Resistance to zinc ofStigeoclonium tenuein the field and the laboratory

Field and laboratory studies have been carried out on the tolerance to zinc of Stigeoclonium tenue growing in flowing waters. Thirty-four different sites in England, France and Germany were included, the majority of them being chosen to represent examples of zinc pollution resulting from past or present mining activities. S. tenue was sometimes abundant in zinc polluted waters, and was found at sites with up to 20·0 mg l-1 of zinc capable of passing through a filter. At sites with field mean zinc levels of about 0·2 mg l-1 and above, populations show increased resistance to zinc in comparison with populations from sites with lower zinc levels, this increased resistance being largely, if not entirely, the result of genetic adaptation. Assays of populations from sites with high calcium levels suggest that these are less tolerant of a particular level of zinc than are populations from sites with low calcium levels. Levels of zinc bringing about a marked reduction in total growth during assay have a relatively greater effect on the erect part of the thallus than on the basal system. Nevertheless in the field S. tenue was restricted to a basal growth form only at the site with the highest level of zinc.