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We subjected a unique set of high-quality paleoecological data to statistical modeling to examine if the biological richness and evenness of freshwater diatom communities in the Falun area, a historical copper (Cu) mining region in central Sweden, was negatively influenced by 1000 years of metal exposure. Contrary to ecotoxicological predictions, we found no negative relation between biodiversity and the sedimentary concentrations of eight metals. Strikingly, our analysis listed metals (Co, Fe, Cu, Zn, Cd, Pb) or the fractional land cover of cultivated crops, meadow, and herbs indicating land disturbance as potentially promoting biodiversity. However, correlation between metal- and land-cover trends prevented concluding which of these two covariate types positively affected biodiversity. Because historical aqueous metal concentrations--inferred from solid-water partitioning--approached experimental toxicity thresholds for freshwater algae, positive effects of metal mining on biodiversity are unlikely. Instead, the positive relationship between biodiversity and historical land-cover change can be explained by the increasing proportion of opportunistic species when anthropogenic disturbance intensifies. Our analysis illustrates that focusing on the direct toxic effects of metals alone may yield inaccurate environmental assessments on time scales relevant for biodiversity conservation.

The aims of the present study were to investigate (1) whether cladocerans living in soft water (operationally defined hardness < 10 mg CaCO(3)/L) are intrinsically more sensitive to Ni than cladocerans living in hard water (operationally defined hardness > 25 mg CaCO(3)/L) and (2) whether a single bioavailability model can be used to predict the protective effect of water hardness on the toxicity of Ni to cladocerans in both soft and hard water. To address these research questions, acute and chronic bioassays were conducted with 10 different cladoceran species collected in soft and hard water lakes in Sweden. Soft water organisms were tested in a 'soft' and a 'moderately hard' test water (nominal hardness = 6.25 and 16.3 mg CaCO(3)/L, respectively). Hard water organisms were tested in a 'moderately hard' and a 'hard' test water (nominal hardness = 16.3 and 43.4 mg CaCO(3)/L, respectively). The results of the toxicity tests in the 'moderately hard' test water revealed no significant differences between the intrinsic sensitivity of soft versus hard water organisms. Modeling exercises indicated that water hardness significantly reduced Ni toxicity to both the soft and the hard water organisms tested. Although predictions of chronic toxicity were sufficiently accurate using the same logK(CaBL) and logK(MgBL) (i.e. the model parameters describing the effect of hardness) for all organisms under consideration, predictions of acute toxicity were significantly more accurate when separate logK(CaBL) and logK(MgBL) values were derived for the soft and the hard water organisms tested. This is due to the fact that the relative decrease of acute Ni toxicity to soft water organisms in 'moderately hard' compared to 'soft' test water was significantly higher than for hard water organisms in 'hard' compared to 'moderately hard' test water.