Calibrating Predicted Mixture Toxic Pressure to Observed Biodiversity Loss in Aquatic Ecosystems

Unlike practices in applied ecology, assessing the impact of chemical pollution on biodiversity depends on species sensitivity data from laboratory toxicity effect tests. There are ~12,000 chemicals with such data, enabling quantification of a metric that characterizes the magnitude of the toxic pressure of chemical mixtures on aquatic organisms. However, the calibration between this lab-based metric and biodiversity effects in the field is lacking. To address this gap, we calibrated both. We quantified mixture toxic pressure levels from extensive water quality monitoring data across 1286 sampling sites and expressed it as multi-substance potentially affected fraction of species (msPAF). We furthermore quantified species abundance and richness loss for those sites. Calibration of both yielded that the observed potentially disappeared fraction of species (PDF) can be quantified from msPAF as biodiversity impact metric. Species abundance and richness generally declined with increasing toxic pressure, and a near 1:1 PAF-to-PDF relationship was derived. Both metrics are key in regulatory chemical policies and comparative biodiversity impact assessments, with PDF also widely used for biodiversity footprinting to assess species loss. Our results imply that the lab-based mixture toxic pressure metric can roughly be interpreted in terms of species loss under field conditions, that assumed regulatory "safe concentrations" may not fully protect exposed species assemblages, and that comparative biodiversity impact assessments can be made based on mixture toxic pressure metrics. These outcomes are highly relevant for biodiversity protection, and support the transition toward a "safe chemical economy" by enabling the design of compounds and products with lower environmental impacts.