Derivation of toxicity parameters from field data: analysis of lake zooplankton species responses to metals and acidity

The WHAM-FTOXβ model describes the toxic effects of mixtures of protons and metal cations towards biological species, using a set of intrinsic parameters for the cations (αH, αM*) and a sensitivity parameter (β) for each species. We applied the model to extensive water chemistry and zooplankton species occurrence data for four lakes contaminated with acidity and metals (Al, Ni, Cu, Zn) at Sudbury, Ontario, over the period 1973-2018, during which cation contamination declined, and zooplankton species numbers increased. Assuming that the appearance of a species resulted solely from decreases in water toxicity, and that αH and αM* values previously derived from laboratory toxicity test data could be applied in the field, we used the field data to estimate values of β for individual lake zooplankton species. Results for lake-species pairs with 20 or more species occurrences (from six samplings per year) were analysed. In most cases, the number of occurrences increased over time from zero to five or six per year, then remained at the high level. For a minority of pairs, occurrences per year increased initially, but subsequently declined, and so data only from the initial period were used to estimate β. The β values derived for the lake zooplankton are reasonably consistent with values derived from laboratory data for a range of other species. The findings support the application of WHAM-FTOXβ to describe toxic effects of mixtures of cations in the field, and the toxicity model might be combined with ecological theory to interpret natural population responses.