Biodiversity & Conservation

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) and to dry out between high neap tides. Saltmarsh plants stabilize and consolidate accreting sediment, reducing erosion and increasing the net accretion rate, so that saltmarsh increases in height over time. Therefore, saltmarsh plants (halophytes) especially pioneer species such as Puccinellia maritima and Salicornia sp. significantly modify the habitat providing benthic habitat as well as plant substratum and habitat for a wide range of species. Dynamic changes, occasional events such as storms and disturbance, and succession, provide a complex habitat for a diverse species assemblage (see habitat complexity below). As accretion causes the saltmarsh to grow upwards in relation to tidal height, seawater influence decreases and the invertebrate fauna, halophytic and algal flora changes. With increasing distance from the sea the fauna and flora change from mainly marine in the lower and pioneer marsh and creeks or pans to mainly terrestrial in origin in the mid to high marsh. Few species are mainly or solely associated with Puccinellia maritima dominated communities themselves. Pioneer saltmarsh communities represent colonizing species early in saltmarsh development (succession) and zonation and occupy a zone between MHWN and MHW. The major environmental relationships are listed below. Puccinellia maritima and other halophytes provide primary productivity to the ecosystem. However, a relatively small proportion of this primary productivity is used directly by grazers and the majority (dead plant material) enters the detrital food chain (Long & Mason, 1983).

Additional primary productivity is provides by mats of filamentous algae (e.g. Rhizoclonium sp. and Vaucheria sp.), mats of cyanobacteria (e.g. Rivularia nitida), epiphytic algae and cyanobacteria, and microphytobenthos. Microphytobenthos may help to bind the surface sediment and facilitate colonization by plants, and is grazed by a variety of invertebrates (Long & Mason, 1983; Adam, 1993). Algal productivity is grazed by several invertebrates, however, the majority is thought to enter the detrital food chain (Adam, 1993).

Detritus (in the form of decaying plant material and organic particulates) may be decomposed by bacteria in the saltmarsh or may provide an important source of organic carbon to the wider ecosystem of the estuary or bay, depending on the local hydrographic regime (Long & Mason, 1983; Adam, 1993; Packham & Willis, 1997).

Hundreds of species of bacteria, fungi, and microalgae may be attached to surfaces of vascular plants and in the sediment. These are grazed by meiofauna (protozoa, foraminifera,) and nematodes.

The epiphytic microalgae on plant stems and the algal mats are probably grazed by gastropods (e.g. Ovatella spp. and Hydrobia ulvae and intertidal mites (acarids), in the lower marsh, by littorinids

The majority of saltmarsh insects occur in the mid the high marsh and 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 leaves of Limonium spp. are eaten by caterpillars of the plume moth Agdistis bennetii. Puccinellia maritima supports a number of species of true bugs (Hemiptera), thrips (Thysanoptera), flies (Diptera), butterflies and moths (Lepidoptera) and beetles (Coleoptera) (Gray & Scott, 1977; Adam, 1993).

Macoma baltica, Corophium volutator and Arenicola marina and numerous oligochaetes are deposit feeders while Hydrobia ulvae grazes the microflora from sediment grains.

The lower shore supports suspension feeding invertebrates such as Mya arenaria, Macoma baltica, Scrobicularia plana and Cerastoderma edule Infaunal or epifaunal predators include the polychaetes Hediste diversicolor and Nephtys hombergi , the nemertines Tetrastemma sp. and Lineus spp. and doliochopodid flies.

Crabs and prawns (e.g. Carcinus maenas) are probably generalist predators or scavengers in the lower marsh or salt pans.

Gobies e.g. Pomatoschistus minutus (sand goby) are significant predators on Corophium volutator and together with the three spined stickleback Gasterosteus aculeatus and juvenile flatfish prey on small invertebrates. In Norwich salt marshes, sticklebacks were found to be a significant part of the diet of the otter (Lutra lutra (Long & Mason, 1983).

Intertidal spider species prey on insects and other invertebrates (Packham & Liddle, 1970; Packham & Willis, 1997) Salt marshes are also used as feeding grounds for wildfowl, grazing the saltmarsh plants directly or preying on the invertebrate fauna. Estimates of the amount of plant material consumed by wildfowl in saltmarsh and seagrass beds range from 1 - 50 percent (Raffaelli & Hawkins 1999). For example, the brent goose (Branta bernicla) grazes Puccinellia maritima and Aster tripolium in high marsh at end of winter, while white fronted geese feed on Agrostis stolonifera and Puccinellia maritima. The shelduck Tadorna tadorna feeds extensively on Hydrobia ulvae.

