Response of the Lake Clair Watershed (Duchesnay, Quebec) to changes in precipitation chemistry (1988-1994)

The chemistry of precipitation and of the lake's outlet (1988-1994) were measured at the Lake Clair Watershed (226 ha, 46 degrees 57'N, 71 degrees 40'W, 270-390 m above sea level), which is located 50 km northwest of Quebec City, Quebec, Canada. In wet precipitation, concentrations of SO4, Ca, and Na decreased from 1988 to 1994 whereas pH increased. In bulk precipitation, only Ca and Na decreased. The lake's outlet SO4 concentration decreased from 1988 to 1994, suggesting that the catchment rapidly responded to the changes in precipitation although a net SO4-S export was observed each year between 1988 and 1994 (4.2 kg.ha(-1).year(-1)). The source of the excess S was apparently located within the soil and most likely originated from SO4 desorption or organic S mineralization. The reduction in SO4 concentration at the outlet was associated with decreases of Ca, Mg, and Na concentrations. However, pH and alkalinity did not show any trends. No trends in NO3 and NH4 concentrations were observed in precipitation or at the lake's outlet during the 1988-1994 period. A major part (90%) of the atmospheric N deposition (9.2 kg.ha(-1).year(-1)) was retained within the catchment, and most of the N export was observed during snowmelt when low soil temperature, high water fluxes, and tree dormancy limited significant N uptake within the catchment. Net base cation export from the catchment was 17.1, 3.7, 2.1, and 6.7 kg.ha(-1).year(-1) for Ca, Mg, K, and Na, respectively. A budget was constructed for Ca and Mg specifically for the soil profile that included input from bulk precipitation, net uptake from vegetation, output from the rooting zone, and weathering rate. This budget indicated net losses of 4.9 and 0.8 kg.ha(-1).year(-1), respectively, for Ca and Mg. Given the size of the exchangeable Ca (200 kg.ha(-1)) and Mg (27 kg.ha(-1)) reservoirs present within the soil profile, these results suggest that forest productivity may be affected in the very near future.