An account is given of the water chemistry and photosynthetic flora of waters in England with a pH value of 3 0 or less. Of the fifteen sites found, thirteen were associated with coal-mining, one with a barytes mine and one was an industrial effluent. Flowing waters predominated, but in some cases pools in direct contact with the streams were also present. All waters carried high levels of heavy metals and silicate, and most carried moderately high levels of phosphate and combined inorganic nitrogen. The total flora consisted of twenty-four algae, two mosses and two flowering plants. Of these, three algae and one flowering plant were found in pools only. Euglena mutabilis was both the most widespread species, and often also the most abundant. The number of species present in a reach showed a positive correlation with pH during late summer, and a negative correlation with total acidity in winter. Only four of the algal species found in England are certainly included in the U.S.A. literature. Some, but not all, the apparent differences between the two floras are very probably due simply to taxonomic problems.* One further coal-mining site (Dovwgang: NY 777428) has been found since completion of this study.
Five species of algae (Chlamydomonas applanata var. acidophila, Euglena mutabilis, Gloeochrysis turfosa, Horrnidium rivulare, Stichococcus bacillaris) were isolated from a stream at pH 2"6-3'1, and their laboratory growth studied. Growth of all species could occur at pH values lower than those from which they were isolated, the lowermost limits being quite similar to those recorded for the particular species growing anywhere in England. Morphological differences were apparent with all five species at the lowermost pH values. These took place with Stichococcus bacillaris at pH values at which there was little reduction in growth rate, but with the other species obvious differences in morphology were correlated with a marked reduction in growth rate below the optimum rate. At the uppermost pH value tested, however, no obvious morphological differences were apparent. The effect of including 10~o stream water in the medium was rather similar for all five species. No influence on growth rate was detectable at the lower pH values, but higher pH values led to a decrease in growth rate as compared with that found in medium lacking stream water.
The toxicity of zinc to a population of Hormidium rivulare isolated from an acid mine drainage was shown to be least at the optimum pH range for the growth of the alga, pH 3.5-4.0; toxicity increases markedly at higher pH values. Calcium clearly antagonizes the toxicity of zinc. Populations of H. rivulare isolated from higher pH values and which are resistant to zinc, are also especially resistant to low pH values, although they are unlikely ever to encounter such values naturally. Nevertheless raised levels of calcium bring about only a slight improvement of growth at very low pH values in the absence of zinc, so the mechanisms of pH and zinc tolerance are not the same. Although the acid stream population grows in the field in an environment with rather similar levels of zinc and copper, copper is less toxic than zinc at pH 3.5, but much more toxic than zinc at pH 6.0.
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