More than 20 years of estimated internal phosphorus loading in polymictic, eutrophic Lake Winnipeg, Manitoba

This study represents the first attempt to estimate internal phosphorus (P) loading over many years in polymictic eutrophic Lake Winnipeg, Manitoba, North America's 7th largest lake. Using long-term data, internal load was quantified by several approaches in Lake Winnipeg's North and South Basins and the connecting Narrows. Internal P loading occurred almost every year in the summer and fall and perhaps under ice in the winter during the study period 1999-2012 and even before (probably as early as a decade before 1969). Several independent approaches (L-int_1, in situ estimates involving TP concentration changes, L-int_2, mass balance, and L-int_3, based on sediment P release rates and active release area) identified an appreciable loading from the sediments that is consistent with redox related processes. Mean gross internal load (L-int_2 and L-int_3) was about 1.3 times the external load for the whole lake in 1999-2012, while the net estimate (L-int_1) was 0.75 times. Internal loads varied between basins: South Basin contributed 18-23% to total internal load and the Narrows about 16-20% for approaches L-int_1 and L-int_3, while South Basin and Narrows combined contributed about 21% in the mass balance approach (L-int_2). North Basin contributed 57-79% to total internal load. Internal load from upstream sections probably affects the North Basin because of the relatively high flushing rate. The potential effect of internal load on phytoplankton and cyanobacterial biomass is highlighted by several significant positive correlations. Internal load can be expected to rise because forecasted regional climatic warming will increase sediment oxygen demand and P release. (C) 2015 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.