Communities of Mediterranean sublittoral estuarine sediments

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). There are also distinct seasonal differences in salinity. In winter the estuary runoff from winter storms and greater flushing reduces the salinity. In spring, runoff becomes small and the estuary gradually returns to marine salinities. One consequence is that the benthos of the sublittoral sediments show rapid transitions from marine to freshwater species.

Typically there is an impoverished benthic macroinvertebrate community in the upper reaches of this estuarine habitat dominated by Tubificidae and Chiromidae. This contrasts with the lower reaches where there is a "salt wedge" community dominated by an abundance of polychates, molluscs and crustaceans. In shallower areas, beds of Cymodocea nodosa may be present. Species diversity is generally low but abundance may be high. Sudden influxes of salt water and drying up in the summer create recurrent disturbances that sometimes cause populations to disappear. In contrast, under oligotrophic conditions and long periods of salt wedge permance, the complexity of the benthic communites can increase. 

Indicators of quality:

Many indicators of quality have been used for this habitat with particular parameters set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis. 

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.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats