Tracking progress toward phosphorus in spring (reducing harmful algal blooms) metric: how it’s calculated

The U.S. and Canada adopted targets to reduce spring phosphorus loading by 40 percent from priority tributaries to control harmful algal blooms in the western Lake Erie basin. Progress is calculated by comparing the average reduction from 2008 levels over the last five years toward the 40 percent reduction target.

A gauge indicator all the way to the left would indicate no reduction in average spring phosphorus, while an indicator gauge pointed all the way to the right would represent an average 40% reduction in spring phosphorus over the last five years.

Sample gauges displaying progress toward goals

Reductions in both total phosphorus (TP) and soluble reactive phosphorus (SRP) are important to reducing harmful algal blooms. Phosphorus loads are influenced by the flow of the river, so another important indicator is the flow-weighted mean concentration, which normalizes for flow and can be used as a benchmark to track progress in load reductions. To provide a comprehensive view of progress toward spring phosphorus reduction, the progress dial represents an average of the reductions in spring TP loading, spring SRP loading, spring TP FWMC, and spring SRP FWMC. For tributaries that do not have targets established for all four of the metrics, only metrics with available targets are included in the average.

Progress is being tracked at three scales:

  • Lake Erie sub-basin: Gauge depicts phosphorus reductions from all Western Lake Erie Basin priority tributaries (phosphorus loads are summed and the average reduction is calculated from the summed phosphorus loads; phosphorus FWMCs are averaged and the average reduction is calculated from the average phosphorus concentrations)
  • State/provincial level: Gauge depicts phosphorus reductions from priority tributaries entirely or partially within individual jurisdiction (phosphorus loads are summed and the average reduction is calculated from the summed phosphorus loads; phosphorus FWMCs are averaged and the average reduction is calculated from the average phosphorus concentrations)
  • Priority tributary: Progress toward the 40% reduction target at each priority tributary is shown beneath the gauge