The properties of saltmarshes generally depend on the specific composition of the vegetation, and factors that affect the vegetation community of a marsh can affect the value of the marsh to birds and to the bio-economy of the estuarine system. There is much still not known about the ecology of saltmarshes, the biology of the plant and animal species, and estuarine ecosystems in general, and consequently conclusions about the potential effects of global warming on saltmarshes must be speculative. Global warming can affect saltmarshes in two broad ways, through change in the climate and by sea-level rise.
Bertness and Pennings (2000) argued that the zonation of saltmarsh plants might be influenced by nutrient enrichment (eutrophication) and by climate. Climate (temperature) affects the rates of biological and chemical processes in saltmarshes, including photosynthesis, transpiration, decomposition, nutrient cycling and the accumulation or organic matter, all of which, together with the direct effect of temperature, may affect plant distributions. Bertness and Pennings (2000) suggested that climate plays a major role in saltmarsh community structure by changing soil salinity. Climate change may increase the rate of evaporation on the soil surface and hence increase salt concentration, or by increasing the rate of precipitation reduce the salinity of the soil. High salinities are usually found at mid marsh elevations, because lower soils are more frequently flushed by the tide and higher elevations are dominated more by rain and less by tidal inundation. High soil salinities may lead to the death of plants and the formation of salt pans, areas of bare mud that may, or may not, retain water. In both cases the habitat is unsuitable for further colonization because the salinity remains high and the pans remain or increase in size as these areas of unprotected sediment are more susceptible to physical erosion than the surrounding vegetated marsh. Salt pans may be colonized by invertebrates usually associated with the mudflats and provide sheltered feeding areas for waders such as Common Snipe.
The productivity of marine plants is generally nitrogen limited (Valiela 1984), and addition of nitrogen, through eutrophication or by increased precipitation caused by climate change, can increase the productivity of a marsh and increase the food value of plants to herbivores (Teal & Howes 2000). Increased precipitation, one forecast effect of climate change, may increase the supply of sediment brought to the marsh by rivers and increase the rate of accretion of the marsh surface, allowing the marsh surface to keep pace with accelerating sea-level rise (see below). Increased precipitation, which lessens the feeding efficiency of sight-feeders, such as plovers, by disturbing the sediment surface, may reduce invertebrate mortality rates and lead to an increase in herbivory/bioturbation. Daborn et al. (1993) described a trophic cascade, in which predation by the Semipalmated Sandpiper Calidris pusilla on Corophium volutator was responsible for an increase in sediment stability by allowing microphytobenthos (otherwise consumed by Corophium) to proliferate. Herbivory and bioturbation by invertebrates, particularly the ragworm Nereis diversicolor, are responsible for some of the extensive loss of marshes in southeast England (Hughes 2001, Paramor & Hughes 2004). Increased temperatures could also increase the activity of invertebrates and by changing their abundances and distributions could affect the rate of herbivory and bioturbation, contributing further to the loss of some saltmarshes (see below).
One response of saltmarshes to sea-level rise is a landward migration, such that the vegetation zonation is maintained relative to sea-level. If this upward progression is prevented by the presence of sea walls the marsh is said to be squeezed, and ultimately could disappear. As sea-level rises the lower limits to the potential niche of each plant species moves upward and the expectation is that the upper species will disappear first and the pioneer zone species, those most able to cope with inundation, will disappear last. However, this progressive loss of species will not necessarily occur at a rate proportional to the rate of rise in sea-level. For example, in a mature flat marsh dominated by Puccinellia, such as at Tollesbury, a small rise in sea-level across the critical threshold of the lower limit of its potential niche would cause much of the marsh vegetation to revert to one dominated by pioneer zone species. This would be detrimental to birds as the short saltmarsh grass preferred by geese and Eurasian Wigeon would be replaced by less palatable, and less productive, pioneer zone species.
Coastal squeeze will reduce the total area of saltmarsh, reduce primary productivity and reduce the time that is available to birds for feeding, roosting and nesting. For example, Brent Geese are relatively small and need to feed for a greater proportion of the time than larger geese. Consequently, they depend more than other species on the higher saltmarsh vegetation that is available for more of each tidal cycle. Brent Geese need to feed at night, particularly following warm days when a larger proportion of the day was spent in non-feeding activities, and on cold nights (Lane & Hassall 1996). Thus increasing temperatures associated with climate change could cause the birds to change their behaviour to depend to a greater extent on the marsh vegetation that could be available for less time because of sea-level rise.
