Flood & coastal defence • Pollution control & water quality • Biodiversity & conservation importance • Palaeoecology & Archaeology • Designations & conservation management

Pollution control and water quality

Several studies have demonstrated that saltmarsh can act as a sink for a number of compounds that are considered as pollutants, including herbicides, pesticides, organochlorines, polychlorinated biphenyls and heavy metals. This is likely to be of increasing significance under the Water Framework Directive and be an important driver for future saltmarsh management. The presence of these pollutants within estuarine and coastal waters can be largely attributed to human activity and their continued persistence in water and sediments can pose an environmental risk (e.g. contamination of shellfish stocks in estuaries). A significant proportion of these pollutants are adsorbed onto sediment particles, which become deposited on saltmarsh surfaces and thus are ‘removed’ from, albeit temporarily, aquatic systems. The combination of adsorption and the burial of pollutants under accreting sediment effectively in-activates these substances, making them unavailable, over time, to organisms and thus reducing the potentially toxic effects of bioaccumulation. As an example, tributyl tin (TBT) has a half-life period in the order of tens of years and burial of sediment contaminated with this substance over this time period can reduce loadings within estuary systems.

However, although these contaminants may be tied up in saltmarsh sediments for relatively long periods of time, shifts in the dynamics of processes can lead to the remobilisation of sediments. Cyclical patterns of erosion and accretion may, therefore, lead to the release and re-deposition of pollutants within a system until (and if) the material is deposited in longer term and system-removed sediment sinks (e.g. marine basins). Nevertheless, it should be stressed that deposition within estuarine systems, of which saltmarsh represents one of main depositional sinks, provides one of the key mechanisms for the removal and natural remediation of potentially harmful substances on a human timescale.

Nutrient cycling within saltmarshes can also have a significant effect on coastal and estuarine water quality. In this respect, healthy, functional saltmarsh habitat may have an important role to play in the control of nutrients which are important in determining water quality and, ultimately, human health. Saltmarshes within estuarine and open coast systems contribute to the cycling and availability of several key elements that are crucial in regulating biological productivity. For example, studies in France, England and The Netherlands, have shown a net export of dissolved nitrogen out of the saltmarshes (Boorman et al., 1994). The same study also showed net exports of phosphorous, albeit in much lower concentrations than nitrogen.

Generally, the release of nitrogen and phosphorous from the saltmarsh occurs during the processes of the decomposition of organic matter, although direct losses by the leaching of nutrients from live plant tissues can also occur. The amounts released are high enough to account for significant increases in the activity of the estuarine plankton community and are thus of potential significance for many other estuarine communities (Boorman, 2000).