Saltmarsh also support large numbers of small birds such as linnets and greenfinch, starlings, pipits and wagtails, feeding on insects and seeds, as well as gulls and birds of prey. Saltmarsh is also used for grazing by rabbits and livestock such as sheep, cattle and horses.

More detailed accounts of the saltmarsh ecosystems are provided by Ranwell (1972), Long & Mason (1983), Adam (1993 )and Packham & Willis (1997).

Seasonal change
Puccinellia maritima remain green over winter but ceases growth, then begins to grow in April, flowering by the end of July - August, although the exact time and duration vary, and growth is fastest in August to October (Gray & Scott, 1977). Filamentous algae, show considerable more temporal and seasonal variation than the vascular plants (see Polderman, 1979; Adam, 1993). In submergent Spartina -Salicornia saltmarsh in Norfolk, UK (Packham & Willis 1997) annelid numbers increased in spring and declined in June-July and increased again in late summer. Many species of insect have short life cycles and/or hibernate over winter as pupae or adults. The sand goby Pomatoschistus minutus entered the marsh in early summer, moved away in August -September but was abundant again in autumn (Packham & Willis 1997). 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.

Succession
Accretion of sediment and accumulation of plant material increases the height of the marsh and reduces tidal influence (with the exception of creeks). Reduced water-logging of the soil, and increased freshwater percolation reduces the salinity of the soil. As a result, plant communities change with shore height and frequency of submergence, eventually reaching effectively terrestrial l but coastal plant communities. Puccinellia maritima appear in pioneer communities and at the lower parts of the marsh as Puccinellia maritima - turf fucoid communities. Towards the mid marsh Puccinellia maritima dominated communities occur with increasing amounts of the Glaux maritima, Limonium vulgare, Plantago maritima and Armeria maritima sub-communities. In the high marsh, except in depressions or pans or the collapsed side of creeks, Puccinellia maritima becomes out-competed by Festuca rubra and Juncus maritimus communities where the soil salinity has decreased. Without intervention (e.g. grazing regimes) or occasional disruptive events (e.g. storms and floods) that erode areas of saltmarsh the saltmarsh communities would essentially move further offshore with the gradual accumulation of sediment.

Spatial complexity

• In areas subject to wave action, the extent of the saltmarsh may be limited to the level of the highest astronomical tides 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 in width 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 Estuary, Wales) 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
• Pioneer saltmarsh communities may be washed away by tides, currents and storms and appear patchy until the vegetation becomes re-established. For example, pioneer Puccinellia maritima communities develop from scattered plants into hummocks, which eventually give way to a continuous sward of Puccinellia dominated communities. The tops of the hummocks may develop a Puccinellia-Salicornia-Suaeda community (Rodwell, 2000).
• Saltmarsh are characterized by a network of creeks formed by freshwater runoff. Growth of pioneer plants on raised areas concentrates water flow into channels that form deepening creeks as the marsh develops. Depressions of salt pans form when areas are surrounded by vegetation or by the die back of an area of vegetation and subsequent erosion. These salt 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.
• Puccinellia dominated communities may colonize the high marsh in slumped creek banks, salt pans or disturbed soil but is otherwise restricted to areas below MHWN by competition with other halophytes, e.g. Festuca rubra.
• 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. Puccinellia maritima favours water logged soil and is out-competed in dryer soils.
• 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.

