Short-term toxicity of zinc to microbenthic algae and bacteria in a metal polluted stream

The response to different levels of metal pollution by assemblages of microbenthic algae and bacteria was investigated in a lowland stream. Colonized glass-discs and samples of natural assemblages on coarse sand were used to test zinc tolerance. Tolerance was characterized by measuring inhibition of 14C-incorporation in microalgae and inhibition of 3H-thymidine incorporation in bacteria. Bacterial assemblages on both glass and sand from a high zinc and cadmium site were resistant to zinc when compared to populations from a cleaner upstream site. Algae from the strongly polluted site were only slightly affected by the highest test concentrations of zinc, in contrast to other algal communities. A change towards metal tolerant bacteria and algae was indicated and seems to occur despite the strong binding of metals by precipitates of iron hydroxide and organic deposits in the biofilm. The observations were compared with published data on short-term and long term zinc toxicity in algae and bacteria. The gradual changes in micro- and macrobenthos under high metal loads is discussed.