Long-term toxicity of zinc to bacteria and algae in periphyton communities from the river Gota Alv, based on a microcosm study

This study aims to clarify the relation between pollution-induced community tolerance (PICT), community structure and net production in periphyton communities exposed to environmentally realistic concentrations of zinc and to determine levels of no-effect (NEC). Therefore, periphyton communities from a relatively uncontaminated river (Gota Alv, Sweden) were exposed to zinc during 4 weeks in a flow-through aquaria system. PICT was estimated as the increase in EC50 for the short-term inhibition of photosynthesis and thymidine incorporation. Community structure was measured as species richness and Bray-Curtis similarity index, and biomass as dry weight and chlorophyll a. NEC values were estimated as the intercept between a regression line and a control base line. NEC values for biomass and biomass dependent variables were 0.12-0.42 mu M which implies that there may be many rivers affected by zinc. However, the PICT response and marked changes in community species composition were found only at much higher concentrations (9.7 mu M). We hypothesise that this discrepancy in effect concentrations between biomass-dependent variables and other structure-related variables (including PICT) is due to an interaction between zinc and phosphorus leading to nutrient depletion. An indirect toxic effect on biomass due to nutrient deficiency should not be detectable as an increased zinc tolerance. (C) 2000 Elsevier Science B.V. All rights reserved.