Community complexity

• Salicornia sp. and Atriplex sp. dominate around MHWN in 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 sp.
• Seventy two species of Bryophytes (mosses & liverworts) are found on British saltmarsh, especially Pottia heimii.
• Macrophytes may predominate the lower saltmarsh, e.g. dwarf Fucus vesiculosus and Ascophyllum nodosum ; Pelvetia canaliculata ecad 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, and mats of cyanobacteria and microphytobenthos may colonize the sediment surface.
• 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. In the lower marsh other infauna include bivalves Mya arenaria, Cerastoderma edule and Macoma baltica.
• The intertidal collembolan Anurida maritima may be confined to the transition zone from mudflat to marsh.
• Several intertidal insects may live in burrows in the sediment, closing off the entrances at high tide, e.g. the beetle Bledius spectabilis, while other insects such as aphids live attached to the stems of vascular plants.
• The plant roots support nematodes, oligochaetes and fungi.
• Salt pans have highly variable salinities from hypersaline after evaporation to hyposaline after rain and support a more marine fauna of Littorina saxatilis, the isopod Sphaeroma sp. and shore crabs Carcinus maenas.
• Epibenthic fish are restricted to pools and creeks at low tide but may feed over a wider area at high tide e.g. Pomatoschistus minutus (sand goby) and Gasterosteus aculeatus (three-spined stickleback).
• Few birds species nest on saltmarsh due to flood risk; shelduck use saltmarsh for rearing young but nest elsewhere. Other bird species nest in middle to higher saltmarsh, e.g. redshank, common terns, skylark, meadow pipit, reed bunting and black-headed gulls.

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 g C /m²/ year (Mann, 1982 cited in Raffaelli & Hawkins, 1999).

Puccinellia maritima is perennial and dioecious, flowering towards the end of July / August. Flowers are wind pollinated and seedlings large enough to survive have been reported in the field by autumn. Seeds germinate on moist surfaces within 14 days of being shed, with maximum germination occurring at 10 °C although germination will occur slowly at lower temperatures (e.g. <2 °C) (Gray & Scott, 1977). Most seed fall within a few centimetres of the parent plant but they are probably dispersed by the tides. Seedling survival depends on successful rooting before the next high spring tide. However, as a pioneer community, it establishes by the rooting of vegetative fragments generated by grazing by wildfowl or livestock and dispersed by the tides.

Plants of the genus Salicornia, Atriplex and Suaeda are annuals, flowers carried in the shoot and have extensive seed banks that persist for more than a year. Plants of these species need 2-3 days without flooding to root effectively. Filamentous algae and cyanobacteria produce enormous numbers of spores, which are carried by the tides and would probably recolonize new habitat within a year.

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). However, polychaetes probably recolonize habitats by a mixture of migration (swimming) and passive transport and thought to be rapid in some species, e.g. Arenicola marina and Hediste diversicolor. Nematodes are ubiquitous are probably colonize by a mixture of larval settlement, and active and passive transport of adults and juveniles. Marine bivalves such as Macoma baltica, Mya arenaria and Cerastoderma edule have sporadic and unpredictable recruitment via settling pelagic larvae or passive bedload transport of juveniles together with significant larval and post-settlement mortality. While a single recruitment event may re-establish the population within a year and longer period (e.g. up to 5 years ) has been suggested for recovery (see individual reviews).

Most insects have short life cycles and hibernate over winter as dormant stages such as eggs or pupae. Although, many of the associated insects have the ability to fly, many intertidal species exhibit partial or complete winglessness. However, the later species are capable of dispersal over considerable distances and a wide area by floating on the incoming tide, e.g. the root aphid Pemphigus trehernei (Treherne & Foster, 1979). Most insect species are not closely associated with Puccinellia maritima communities an would probably recolonize available habitat from the surrounding area. Similarly, mobile species such as shrimp, crabs and shore fish species are not closely associated with the Puccinellia maritima communities and would occupy new habitat by migration from other habitats.

Gray & Scott (1969) reported that Puccinellia maritima established 5 -30% cover in bare areas left by turf cutting within 1-2 years. Packham & Willis (1997) reported that Puccinellia maritima formed radially spreading hummocks (clonal growth) and that in the newly expanding Cefni Marsh, Anglesey, roughly circular patches of 120-150cm in diameter formed in the newly developed Puccinellia community with 3 -4 years.

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, e.g. Puccinellia maritima showed the greatest abundance after 4 years after water-logging, 6-10 year chemical destruction by herbicides), 5-7 years after changes in tidal regime.

Rodwell (2000) noted that Puccinellia maritima colonizes ruts left by cars or livestock. Glaux maritima and Limonium vulgare sub-communities representing further stages in succession form Puccinellia maritima and would probably take longer to establish.

Recolonization of the lower marsh communities by marine infauna would probably be rapid (ca <5 years). The insect fauna would probably recolonize recovered plant communities in the mid to high marsh rapidly but avoid pioneer communities until they had built up sufficient height, which could itself take many years.

None entered.