Hydrological forcing of a recent trophic surge in Lake Winnipeg

► Phytoplankton surged and cyanobacteria became dominant in Lake Winnipeg in 1990s. ► Phosphorus concentration doubled from 1992 to 1999 and has remained high since. ► Phosphorus concentration in the major source, the Red River, is 2 × greater in floods. ► Increased runoff contributed more than anthropogenic loading to higher phosphorus. ► 85% of phosphorus from flooded, flat farm land is dissolved (highly bioavailable).