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Long-term declines in phosphorus export from forested catchments in south-central Ontario

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Total phosphorus (TP) levels in many Canadian Shield lakes in central Ontario have declined over recent decades, despite increases in human activity in most watersheds. To investigate the contribution of changes in catchment export to long-term declines in lake TP, we examined temporal and spatial patterns in TP concentrations and export (1980–1981to 2001–2002) across 11 subcatchments that drain into three lakes in which average ice-free TP levels have declined by approximately 35%. Annual stream export of TP decreased significantly by 30%–89% in eight of the 11 subcatchments, and decreases in export were driven by declines in TP concentration, not changes in stream flow. Annual average TP concentrations varied fivefold among adjacent subcatchments, and temporal patterns in annual average TP concentrations were poorly correlated. Seasonal patterns of TP concentration were most similar among streams in the spring (March–April–May), and TP export in the spring declined significantly in 10 of the 11 subcatchments. Because spring melt is the principal hydrologic event in these seasonally snow-covered basins, decreases in TP export during the spring were primarily responsible for declines observed in annual export. The drivers of changes in TP over time are unclear at this point but are the focus of current research.

Les concentrations de phosphore total (TP) dans plusieurs lacs du Bouclier canadien du centre de l’Ontario ont diminué au cours des dernières décennies, malgré l’augmentation des activités humaines dans la plupart des bassins versants. Afin d’étudier la contribution des changements dans l’exportation de TP des bassins versants aux déclins à long terme du TP lacustre, nous examinons les patrons temporels et spatiaux de concentrations et d’exportation de TP (1980–1981 à 2001–2002) dans 11 sous-bassins qui se déversent dans trois lacs dans lesquels les concentrations de TP durant la période sans glace ont chuté d’environ 35 %. L’exportation annuelle de TP dans les cours d’eau a décru significativement de 30–89 % dans huit des 11 sous-bassins et les déclins de l’exportation de TP s’expliquent par des diminutions des concentrations de TP et non par des changements de débit des cours d’eau. Les concentrations annuelles moyennes de TP varient par un facteur de cinq dans des sous-bassins adjacents et les patrons temporels des concentrations annuelles moyennes sont faiblement corrélés. Les patrons saisonniers des concentrations de TP entre les cours d’eau ont la similarité la plus forte au printemps (mars–avril–mai) et l’exportation de TP au printemps a diminué significativement dans 10 des 11 sous-bassins versants. Parce que le dégel printanier est l’événement hydrologique principal dans ces bassins versants couverts de neige en saison, la diminution de l’exportation de TP au printemps est la cause principale des déclins observés dans l’exportation annuelle. Le ou les facteurs explicatifs des changements de TP au cours du temps restent obscurs, mais font actuellement l’objet de recherches.

Document Type: Research Article

Publication date: 2009-10-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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