Parasites as biological indicators of sewage sludge dispersal
This investigation has demonstrated that levels of parasitism may provide a sensitive, biologically-based index of marine environmental pollution. Flatfish were initially selected as indicator hosts in field studies conducted at an 'accumulating' and two 'dispersing' sewage sludge dump sites in Scottish coastal waters, but the results indicated that they were unsuitable for monitoring sewage sludge dispersal. The common whelk, Buccinum undatum , was selected as an alternative host-parasite system in a seasonal study at the Garroch Head sewage sludge dump site in the Firth of Clyde, where marked spatial concentration gradients exist for a variety of trace contaminants in the sediments. Buccinum is first intermediate host to several species of larval digenean parasites. Parasite prevalence in Buccinum declined significantly along the gradient of increasing contamination of the sediments from approx. 15&'37 at 3km north of the dump to approx. 2&'37 on its periphery. At the reference site approx. 20&'37 of Buccinum were parasitised. Site, seasonal, sex and parasite effects on growth and the effects of exposure to sewage sludge on the age-prevalence relationship and host response to infection were examined. The age structures of Buccinum populations were used to compare mortalities due to fishing, parasitic infection and proximity to the dump site. Patterns in parasite prevalence recorded at the dump site were not correlated with any natural environmental or host related factors that were examined. The most commonly recorded parasite of Buccinum was Zoogonoides viviparus . Experimental studies demonstrated that the free-living miracidium and cercaria of Z.viviparus were the life-cycle stages that were most susceptible to sewage sludge. There was no evidence of sewage sludge toxicity to the parasitic stages within intermediate or flatfish definitive hosts. However, the survival of Baccinum was reduced by parasitism and experimental exposure to sewage sludge, and in a combination these two factors acted synergistically. The gradient in parasitism in Buccinum at the Garroch Head dump site is considered to result primarily from the toxic effects of trace metals on the miracidium, reducing parasite transmission to the molluscan host. The Buccinum -parasitesystem may therefore provide a sensitive and valuable index for monitoring the dispersal of these contaminants around the Garroch Head sewage sludge dump site. The potential role for a parasite-based index in marine pollution monitoring studies is discussed in the light of this investigation.