The climate benefits of improving water quality.

Eutrophication enhances emissions of greenhouse gases (GHGs) from surface waters. Policies designed to ameliorate eutrophication by limiting nutrient loadings to surface waters can reduce these GHG emissions and, in turn, reduce future climate damages (e.g., from heat stress, sea-level rise, etc.)-yet this benefit has not been considered in benefit-cost analyses of water quality policies. We address this gap by using a set of linked watershed, lake, and aquatic GHG models to estimate emission reductions from a large-scale nutrient management program in the America's largest estuary, the Chesapeake Bay. The modeling system predicts reductions in chlorophyll-a, total phosphorus, and GHG emission rates in waterbodies throughout the watershed, but those in the southern portion of the watershed are predicted to exhibit greater reductions than those in the north, likely due to strong climate (e.g., ice-cover duration) and land-cover gradients across the domain. We estimate climate benefits from changes in GHG emissions from these water bodies of over $300 million over the first 50 years of the program (2025-2075)-similar in magnitude to commonly quantified categories of water quality benefits. We then extrapolate our results to the third largest drainage basin in the world-the Mississippi-Atchafalaya River Basin-to estimate climate benefits of reduced GHG emissions from lakes and reservoirs in the basin resulting from a similarly stringent nutrient management policy. Our findings suggest that reductions in GHG emissions from nutrient management programs should not be overlooked when evaluating the societal benefits of such policies.