The US and Canada have agreed on targets for a 40% reduction of phosphorus discharges into Lake Erie, the second smallest of the five so-called Great Lakes that mostly separate the two countries. Since the 1990s, the lake has seen an increase in algal growth that has compromised water quality.

Among other things, the new targets designed to minimize the extent of low oxygen “dead zones” in the central basin of Lake Erie are expected to hold algae growth to levels consistent with healthy aquatic ecosystems and maintain algae biomass at levels that do not produce toxins threatening human or ecosystem health, said the heads of the US Environmental Protection Agency (EPA) and Canada’s agency for Environment and Climate Change.

With their 2012 the Great Lakes Water Quality Agreement, the US and Canada set the stage for the pact just concluded. Both committed to combat the growing algae threat and develop updated binational phosphorus reduction based on 2008 loading levels. A binational public consultation process was held in summer 2015 and final targets were established following what the environmental agencies said was widespread support for the draft targets and the target setting process.

Modeling experts used nine different computer simulation models to correlate changes in phosphorus levels with levels of algal growth in order to determine phosphorus load reduction targets.

The surface area of the five Great Lakes combined is roughly equal to the size of the UK, while the surface area of the entire basin, including the lakes and the land they drain, is about the size of the UK and France combined.