Integrating Bioassessment and Ecological Risk Assessment: An Approach to Developing Numerical Water-Quality Criteria

Bioassessment is used worldwide to monitor aquatic health but is infrequently used with risk-assessment objectives, such as supporting the development of defensible, numerical water-quality criteria. To this end, we present a generalized approach for detecting potential ecological thresholds using assemblage-level attributes and a multimetric index (Index of Biological Integrity—IBI) as endpoints in response to numerical changes in water quality. To illustrate the approach, we used existing macroinvertebrate and surface-water total phosphorus (TP) datasets from an observed P gradient and a P-dosing experiment in wetlands of the south Florida coastal plain nutrient ecoregion. Ten assemblage attributes were identified as potential metrics using the observational data, and five were validated in the experiment. These five core metrics were subjected individually and as an aggregated Nutrient–IBI to nonparametric changepoint analysis (nCPA) to estimate cumulative probabilities of a threshold response to TP. Threshold responses were evident for all metrics and the IBI, and were repeatable through time. Results from the observed gradient indicated that a threshold was ≥50% probable between 12.6 and 19.4 μg/L TP for individual metrics and 14.8 μg/L TP for the IBI. Results from the P-dosing experiment revealed ≥50% probability of a response between 11.2 and 13.0 μg/L TP for the metrics and 12.3 μg/L TP for the IBI. Uncertainty analysis indicated a low (typically ≤5%) probability that an IBI threshold occurred at ≤ 10 μg/L TP, while there was ≥95% certainty that the threshold was ≤ 17 μg/L TP. The weight-of-evidence produced from these analyses implies that a TP concentration > 12–15 μg/L is likely to cause degradation of macroinvertebrate assemblage structure and function, a reflection of biological integrity, in the study area. This finding may assist in the development of a numerical water-quality criterion for TP in this ecoregion, and illustrates the utility of bioassessment to environmental decision-making.