Internal phosphorus load assessment of Lake Nairne, Charlevoix, Quebec
Cyanobacterial blooms have been recorded in Lake Nairne (Charlevoix, Quebec) since 2001. These blooms are a symptom of the lake’s eutrophication caused by nutrient enrichment. In 2005, a study suggested that a significant portion of Lake Nairne’s phosphorus loading originates from the lake’s bottom sediments. The goal of this study was to estimate Lake Nairne’s internal phosphorus loading so that local water management authorities could consider this phosphorus source in their lake restoration plan. The internal phosphorus load of Lake Nairne was estimated using two methods during the summer of 2010. First, phosphorus release rates were calculated based on the concentrations of two phosphorus fractions and organic matter in the sediments. Release rates were then multiplied by an anoxic factor, which represents the active release area and period. The second method yields an internal load estimate using the variation of total phosphorus concentrations in the water column throughout the summer of 2010. The mean total phosphorus concentration of the sediments in the deepest part of the lake basin was 2.17 ± 0.04 mg•g-1 dry weight. Total phosphorus concentration in the hypolimnion (at a depth of 11 m) rose significantly from 28 μg•L-1 on July 27th to 200 μg•L-1 on 8th September 2010. Based on the different methods, the internal phosphorus load of Lake Nairne was estimated to range between 83 and 170 mg•m-2•summer-1.