess the ecological status of coastal waters, including estuaries, according to the Water Framework Directive, include physical indicators, water quality indicators and measures of benthic diversity, species richness and abundance. The latter group, which is particularly relevant to benthic habitats, includes a Benthic Quality Index, an Infaunal Trophic Index, a Marine Biotic index based on ecological groups, and the Benthic Opportunistic Polychaetes/Amphipods Index.

Species typical of environments with high organic loads such as Capitella capitata, Heteromastus filiformis and Polydora spp. may be potential indicators of degraded quality. Indicators used within the Water Framework Directive may also be applied for estuarine habitats as a whole. 

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats