LAUSR

Abstract Cultural eutrophication of the Great Lakes resulted in actions to reduce phosphorus loading beginning in the 1970′s. Despite these measures, a resurgence of harmful algae blooms began in the 1990s. Non-point sources and changing agricultural practices are frequently cited as contributing to this resurgence. We examined changes in total phosphorus concentrations ([TP]) and several other water quality parameters from April through November from 1979 to 2011 using 56 streams throughout Ontario, including agricultural, undeveloped, urban, and mixed-use watersheds. Growing season median [TP] was not significantly different among stream types, but decreased in 32 of the 56 sites; only one urban site increased. There were weak differences among land uses; mixed sites typically had the highest [TP] and steepest declines. Soluble reactive P concentration [SRP] decreased at 12 sites, and increased at one agricultural and three urban sites. Where trends were significant, suspended solids typically decreased, while [Cl − ] and alkalinity typically increased. As adequate winter data were not available and there may have been changes in the seasonal distribution of high flow during the period we studied, the changes we observed in median growing season [TP] may not reflect changes in loading. However, seasonal discharge trends were not significant except for one site with a decreasing trend in spring discharge. Possible mechanisms for pervasive downward trends in TP include improved rural and urban land use practices, reduced acid precipitation, increased nitrogen availability in terrestrial ecosystems, and ongoing forest regrowth after initial clear-cutting.