Grazing • Access and amenity • Pollution • Coastal defence maintenance • Alternative uses

Pollution (cont)

Oil • Chemicals • Sewage and other enrichment • Litter

Chemical

Industrial effluents are potentially very damaging to the environment and often unseen. Their impact on saltmarsh vegetation and its associated fauna are little understood. Some studies from the US suggest that heavy metals may affect saltmarsh plants and the invertebrates that ingest contaminated material. However, there is little information on the way in which the saltmarsh acts as a sink for some of these toxins and whether they are then transported to the estuarine system.

In the Mersey Estuary, the deaths of large numbers of waders and other birds in 1979 provides an indication of possible effects where acute pollution has occurred. Here trialkyl lead appears to have built up in the food chain through uptake by the benthic fauna, which was subsequently eaten by the birds. Whilst these incidents are relatively rare, the fact that saltmarshes and their sediments accumulate heavy metals should be a cause for long-term concern. This is particularly important where rising sea level or potentially erosive forces are causing increased loss of saltmarsh and possible re-suspension of toxic substances.

Studies of these relationships, including the impact of pesticide and herbicide residues on the biological components of the marsh, have been very few. Work at Imperial College, London, on the herbicide Mecoprop® shows that rain soon after agricultural applications, may lead to the contamination of drainage ditches and subsequent transport of polluted water onto marsh surfaces. There is also some evidence that vegetated saltmarsh areas remain contaminated from one season to the next. As yet there is no direct link established with possible toxic effects on marsh flora.

Other agricultural chemicals, including herbicides such as atrazine and organo-chlorine insecticides, occur widely in the environment and have been detected in saltmarsh sediments (Scrimshaw et al., 1994). Later research by Scrimshaw et al. (1996) has shown that these substances are widely distributed in the sediments along the Essex coast, with higher levels in certain areas. Assessing the impact of these chemicals on the biota is, however, difficult as they only occur at very low levels with most of the data relating to the effects on plants and animals at higher concentrations.

Tributyltin (TBT) and its breakdown derivatives are another common polluter of saltmarsh sediments. TBT was formerly used extensively in anti-fouling paint before its harmful effects on marine invertebrates, particularly molluscs, became evident in the 1980’s (Bryan et al., 1986). Although TBT rapidly degrades in the water column (with a half life of days to weeks) it is much more persistent in anaerobic sediments (with a half life of tens of years). Although little research has been conducted on the effects of TBT on saltmarsh vegetation, there have been reports that it does affect the growth of some saltmarsh species (Boorman, unpublished data).