Chesapeake Bay targets should count bioavailable loads, not just total mass
Chesapeake Bay progress is scored in total kilograms of nitrogen and phosphorus. The Bay responds to the forms algae can use.
On paper, the Chesapeake Bay is close to its phosphorus goal: the Bay Program’s models put it at 90% of the way there. In the water, the picture is different. The Bay meets less than 30% of its water quality standards, and the Program’s own monitoring showed phosphorus rising in four of the nine major rivers.
Part of the reason is what gets counted. Targets are set in total kilograms of nitrogen and phosphorus, and every kilogram counts the same. Algae do not see it that way. They grow fastest on dissolved forms and far more slowly on particles and slow-to-break-down organic matter. A farm practice can cut the total while the dissolved share rises, and the ledger records only the cut.
In this Viewpoint, Suzanne Dorsey and I propose that the Bay’s 2030 targets count dissolved nitrogen and phosphorus alongside the totals, as Lake Erie has done for phosphorus since 2016. The river monitoring stations already measure these forms, so no new monitoring is needed. Credits for farm practices should also reflect how water actually leaves a field: over the surface, through tile drains, or through ditches.
Air-quality rules made a similar shift decades ago, from total airborne particles to the fine particles (PM2.5) that do the harm. The point is not to stop counting mass, but to stop scoring with mass alone.