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TRANSFORMATION DE LA QUALITE D’UNE MEME EAU DE RIVIERE LORS DE SON SEJOUR DANS DEUX RESERVOIRS DIFFERENTS: (A) LE LAC BOIVIN ET (B) LA RESERVE DE GRANBY, QUEBEC, CANADA

1982; Taylor & Francis; Volume: 7; Issue: 1 Linguagem: Inglês

10.4296/cwrj0701127

1918-1817

Peter G. C. Campbell, Daniel Cluis, Patrice Couture,

Soil and Water Nutrient Dynamics

Nitrogen and phorphorus dynamics were studied in two adjacent reservoirs (Fig. 1) used by the city of Granby, Quebec, for domestic and industrial water supply. The two impoundments have a common water supply, the North Yamaska River, but differ considerably in their physical characteristics (Table 1) and at the biological level (primary producers); aquatic vascular plants are abundant in the shallow main-stream reservoir (A) but are totally absent from the deep off-channel reservoir (B). The study covered 20 consecutive months (January 1975–October 1976); prolonged drought conditions occurred during the first summer (1975), whereas slightly higher than normal discharges prevailed during 1976 (Fig. 2).The river water entering the two reservoirs was highly enriched in phosphorus (Table 3), reflecting land use practices in the upstream watershed. Concentrations of dissolved phosphorus at the inlet greatly exceeded the generally accepted water quality criteria for standing waters; the N:P atomic ratio, calculated for dissolved inorganic nitrogen and dissolved phosphorus, varied from 3 to 22, being < 10 during much of 1975. During summer stagnation, a lagged relationship was observed between indigenous phytoplankton chlorophyll-a and algal growth potentiel (AGP: using Selenastrum capricornutum), provided that the observed concentrations of chlorophyll-a were offset by + 14 days with respect to the AGP values (Fig. 3).Fluxes through each reservoir for a variety of parameters were calculated from data on concentrations and discharge measured at the inlet and outlet of each impoundment: C1, conductivity; inorganic N, total P, AGP and chlorophyll-a. Mass balances (input-output) were then determined both for the hydrological year October 1975 – October 1976 and for the summer periods of maximum biological activity (Tables 4, 5). For the conservative parameters (e.g. C1), the input-output budgets balanced within < 2% for each reservoir and for each period considered. The budgets for inorganic N were systematically negative (losses of − 14 to − 41% within the impoundments), those for P were generally either balanced (reservoir A) of slightly negative (reservoir B), whereas those for chlorophyll-a were strongly positive (+ 12 to + 83%) in both systems. With respect to the algal growth potential, however, the two inpoundments differed significantly: the budgets were balanced in the main-stream reservoir but were strongly negative in the off-channel reservoir (− 25% in 1975; − 36% in 1976). In the former case, the chlorophyll-a mass balance increased without a concomitant decrease in AGP, presumably as a result of water-sediment-macrophyte interactions and the resulting nutrient exchange. In the off-channel reservoir, reasonable agreement was found between the chlorophyll-a and the AGP budgets; the decrease in AGP was associated with an appreciable increase in the mass balance for chlorophyll-a. The implications of these results for the modelling of water quality in other reservoirs are briefly discussed.