Ecological and functional relationships
Few grazers feed on the saltmarsh plants directly. In spring and summerSpartina sp. are highly productive and in autumn leave die back and decompose on the stalk. Therefore, the majority of Spartina sp. productivity, and presumably other vascular plant productivity, enters the food web as detritus.
Benthic algae and microphytobenthos play an important role in cycling nutrients, and hundreds of species of bacteria, fungi, and microalgae may be attached to surfaces of vascular plants and sediment. These are grazed by meiofauna (e.g. protozoa, foraminifera, nematodes). There are significant numbers of marine macrofauna species present.
The majority of saltmarsh insects are sap sucking aphids or chewing grasshoppers, e.g. the saltmarsh aphid, Sipha littoralis feeds mainly on Puccinellia maritima and Spartina anglica, and the aphid Macrosiphonella asteris feeds on stems of Aster tripolium with lowest salt content, but may not be found in pioneer saltmarsh biotopes.
The brent goose (Branta bernicla) grazes Puccinellia maritima and Aster tripolium in high marsh at the end of winter. Estimates of the amount of plant material consumed by wildfowl in saltmarsh and seagrass beds range from 1 to 50% (Raffaelli & Hawkins, 1999).
Macoma baltica, Corophium volutator and Arenicola marina are deposit feeders, while Nereis diversicolor and Nephtys hombergi act as predators.
Hydrobia ulvae grazes the microflora from sediment grains and epiphytes.
Several birds species feeding on intertidal flats probably also feed on adjacent saltmarsh, e.g. shelduck which feed extensively on Hydrobia ulvae together with Macoma baltica and Corophium volutator.
Gobies e.g. Pomatoschistus minutus (sand goby) are significant predators on Corophium volutator.
Seasonal and longer term change
In submergent Spartina -Salicornia
saltmarsh in Norfolk, UK annelid numbers increased in spring and declined in June-July and increased again in late summer (Packham & Willis 1997). The sand goby entered the marsh in early summer, moved away in August -September but was abundant again in autumn.
Germination of salt-marsh plants tends to occur in spring, encouraged by low salinities although Salicornia
spp. can germinate at high salinities.
The filamentous green algae Ulothrix
is found on exposed mud in spring but disappears in summer.
There is a continual change in bird species in the coastal zone. January brings wildfowl back from their annual moult migration e.g. shelduck, wigeon, mallard, teal and pintail. Waders become conspicuous in May e.g. godwits, grey plover, and spotted redshank. Terns, ringed plover, oystercatcher and shelduck breed in June. However, the exact array of species varies between sites depending on the types of coastal habitats and feeding grounds present, disturbance and availability of nesting sites.
Habitat structure and complexity
Pioneer saltmarsh occurs in sheltered, low energy habitats at the top of the intertidal where sediment has built up above mean high water of neap tides (MHWN). The sediment becomes colonized by halophytic vascular plants which themselves stabilize the sediment, slow water movement and promote additional accretion of sediment, until the height of the marsh is only covered by the highest tides. Pioneer saltmarsh communities represent colonizing species early in saltmarsh development (succession) and zonation and occupy a zone between MHWN and mean high water (MHW).
- In areas subject to wave action the saltmarsh may be limited to the highest astronomical tides (HAT) but in very sheltered areas may extend to MHWN.
The extent of saltmarsh is affected by topography and may be extensive on flat, gently sloping shores or limited to a few metres on steep shores.
Sedimentation rates, and hence accretion rates vary between sites e.g. 8 mm/yr. at Scolt Head, east England and 78 mm/yr. in the Dovey, Wales (Packham & Willis, 1997), and is determined by the hydrographic regime, and sediment supply from eroding cliff or riverine sources. Sediment may be bound by mucilaginous diatoms of the microphytobenthos, tubes of burrowing polychaetes, vegetation, or destabilized by bioturbation due to infauna e.g. Corophium volutator or Hydrobia sp.
Pioneer saltmarsh communities may be washed away by tides, currents and storms and appear patchy until the vegetation becomes established.
Saltmarsh are characterized by a network of creeks formed by freshwater runoff and salt pans. Growth of pioneer plants on raised areas concentrates water flow into channels that form deepening creeks as the marsh develops. Depressions surrounded by vegetation (pans), hold water that evaporates after high tide, in many respects, the saltmarsh equivalent of rockpools. However, typically 70% of the surface is dominated by saltmarsh flat (Ranwell, 1972; Long & Mason, 1983; Adam, 1993; Packham & Willis; 1997).
