Changes in total phosphorus (TP), chlorophyll a (Chl a), and total nitrogen (TN) concentrations were monitored in a lake chain in northern Quebec to which sewage loading had been reduced by about 80% over 8 yr. Summer concentrations of TP and Chl a fell by 70 and 78%, respectively, in Lake Pearce, the uppermost lake most affected by eutrophication. Reductions in the two downstream lakes were 64 and 55% for TP and 45% for Chl a. The rapid response of the lakes to reduced sewage loading is related to the short residence time of water (60 d in winter to 2 d during snowmelt in Lake Pearce). Sediments in Lake Pearce showed thick (20–70 cm) accumulations of soft, organic-rich material (loss on ignition 10–50%), containing 4–20 mg P∙g−1; however, winter P release rates were small (about 0.2 mg P∙m−2∙d−1), possibly associated with the Fe-rich nature of the sediments. Although anoxic conditions still develop under the thick ice cover (1 m), the depth and intensity are less than before sewage reduction. Reductions in TP have been accompanied by increases in TN and TN:TP ratios (from < 10:1 to > 30:1), suggesting that there has been a shift from N to P limitation.
