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Although the free ion activity model (FIAM) has been well-established in laboratory studies, there remains the need for field data in order to validate the applicability of this model in natural systems. The objective of this study was to investigate the response of copper and zinc accumulation in periphyton to short-term variations in metal concentration and speciation in freshwater. During heavy rain events, dissolved Cu in the Furtbach stream increased from 40 to 118 nM, while dissolved Zn increased from 45 to 147 nM due to the release of metals from contaminated sediments. Increases in free copper and free zinc ions in the water (from 10(-14) to 10(-11.5) M for Cu2+; from 1 to 15 nM for Zn2+) were observed during the onset of heavy rain events. Periphytic algae collected from artificial substrates had an intracellular copper content (0.2-2.8 micromol/g dry weight (dw)) that varied as a function of the exchangeable copper in the water (labile form) rather than the free Cu2+. Intracellular zinc content (1.5-8.0 micromol/g dw) was found to follow the same trend as the free zinc ion concentration. Adsorbed Cu and Zn on periphyton showed a very dynamic response to variations in dissolved metal concentration. Different concentrations of dissolved manganese during the two time periods may affect the accumulation of zinc and copper by competition for metal uptake.

The free ion activity model (FIAM) has already been confirmed under laboratory conditions for many trace metals but has still to be validated under natural conditions where the presence of natural organic ligands influences metal speciation and bioavailability. The goal of this study was to test if the FIAM is followed under natural conditions by measuring copper and zinc speciation as well as metal accumulation in periphyton. Periphyton was exposed in microcosms to natural river water with different added concentrations of copper (25-258 nM) or zinc (18-501 nM) and additions of a synthetic ligand (NTA). Free Cu2+ was in the range of 10(-16.5)-10(-14.5) M and Zn(2+) was in the range of 0.7-8.7 nM, as measured by competitive ligand exchange coupled with cathodic/anodic stripping voltammetry. Other metal complexes were either measured or computed. Bioaccumulation of zinc in periphyton appeared to be controlled by the free zinc ion concentration, confirming the FIAM. In contrast, bioaccumulation of copper was controlled by weakly complexed copper (including Cu2+ plus inorganic and weak organic complexes), which is in disagreement with the FIAM, and appears to be caused by limitation of copper diffusion due to very low free Cu2+ occurring in natural environments.

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