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

Abstract

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.

Keywords: biodiversity loss; ecosystem services; ecotoxicity; life cycle impact assessment; mixture toxic pressure; risk assessment; species richness.

FIGURE 1

Overview of the workflow followed in compiling, harmonizing, and analyzing field monitoring data for the Netherlands and for various separate waterboards in order to characterize the calibration between mixture toxic pressure and structural biodiversity damage responses, that is, the multi‐substance potentially affected fraction to the observed potentially disappeared fraction of species (msPAF‐to‐PDF) calibration, with emphasis on characterizing (absence of) damage at the regulatory protective threshold exposure and at the impact working point (msPAF = x = 0.05 and x = 0.2, respectively). The msPAF to PDF relationship was derived primarily for species that are present in the cleanest sites and sensitive, but also (as robustness check) when considering all species and when considering invasive species.

FIGURE 2

Characterization of the chemical monitoring data for surface water samples of the Netherlands (left) and the relatively data‐rich Delfland waterboard (right) covering the years 2000–2015 for the data included in this study across 622 unique chemical substances. (a) The top row represents the proportion of identified chemical use categories, whereby the “others” category represents chemicals with unidentified use or metals. (b) The bottom row represents the proportions of metals and non‐metal chemicals. Percentages on top of each Delfland bar represent the proportion of Delfland data points relative to the total number of data points in the entire Netherlands for each bin.

FIGURE 3

Cumulative distributions of mixture toxic pressure variability (msPAF_EC10) across the Netherlands and the Delfland regional waterboard (Delfland data stretched over the same X‐axis range as the Whole‐NL data). The mixture toxic pressure level is shown as a “dot” per XY‐site (rank‐ordered based on the mean calculated value), and the vertical bars (error bars) indicate temporal variability of mixture toxic pressure in the dataset for those sites over the years 2000–2015.

FIGURE 4

The annual number of sampling data points for invertebrates records for 18 waterboards from the Netherlands (left) and the Delfland regional waterboard (right); n values indicate the total number (rows of records) of available data across the monitored years. The colors represent the considered taxonomic phyla. Data from 2000 to 2014 were used for the multi‐substance potentially affected fraction to the observed potentially disappeared fraction of species calibration study.

FIGURE 5

Illustration of trends in raw species abundance data with increasing mixture toxic pressure for selected species (9 out of 1217) in the dataset for the entire Netherlands from three taxonomic phyla with the highest number of data records. The X‐axis represents the mixture toxic pressure (msPAF_EC10), and the Y‐axis represents species‐specific abundance data (as a structural biodiversity metric). Sparse or absent dots in the upper right corner indicate that mixture toxic pressure acts as a pressure that limits species abundance. The colors represent different taxonomic phyla.

FIGURE 6

(a) Trends in absolute species richness for all observed species with increasing mixture toxic pressure for the Netherlands (left) and the Delfland regional waterboard (right). Similar to species abundance (Figure 5), high X‐high/high Y values are scarce or absent, indicating that an increased mixture toxic pressure limits species richness. Each dot indicates the number of unique species counted at a particular site across the years, and n indicates the total number of unique sites. (b) Trend analysis of relative species richness change for only sensitive species, with indications for GAM‐interpolated values of the species richness at X = 0.0 (clean sites, blue printed value, and dot) and X = 0.05 (regulatory protective threshold, orange printed value, and dot) and X = 0.2 (impact working point, red printed value and dot). The mean species richness calculated from all sites of the Netherlands (left) and Delfland (right) is taken as a reference point, defining Y = 0.

FIGURE 7

Illustration of species richness patterns in the Netherlands (top) and in the Delfland regional waterboard (bottom), using box and violin plots, after binning the sites into five (almost equal number of samples per bin; left) or three toxic pressure groups (considering the regulatory protective threshold and the impact working point, defined by X = 0.05 and X = 0.2, respectively; right). Data analyses were made after excluding opportunistic species and species occurring in fewer than five sites. The box and violin plots highlight differences in sample densities, especially species numbers (μ) across bins. p‐values denote the significance of the differences between bins in posterior tests. The sites are categorized by mixture toxic pressure levels, from minimal (Group 1) to high mixture toxic pressure sites (Group 5), with box and violin plots indicating data distributions. Subfigure (a) shows the species richness patterns for 783 species in the Netherlands. Subfigure (b) displays the same for the Delfland regional waterboard. “μ” denotes mean species count, and “n” indicates the total number of sample sites in each mixture toxic pressure group. Groups are characterized by their specified mixture toxic pressure (msPAF_EC10)‐ranges (top left in each panel).

FIGURE 8

Deriving the multi‐substance potentially affected fraction to the observed potentially disappeared fraction of species (PAF‐to‐PDF) relationship based on the observed fraction of disappeared aquatic invertebrate species (mean Y) per mixture toxic pressure bin (X‐group) based on varying numbers of bin groups. The relative species richness values are interpolated from the data at X = 0, X = 0.05, and X = 0.2 and depicted with blue, orange, and red colors. In practical use, these estimated values allow us to characterize the species richness loss for the average, randomly selected site in a study area, given an ambient mixture toxic pressure. Grey bands represent the 95%‐confidence intervals for the Generalized Additive Model predictions. The confidence interval is not plotted beyond the mean mixture toxic pressure value of the highest bin.