Elucidation of Environmental Factors that May have Caused the Decline in the Intermediate Hosts

In order to elucidate the possible causes for the declines in parasite diversity and in the numbers of intermediate hosts such as S. corneum and other sediment dwellers such as ammocoete larvae and Gammarus pulex the physicochemical factors that might be implicated were investigated.

The relatively low range of ammonia, nitrite, nitrate, phosphate and BOD values (Table 1) indicate that neither river is likely to suffer from eutrophication or organic pollution. To the contrary, the nutrient and organic loading may be beneficial in these acid waters as they facilitate the release of bases and hence deacidification during decomposition in the sediments (Davison, 1986,1987).

Changes in pH associated with changes in the volume of water are more likely to be harmful. The pH values, which varied between 5.5 and 7.7 and 4.8 and 6.9 in the Teifi and Pysgotwr, respectively, were inversely correlated with water level or volume (Table 1). However, there is evidence that in times of heavy spate the pH values often declined to values even lower than those given in Table 1 (Wade, verbal communication). It is well established that under these conditions the mobilisation of heavy metals, such as aluminium, zinc, lead and copper into the labile, ionic form will be favoured (Hutchinson and Sprague, 1986; Howells etal. 1990).

The results of analyses carried out on sediment cores in the River Teifi suggest that heavy metal toxicity may be involved in causing the decline in the distribution and abundance of intermediate hosts. Thus, according to the United States Environmental Protection Agency (US EPA) guidelines (Giesy and Hoke, 1990) for sediments the metal concentrations (Figure 24a-d) are indicative of moderate pollution in the case of copper and chromium (> 25 mg/kg dry weight) and of heavy zinc (> 200 mg/kg dry weight) and lead pollution (> 60 mg/kg dry weight). These concentrations also fall within the 'action levels' of 40 and 200 mg/kg for lead and zinc, respectively, according to the equilibrium partitioning approach of Webster and Ridgway (1994). They are also outside the limits of tolerance (250 and 800 mg/kg for lead and zinc, respectively) set by the Ontario Ministry of the Environment (Giesy and Hoke, 1990).

The levels of metal pollution found in the River Teifi sediments are comparable to those in the sediments of industrially polluted aquatic habitats. Thus, Che and Cheung (1998) found lead and zinc levels of 100-155 and 226-421 mg/kg, respectively, in the Mai Po marshes in Hong Kong whilst Bubb et al. (1991) found lead and zinc levels of 100-155 and 226-421 mg/kg, respectively, in the River Yare, Norfolk.

Heavy metals such as lead and zinc are readily accumulated by invertebrates and hence by their fish predators (Woodward et al., 1994). As low pH conditions favour the mobilisation of heavy metals, such as aluminium, zinc and lead, into the labile, ionic forms (Muniz, 1991) it might be expected that the rate of bioaccumulation and hence toxicity would be enhanced by increased acidity. However, the relationship between pH and metal toxicity is not simple. In the case of copper, for example, although the concentration of the Cu2+ ion, which is the major toxic species, increases with increase in H+ concentration its toxicity is reduced by competition from H+ at the active sites (O'Sullivan et al., 1989). It is also noteworthy that high H+ concentrations on their own can also be toxic to aquatic organisms as they disturb acid-base homeostasis and ionic regulation (Wood, 1987).

Although nothing specific can be said about the toxic effects of heavy metals in the sediments as the pore water concentrations are unknown some conclusions can be drawn from the chemical analysis of the water column (Table 1). The total hardness values for both the Pysgotwr and the Teifi are very low (2.5-5.1 mg 1_1 and 6.5-25.5 mg H, respectively). As a result Table 1 shows that the environmental quality standards (EQS) for aluminium, zinc, lead and copper (100, 8, 4, and 1 |ig 1_1, respectively) are often exceeded in these base-deficient rivers (Department of the Environment and Welsh Office, 1989).

The potential dangers to the biota of base-deficient water bodies from aluminium toxicity are well documented (Turpenny, 1989; Howells et al., 1990; Merret et al. 1991; Department of Water Affairs and Forestry, 1996). Toxicity due to aluminium is most likely to occur during episodic, spate conditions when toxic aluminium and H+ concentrations rise by an order of magnitude whilst Ca2+concentrations, which have an ameliorating effect, fall (Reader and Dempsey, 1989). It is most toxic at pH 6.5-5.0 when the hydroxy forms Al(OH)2+ and Al(OH)2+predominate but within that range the toxicity is independent of pH (Howells et al., 1990).

The pH values fall to 5.5 and 4.8 in the Teifi and Pysgotwr, respectively, under spate conditions (Table 1). It is probable that under these conditions the aluminium concentrations (Table 1) will exceed the target water quality range (5 ng I"1), the chronic effect value (10 |ig H) and the acute effect value (100 |ig

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