Salt marshes in Europe and temporal variability
A salt marsh (Figure 1) is a tidal wetland in the upper coastal intertidal zone between salty or brackish water. They replace mangroves in temperate and arctic regions. Their flora is dominated by halophytic (salt-tolerant) vegetation (Adam, 1990 ), such as grasses, shrubs and herbs . The sediment consists of mud and sand. Salt marshes are normally associated with mud flats, these mud flats are sometimes dominated by algae. However, salt marshes can also occur on sand flats.
Salt marshes are regularly subjected to tidal movement and wave action. Tidal channels control the drainage of this seawater and allow detritus, dissolved nutrients, plankton and small fishes to be flushed in and out the salt marshes. Salt marshes protect the lowlands from marine flooding by damping storm and waves and by slowing flows pushing inland (Allen, 2000).
The total number and area of salt marshes have been declining for many years. The main cause is enclosure, which removes the habitat from tidal inundation. Another major problem, especially across Europe, is lateral erosion which leads to a loss of salt-marsh habitat. (More info)
European-scale distribution of salt marshes
Species composition and zonation of marsh vegetation
Species compositions of vegetation communities are governed by species’ ability to compete and their tolerance to site‐specific environmental conditions and physical disturbance (Grime 1979 ). Most classification schemes recognize five vegetation zones in salt‐marshes along the vertical gradient in seawater exposure (inundation frequency) (Figure 2). To some degree, this vertical zonation reflects a general decline from the low to high elevations in the tolerance of species to seawater flooding (Allen 2000). Thus, the grazing-marsh above the intertidal zone has less predominance of salinity‐tolerant species than the salt marsh. However, the predictable vertical alignment of species along the salinity gradient is strongly modified by the species’ competitive abilities, as well as their tolerances to site‐specific conditions, including tidal range and climate (e.g. rainfall), as well as salinity, nutrient and disturbance (e.g. waves or grazing) regimes (Allen 2000; Bertness et al., 2002 ). For instance, intense grazing disturbance may cause less competitively strong mid‐marsh species to become more common in the high marsh (Kiehl et al. 1996 ). Traits for competition, environmental and disturbance tolerance often exist in trade-off. Thus, while competitive fitness generally increases up‐shore, some low marsh species compensate for lack of competitive ability by being more tolerant to wave disturbance than high shore species (Pennings and Calloway 1992 ).
European salt‐marsh distribution and vegetation composition
Although salt‐marshes are not the most diverse ecosystems, due to the influence of the salinity regime, they are highly productive. Along the European coasts salt‐marsh vegetation composition differs between four regions (Figure 3) from which a large area is protected within Natura 2000: 1) North Atlantic; 2) South Atlantic; 3) Mediterranean; 4) Baltic and Boreal regions (European Commission, 2007). A short description of these regional vegetation differences in vegetation composition is given in Table 1, Table 2 and Table 3. In general the salt marshes in North-West Europe can be considered to belong to the North Atlantic. In contrast to the other types, these marshes are mostly exposed to large tidal amplitudes. In general the salt marshes and meadows along the Baltic Sea experience minimal tidal differences, and can be considered micro-tidal. Most of the Baltic and boreal coastal areas were traditionally used for mowing or grazing, thus enlarging the areas and keeping the vegetation low, rich in vascular plants, characteristically the vegetation occurs in distinct zones, with saline vegetation closest to the sea. The Mediterranean and South Atlantic coastal areas consist largely of the same species composition. This is due to the temperature, although Mediterranean species higher up on the shore are usually more dessication resistant. In general the salt marshes and meadows along the Mediterranean experience minimal tidal differences, and can be considered micro‐tidal.
