Comparison of Phosphorus Turnover Times in Northern Manitoba Reservoirs with Lakes of the Experimental Lakes Area

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Abstract
The turnover time, τ, of [32P]PO4was measured during July and September in a natural lake and two lakes in northern Manitoba that were undergoing impoundment in 1976. The τ values were 10–1000 times longer than those observed in the epilimnia and hypolimnia of two lakes being experimentally enriched in northwestern Ontario (Experimental Lakes Area) in the same summer. Algal populations in the Experimental Lakes Area exhibited short τ values over a large range of temperature and light values, which exceeded the ranges of temperatures and light intensities measured in northern Manitoba. In northern Manitoba the shortest τ values occurred in July when an independent indicator of sestonic P deficiency, the particulate alkaline phosphatase activity per unit weight ATP, measured moderate to severe P deficiency. In September in northern Manitoba, τ values were at least 10 times greater than in July and were among the highest reported in the literature. The 2-yr-old Notigi Reservoir had the highest τ values in both July and September, and it deviated strongly from the expected chlorophyll–phosphorus relation for north temperate lakes. These new reservoirs in northern Manitoba are not P limited and even the natural lakes may be only moderately P limited.

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