Sewage

Biocides: Tributyltin

Biocides: Ivermectin

Chronic impacts are those which have a long duration, on a scale of years. The effects of these impacts are often subtle but can, nevertheless, have severe consequences for biological communities. Any long term climatic change which results from human activity could cause chronic impacts on rocky shore communities as discussed in Chapter IV. Low intensity pollution and physical disturbance are the main sources of chronic impact on rocky shores. Shorelines are especially susceptible to the effects of chronic pollution since discharges often occur close to the shore or into rivers and the shallow water limits the potential for dilution of pollutants.

The most severe effects of sewage effluent discharge occur in semi-enclosed areas such as estuaries and sheltered bays. Effects on high-energy rocky shores are negligible, while effects on low- to medium-energy rocky shores can be more pronounced. Water movement limits the build up of particulates and prevents eutrophication. Thus, the ecological effects of large sewage outfalls may stretch to a few hundred metres while the effects of smaller discharges are usually confined to within about 10m of the pipe (Raffaelli and Hawkins, 1996). Such discharges can, for example, encourage the growth of ephemeral green algae in the affected area. Sewage outfalls can introduce plastics and other solids to the marine environment. These can disfigure large areas of shore, though their effects are mainly aesthetic. From 1998, the U.K. government will be enacting E.U. law to progressively reduce sewage discharge into the sea.

The toxic effects of tributyltin (TBT) on molluscs, especially the dogwhelk Nucella lapillus, are well documented (Bryan et al., 1986, 1987). TBT, an organotin, was extensively used in antifouling paints specifically to kill marine fouling organisms. Unsurprisingly, it therefore had an ecological impact. Many rocky shore communities have been and continue to be impacted by the sublethal effects of TBT and its breakdown products which can occur at concentrations of less than 1ng Sn l^-1 (Bryan et al., 1986). TBT is lipophilic in nature. Consequently it becomes concentrated in the water surface microlayer which is rich in lipids. It is this microlayer which washes over rocky shore organisms each time the tide rises and falls. Effects on settling larvae that also accumulate in the surface layer are less well known. Given the original purpose of TBT as antifouling agent such effects are likely to be considerable.

Many shallow coastal waters escape the pollution associated with busy harbours and industrialisation. However, the expansion of recreational boating exposed previously clean areas to the effects of TBT. The impact was greatest in areas with heavy boat traffic and close to marinas, where boat moorings and maintenance activities were concentrated.

Very low concentrations of TBT can lead to the condition known as imposex in dogwhelks. Imposex is the development of male sexual characteristics, including a penis and vas deferens in females. At concentrations of around 2-3ng Sn l-1, sterilisation of females can occur as the vas deferens blocks the genital pore, preventing egg capsules from being released. At higher concentrations, virtually all females are sterile (Bryan et al., 1987). This results in a change in the structure of dogwhelk populations. Since dogwhelks have no larval dispersal phase, local populations are sustained by their own fecundity. Therefore these populations become less abundant and increasingly dominated by adults as exposure to TBT increases.

Dogwhelks are an important predator on rocky shores and their decline might be expected to have profound effects on the rest of the community. Dogwhelks are less abundant on sheltered shores where their prey are less common (Spence et al., 1990). The main sources of TBT release (fish farms, harbours and marinas before legislation) are usually in sheltered sites. Thus the gradient of reduced abundance due to impact runs in the same direction as the gradient of reduced abundance due to habitat features. This fact might potentially mask subtle effects of TBT on dogwhelk populations. Dogwhelks are an important predator on rocky shores and might determine the timing of the switch from barnacle to algal domination in areas of these shore (Section III.C). TBT also affects mussels, an important space occupying species on rocky shores and may therefore have important effects on community structure. Evidence for community-wide effects is, however, limited (Hawkins et al., 1994).

The use of TBT paints on small boats was banned in the late 1980s. However, recovery has been impeded because dogwhelks are slow to recolonise areas which have suffered population decline. Furthermore, it is now recognised that TBT originating from larger vessels also has detrimental effects (Davies and Bailey, 1991).

Ivermectin is a pesticide which is used to control sea lice on farmed salmon. It is currently licensed to 15 salmon farms in Western Scotland. Ivermectin targets the neuromuscular system of invertebrates and is acutely toxic to lugworms (Thain et al., 1998), reducing activity at concentrations as low as 6ppb and causing death at 23ppb. Ivermectin can reduce the abundance of infaunal polychaetes (Black et al.,1997) and is also lethal to starfish and shrimps (‘No Free Lunch’, New Scientist, 7 February 1998). The effects of ivermectin on rocky shore communities remain to be studied. However, since the pesticide is toxic to a wide taxonomic range of invertebrates, its wider use could potentially impact rocky shore communities, either by reducing the competitive abilities of susceptible animals or by causing death.

Next Section                     References