The substratum varies but contains more silt and clay than underlying intertidal sediment e.g. saltmarsh soil at Bull Island, Dublin Bay was 75% sand whereas at Colne Point, east England it was 5%. The relative composition of sand affects porosity and water holding capacity.
Organic matter is derived from deposited detritus and particulate matter together with degraded plant material from saltmarsh vegetation. Therefore the organic content increases with time and shore height.
The high organic content encourages microbial activity, which together with poor oxygen exchange in silty sediments results in anoxic conditions, releasing toxic methane and hydrogen sulphide. Typically, saltmarsh soil has a high salinity, is commonly anaerobic, and has low levels of nitrogen and phosphorus compared to other terrestrial soils.
Salicornia sp. and Atriplex sp. dominate around MHWN is presence of wave action but where occasional smothering by marine debris keeps vegetation open. Low water marsh is dominated by Spartina sp., Aster tripolium, and Puccinellia phyganodes.
Seventy two species of Bryophytes (mosses & liverworts) are found on British saltmarsh, especially Pottia heimii.
Macroalgae may predominate the lower saltmarsh, e.g. dwarf Fucus vesiculosus and Ascophyllum nodosum ; Pelvetia caniculataecad libera is found in pans entangled in vascular plant stems, and Ulva nana and Catanella repens may be epiphytic on vascular plant stems. Filamentous brown algae colonize steep creek banks (e.g. Vaucheria thuretti); cyanobacteria may be found amongst vascular plants (e.g. Calothrix sp.); exposed mud may be colonized by filamentous green algae e.g. Ulothrix (found in spring but disappearing in summer).
The sediment in saltmarsh of the Stour estuary was found to support the polychaete Nereis diversicolor, the oligochaetes Tubificoides benedini and Tubiflex costatus, the crustacean Corophium volutator, and the mud snail Hydrobia ulvae. The intertidal collembolan Anurida maritima may be confined to the transition zone from mudflat to marsh.
Epibenthic fish are restricted to pools and creeks at low tide but may feed over a wider area, including pioneer saltmarsh, at high tide e.g. Pomatoschistus minutus (sand goby) and Gasterosteus aculeatus (three-spined stickleback).
Primary producers include the vascular plants and microalgae and any filamentous algae and macrophytes present. Adam (1993) suggested that algae made an important contribution to net productivity. Saltmarsh is highly productive, although most of the productivity is consumed secondarily. Dead plant material is broken down by bacteria on the surface of the sediment. This increases its food value by degrading cellulose in digestible carbohydrates. The remaining detritus forms the basis of a food chain for a wide variety of organisms and may be a major source of organic carbon for surrounding communities, depending on the hydrographic regime. For example, primary productively for Spartina, Salicornia and Limonium saltmarsh in the UK was estimated to be 400 gC /m²/ year (Mann, 1982 cited in Raffaelli & Hawkins, 1999).
Spartina anglica is a perennial and can spread over large distances by means of fragments carried to new sites with the tide. Gaps in colonies rapidly fill with seedlings once the colony is established, although it produces seed erratically which lose viability if they dry out. Spartina maritima however, sets little fertile seed and widely separated clones develop from the occasional fragment and it therefore has a restricted distribution. Plants of the genus Salicornia, Atriplex and Suaeda are annuals with flowers carried in the shoot and extensive seed banks that persist for more than a year. Plants of these species need 2-3 days without flooding to root effectively. Recruitment in infaunal sediment dwelling invertebrates may be patchy and sporadic depending on the hydrographic regime and post-settlement mortality (from scour, smothering and predation).
Time for community to reach maturity
Beeftink (1979) reviewed the effects of disturbance on Haliminone portulacoides saltmarsh communities in the Netherlands. After die back of the Haliminone portulacoides communities a successional recolonization occurred, beginning with Suaeda maritima (and sometimes Salicornia sp.) followed by Aster tripolium, then Puccinellia maritima until Haliminone portulacoides returned. The time take for recovery depended on the initial level of disturbance to the Haliminone portulacoides community, taking less time after minimal disturbance. For example, Suaeda maritima recolonized within a year after waterlogging, and Suaeda maritima and Salicornia europaea recolonized within three years of chemical destruction of the Haliminone portulacoides community.
This review can be cited as follows:
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=25&code=2004>