Atlantic coastal salt marsh

This habitat comprises natural grasslands on saline, clayey substrates along the coasts of the Atlantic Ocean, including the North Sea, characterized by halophytes. The northern limit of the habitat lies in northern Iceland and northern Norway (see habitat A2.5a), the southern limit lies in central Portugal (estuary of river Mondego),from which point southwards the salt marshes have predominantly Mediterranean characteristics. The most important factor for this azonal habitat is flooding by sea water, creating a gradient from lower, daily flooded parts towards higher parts with a low flooding frequency and duration and a more fluctuating soil salinity. With the flooding, sediments are brought in, and differences in sedimentation patterns (sand is deposited in the more dynamic parts, clay in low dynamic areas) creates a geomorphology of deep drainage channels (creeks), bordered by higher, sandy levees, with lower, clayey depressions behind. The salt marshes form transitions to bare mud flats on the lower elevation, and dunes or dune slacks on the higher part. These salt marshes are found in sheltered parts along the coast, like in lagoons, in sea inlets behind dunes, on the inland side of barrier islands, in estuaries, and in the Wadden Sea also on so-called “green beaches” (transition areas flooded both from North Sea and Wadden Sea) and rarely on sheltered parts of broad beaches.

The typical zonation of the North-Western Atlantic contains very open annual Salicornia species and perennial Spartina fields in the pioneer marsh (alliances Thero-Salicornion, Spartinion maritimae), open communities of Puccinellia maritima, Limonium vulgare, Halimione portulacoides , Triglochin maritima , Spergularia maritima and Plantago maritima in the lower salt marsh (alliance Puccinellion maritimae) and more closed communities of Juncus gerardi, Festuca rubra, Agrostis stolonifera, Armeria maritima, Elytrigia pycnanthus, Juncus maritimus and Artemisia maritima in the higher salt marsh (alliance Armerion maritimae). The latter form transitions towards brackish Potentillion anserinae communities (sometimes considered as Loto tenuis-Trifolion fragiferi Westhoff et Den Held ex de Foucault 2009) or towards Plantago coronopus, Sagina maritima and Cochlearia danica ommunities (alliance Saginion maritimae) on rarely flooded sand dunes. In places with seepage of freshwater, Blysmus rufus, Phragmites australis or Bolboschoenus maritimus may dominate the vegetation (alliance Scirpion maritimae). Also Apium graveolens and Oenanthe lachenalii are characteristic for such brackish transitions. Alpha diversity is in general low and rare species are few, for example Halimione pedunculata, Salicornia disarticulata, Scirpus americanus, Hordeum maritimum, Puccinellia fasciculata  and Bupleurum tenuissimum. In the thermo-Atlantic regions of Southwestern France, Northern Spain and Portugal, several other species with an Atlantic-Mediterranean distribution are found. In the first place several perennials chenopods grow here, like Sarcocornia perennis, Arthrocnemum fruticosum and Suaeda vera. Other ‘southern elements’ include Inula crithmoides, Juncus acutus, Frankenia laevis, Hutchinsia procumbens, Triglochin bulbosa subsp. barrelieri, Parapholis incurva, and Spartina maritima on the lower parts of the salt marsh.

Along the Atlantic coast saline meadows are also found in sites that have been embanked, sometimes centuries or decades ago, but still are fed with saline or brackish water. Such salt marshes often lack the typical  geomorphology but contain almost the same species composition as freely flooded salt marshes.  They are described here as part of Atlantic salt marshes; in most cases they only represent a small part of the area of the habitat type.

Many of the salt marshes along the Atlantic coast have a long tradition of extensive grazing. Ungrazed salt marshes are rare, but large examples exist, for instance in the Netherlands. No management leads, because of progressing clay sedimentation and nutrient enrichment, towards relatively species poor salt-marshes, were the lower belts are dominated by Halimione portulacoides, and the higher parts by Elytrigia atherica  (pycnanthus). Close to dunes or dikes Phragmites australis may dominate, but unlike in the Baltics, salinity and larger tidal fluctuations prevent reed to dominate large areas and threaten the diversity of the salt marshes. Therefore, reed beds form part of the Atlantic salt marshes, end in almost all cases these reed beds contain several halophytes. In general it is expected that low extensive grazing causes the highest biodiversity in Atlantic salt marshes.

Salt marshes are important stop overs on the fly ways of migrating birds. Puccinellia maritima for example provides a consistent diet for goose species. The habitat also is an important breeding and resting site for many waders. Besides for birds, salt marshes are important for specialized invertebrates (insects), many of them living just on one or a few halophytic plant species. They are considered as very productive ecosystems and the creeks form nurseries for some fish species, like Dicentrarchus labrax.

Salt marshes are threatened by drainage, building of hard sea defense systems (dikes), modification of the topography, invasive species (Spartina anglica, Spartina alterniflora) and eutrophication of the sea water. Sea level rise, resulting from climatic change, may cause erosion, especially of the pioneer belts with Salicornia where vegetation cover is very low.

Indicators of good quality:

The following characteristics are considered as indicators of good quality:

• Pattern of branching creeks, with levees and depressions
• Regularly flooding with sea water (absence of dikes…)
• Presence of rare species
• Low level or absence of invasive and nitrophilous species
• Complete set of zonation belts, with no overrepresentation of certain belts
• Absence of erosion

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats