The rapid eutrophication of Lake Winnipeg: Greening under global change

Nuisance blooms of heterocystous Cyanobacteria in Lake Winnipeg have nearly doubled in size since the mid 1990s. The increases are the result of a recent rapid increase in loading and concentration of phosphorus. The rapid increase in phosphorus is largely the result of two factors. The first factor is the result of rapidly increased livestock production and use of synthetic fertilizer in the Red River Valley, with smaller contributions of phosphorus from the city of Winnipeg and other human development in the Red and Winnipeg river basins. The second factor is the increased frequency and intensity of spring floods in the Red River watershed in recent years, which have greatly enhanced the transfer of phosphorus from the landscape to the lake, as well as slower increases in nitrogen. Because the low ratio of nitrogen to phosphorus in the increased inputs favors nitrogen fixing species of Cyanobacteria, these nuisance forms account for most of the increase in phytoplankton. Recovery of the lake will require reducing both agricultural and major urban sources of phosphorus and, if possible, the frequency and intensity of flooding in the Red River watershed. Flooding will be increasingly difficult to control if modeled predictions for increased precipitation under climate warming materialize. Even with targeted reductions in phosphorus inputs of 50% and measures to control flooding, recovery of the lake is expected to be slow because of phosphorus recycled from sediments and the climatic sensitivity of this shallow lake and the flooding of the Red River. (c) 2012 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.