Abstract This study estimates the marginal contribution to environmental performance made by six separate macroinvertebrate metrics included in an additive index developed to measure wadeable stream condition. We model the metrics as outputs in a quadratic directional output distance function, which serves as an alternate measure of environmental performance, and then derive the marginal performance for each metric. We use the resulting estimates of marginal performance to develop a metric weighting scheme that incorporates each metric's marginal contribution to performance, at each site, and then construct a weighted version of the original index. The performance-weighted index provides a similar measure of overall performance in this application, while also adding new information on the relative importance of each of the index metrics. This represents a new application of existing methods to measure productivity, and is closely related to the environmental shadow price literature.