Effects of heavy metals on freshwater Chlorophyta
A study was carried out on the chemistry and vegetation of two streams containing elevated levels of heavy metals. In neither stream had the presence of these metals prevented the development of an algal flora, though species numbers were low in comparison with uncontaminated streams. The algal biomass was high in both streams, probably due to the lack of invertebrate grazers. Green algae were dominant. One of these streams, a highly calcareous mine effluent in Northern England (Durham code no. 0097), was studied throughout its annual cycle. Concentrations of heavy metals accumulated by the dominant algae (Mougeotia spp.) were equated with the physical and chemical properties of the water. Zn was supersaturated in the stream water (6.84 mg 1 (^-1) at pH 7.85); this was shown to be the major factor which influenced Zn accumulated by the algae. Samples from the other stream, a smelter tip seepage in South-East France (Durham code no. 3026), were available from an earlier visit. This stream contained extremely high levels of heavy metals (Zn = 3840 mg 1(^-1); Cd = 345 mg l(^-1)) and was dominated by the green alga Hormidium rivulare. Ten strains of green algae were isolated from these two streams and were shown to be resistant to Zn in the laboratory, probably as a result of genetic adaptation. Environmental factors which were likely to be affecting Zn toxicity were investigated for isolates of the dominant algae. Mg reduced Zn toxicity in both streams and may have an important role in the development of resistance by these algae. Field levels of Cd did not influence algal growth at field levels of Zn. The role of carboxylic acids in algal Zn resistance was investigated but could not be established. Accumulation studies suggest that Zn resistance by green algae involves internal detoxification of the metal and not exclusion.