Influence of zinc on lotic plants: I. Tolerance of Hormidium species to zinc

Field and laboratory studies have been carried out on the tolerance to zinc of the filamentous green algae Hormidium rivulare Kütz., H. flaccidum Kütz. and H. fluitans (Gay) Heering growing in flowing waters. Forty‐seven different sites in England, France, Germany and Ireland were included, the majority of them chosen to represent examples of zinc pollution resulting from past or present mining activities. All three species were widespread both in waters free of zinc pollution and waters with zinc pollution, and H. rivulare and H. flaccidum were frequent at a site with the highest zinc level included in the survey (mean of 22.8 mg l −1 , maximum of 30.2 mg l −1 Zn passing through a filter). At sites with higher zinc levels, populations showed increased resistance to zinc of the filamentous green algae Hormidium rivulare Kütz., H. flaccidum creased resistance being largely, if not entirely, the result of genetic adaptation. An increased frequency of geniculations can occur both as an environmental and a genetic response to very high zinc levels. The application of various statistical analyses to the data suggests that two groups of chemical factors may be influencing the toxicity of zinc in the field. The first group which appears to increase the toxicity of zinc includes cadmium and lead, while the second group, which decreases the toxicity of zinc, includes magnesium, calcium and various hardness factors. As the occurrence of zinc, cadmium and lead in the field are all strongly intercorrelated, it is not possible to distinguish clearly the effects of one from the other, although the data do suggest that cadmium is more likely than lead to influence zinc toxicity. The ‘amelioration’ factors are also strongly intercorrelated in the field, so it is difficult to comment on their relative importance. There is some indication for H. rivulare at least, that an increase in PO 4 ‐P and a decrease in pH may also reduce zinc toxicity.