Successful rearing of young from nests on saltmarshes will decline as progressively less of the available vegetation will remain above the spring tides in the summer months. Birds are capable of moving large quantities of nutrients from their feeding grounds, mudflats and terrestrial sites, to their roosting sites. Post et al. (1998) estimated that geese were responsible for 40% and 75% of the imported nitrogen and phosphorous, respectively, into a freshwater wetland. The significance of the defecation of roosting waders and wildfowl on nutrient enrichment of saltmarsh vegetation is likely to be less than this freshwater example, but a decline in primary productivity from this source is one potential consequence of coastal squeeze.
There could also be more subtle but important consequences for the ecology of the marsh and the surrounding estuary, as plant species within saltmarshes have different properties, including primary productivity, ability to accrete and bind sediment, and their effects on nutrient cycling. In general, primary productivity in marine and estuarine ecosystems, including saltmarshes, is nitrogen limited (see above), and one of the important ecosystem processes that occurs within saltmarshes is nitrogen cycling. Thomas and Christian (2001) compared the ability of different plant communities within a saltmarsh to cycle nitrogen in the context of rising sea-levels. They concluded that under coastal squeeze, not only will there be a reduction in total marsh area, but the marsh will cycle less nitrogen per unit area because the high marsh will decrease in size relative to the low marsh.
Sea-level rise may reduce the intertidal area of mudflat within an estuary, especially in an estuary constrained by sea walls. However, mudflat area per se may be unimportant for birds. What is important is the biomass and availability of invertebrate food, which depend ultimately on primary productivity elsewhere, and the time the mudflat is exposed by the tide and available to birds. Thus sea-level rise could carry a double problem for waders, a reduction in saltmarsh productivity and a decline in the time their potential food is available. The outcome is a reduction in the carrying capacity of the estuary for birds.
One potential consequence of sea-level rise is the construction of sea walls behind saltmarshes where currently they do not exist. This could lead to several long-term consequences, including those associated with coastal squeeze, and they may lead to changes in the abundance and distribution of plants. Sea walls reflect wave energy back onto the marsh, increasing the physical erosion of the vegetation, and this perhaps together with an increase in the formation of erodable salt pans may be one reason why many marshes of southeast England are characterized by the presence of deep basins close to the sea walls (Fig. 3). Sea walls separate saltmarshes from the adjacent terrestrial habitats and one consequence of this is the lack of runoff or percolation of groundwater. This water is of relatively low salinity, which may have led to an increase in salinity of the upper and mid marsh soils, thereby contributing to the formation of salt pans (see above).
Where saltmarshes are still in contact with terrestrial communities they may intercept nitrogen in the groundwater (especially in fertilized arable areas), increasing productivity and acting as a buffer between the land and the estuary. Thus these saltmarshes may have a beneficial effect on, for example, intertidal eelgrasses (Zostera spp.), which are affected negatively by enriched nitrogen concentrations (Touchette & Burkholder 2000, Valiela et al. 2000). In the agricultural estuaries of southeast England the sea walls may have reduced this buffering capacity, as groundwater usually collects in borrow dykes inside the sea walls and flows to the estuary through sluices protected on their seaward side by simple flap-valves. This flow occurs mostly on the ebbing tide and the nutrients do not reach the marsh vegetation immediately, if at all, and when they do, on subsequent rising tides, they will be much diluted. Instead, the water flows onto the mudflats where its nutrients may continue to contribute to the demise of eelgrasses, with concomitant effects on system productivity, sediment stability and direct effects on herbivorous birds such as Brent Geese and Eurasian Wigeon. However, nutrient enrichment may promote the growth of opportunistic green algae on the mud/sandflats (see Raffaelli 1999 for a review). The growth of these algae may also be promoted by climate change, as indicated by the fact that in the Ythan Estuary of northeast Scotland macroalgal blooms were correlated with higher spring temperatures (Raffaelli 1999). Although these algae increase system productivity and are consumed by herbivorous birds, algal mats have negative effects on mudflat invertebrates, particularly Corophium, a common prey item of waders and fish. When algal mats were common, the food intake of Common Redshank, Bar-tailed Godwit Limosa lapponica and Eurasian Curlew Numenius arquata declined (Raffaelli 1999).