|Glasswort swards (Thero-Salicornietalia): annual glasswort (Salicornia spp., Microcnemum coralloides), seablite (Suaeda maritima), or sometimes saltwort (Salsola spp.) formations colonizing periodically inundated muds of coastal salt marshes and inland salt‐basins.|
|Mediterranean halo-nitrophilous pioneer communities (Frankenion pulverulentae): formations of halo‐nitrophilous annuals (Frankenia pulverulenta, Suaeda splendens, Salsola soda, Cressa cretica, Parapholis incurva, P. strigosa, Hordeum marinum, Sphenopus divaricatus) colonizing salt muds of the Mediterranean region, susceptible to temporary inundation and extreme drying.|
|Atlantic sea‐pearlwort communities (Saginion maritimae): formations of annual pioneers occupying sands subject to variable salinity and humidity, on the coasts, in dune systems and salt marshes. They are usually limited to small areas and best developed in the zone of contact between dune and salt marsh.|
|Central Eurasian crypsoid communities : Sparse solonchak formations of annual grasses of genus Crypsis (Heleochloa) colonizing drying muds of humid depressions of the salt steppes and saltmarshes (15.A) of Eurasia, from Pannonia to the Far East.|
|Flat‐leaved cordgrass swards: perennial pioneer grasslands of coastal salt muds, dominated by flat-leaved Spartina maritima, S. townsendii, S. anglica, S. alterniflora.|
|Rush‐leaved cordgrass swards: perennial pioneer grasslands of southern Iberian coastal salt muds, dominated by the junciform‐leaved Spartina densiflora.|
|Tall rush (Juncus maritimus and/or J. acutus) dominated salt|
|Short rush, sedge and clover saltmarshes (Juncion maritimi) and humid meadows behind the littoral, rich in annual plant species and in Fabacea (Trifolion squamosi)|
|Mediterranean halo-psammophile meadows (Plantaginion crassifoliae)|
|Iberian salt meadows (Puccinellion fasciculatae)|
|Halophilous marshes along the coast and the coastal lagoons (Puccinellion festuciformis)|
|Humid halophilous moors with the shrubby stratum dominated by Artemisia Coerulescens|
|Agrostis stolonifera, Blysmus rufus, Bolboschoenus maritimus, Calamagrostis stricta, Carex nigra, C. paleacea, Centaurium littorale, C. pulchellum, Eleocharis uniglumis, E. parvula, Festuca rubra, Juncus gerardii, Odontites litoralis, Ophioglossum vulgatum, Plantago maritima.|
Examples of temporal variability
Salt marshes can show large spatial and temporal variability. Due to the interaction of hydrodynamic forces and vegetation‐sedimentation feedbacks, complex patterns of marsh establishment, development and destruction can occur at the same time spread across the marsh. Moreover, salt-marshes can go through cycles of large-scale marsh build-up alternated with lateral erosion resulting in a spatial shift of the leading vegetation edge and eroding cliff in time. Consequently, it will be a challenge to separate development trends in salt-marsh extant caused by environmental changes, by e.g. climate change and sea level rise, from changes due to the natural variability of this ecosystem.
Case study 1: Salt marshes along the Western Scheldt, Netherlands
Recently the spatiotemporal variability of eight salt marshes along the Scheldt estuary have been investigated (Figure 4, van der Wal., et al 2008 ). The study revealed the significance of intrinsic processes in salt marsh development, and the necessity to consider the local feedback mechanisms between plant growth, morphology and hydrodynamics of both the salt marsh and the mudflat, when assessing the status of salt marshes. Furthermore, the importance of assessing salt marsh changes in a spatial context is highlighted, rather than looking at changes in total salt marsh area. Most salt marshes showed simultaneous expansion of Spartina anglica tussocks, and lateral retreat of the mature salt marsh plateau, resulting in salt marsh rejuvenation, which support these conclusions. However, based on this study no clear relationship could be found between the net expansion or erosion and the hydrodynamic conditions at the edge of the salt marsh. Hence, additional mechanistic insight is required to determine which and how hydrodynamic and sedimentary conditions are important for the development and erosion of salt marshes.
Case study 2: Salt marshes along the Yangtze Esturary, China
The Yangtze Estuary is a typical medium tidal estuary with multi-order bifurcations, shoals and sand bars. The enormous quantities of sediment produced by the Yangtze River have created extensive areas of shoals and tidal flats in the estuarine region, which have been colonized by various types of salt-marsh vegetation. According to a recent study based on remote sensing mapping (Huang et al., 2008 ), the salt marsh vegetation in the Shanghai region amounted to 18314.8 ha (Year 2008) and more than 95% of salt marsh vegetation belonged to the four major plant communities, i.e. the reed (Phragmites australis) community, the smooth cord-grass (Spartina alterniflora, an exotic species) community, and two types of sedge (Scirpus mariqueter and Carex scabriflora) communities.
The tidal flats closest to the low water mark, elevation less than 2 m, are characterized by mud flats that are devoid of any vascular plants. As sedimentation and succession progressed, the Phragmites australis community replaced the Scirpus mariqueter community above the 2.9 m elevation. An additional species in this zone was Spartina alterniflora, which was introduced to the Yangtze Estuary in 1990s. Over the last two decades this species has gradually invaded large areas which were previously covered by P. australis and has also started to invade the upper parts of the S. mariqueter zone (Figure 5). This expansion of S. alterniflora, is a prime example of a spatially‐structured invasion in a relatively simple habitat, for which strategic control efforts can be modeled and applied.
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