Industrialised and urbanised estuaries and coastlines receive effluent discharges which contain conservative contaminants, i.e. those with a long half-life, are likely to bioaccumulate (remain within the food-chain) and thus have a toxic effect (Clark, 1997). Such contaminants include heavy metals, both essential (e.g. copper, zinc) and toxic (e.g. mercury, cadmium), radionucleides and synthetic organic compounds (e.g. dieldrin and polychlorinated biphenyls). The lethal and sub-lethal effects of these pollutants vary according to the state and availability of the compound and the characteristics and organisms of the receiving systems. Some effects may be lethal, by removing individuals and species and thus leaving pollution tolerant and opportunistic species. Other effects may be sub-lethal, in affecting the functioning of organisms such as the reproduction, physiology, genetics and health, which will ultimately reduce the fitness for survival (Nedwell, 1997).

Sheltered, low-energy areas such as intertidal mudflats in enclosed bays or estuaries will be most susceptible to these pollutants as dispersion is low and the finer substrata in these areas will act as a sink (e.g. McLusky, 1982; Nedwell, 1997; Ahn et al, 1995; Somerfield et al, 1994). The pollutants will then enter the food chain and be accumulated by predators, as shown by the seasonal loading of heavy metals in tissues of wading birds in the Wash (Parslow, 1973). However, the industrialisation and urbanisation of these areas results in them receiving complex mixtures of pollutants. For example, intertidal areas in Southampton Water (Coughlan, 1979) and the Tees (Langslow, 1981) had reduced benthic communities through contamination by phenols, oil effluent, sulphides and nitrogen compounds.

In contrast to the low-energy areas, the higher-energy sedimentary biotopes are less likely to receive and/or retain these contaminants. The coarser sediments of exposed intertidal sandflats and the hydrodynamic characteristics, including high dispersion, at subtidal sandbanks dictates that there are few cases of severe pollution in these areas. However, chemical pollution of intertidal sands can occur and will remove elements of the fauna, for example effluent from a pharmaceutical plant created a degraded community in intertidal sands near Montrose (SEPA East, unpublished).

Next Section                     References