Évolution du cheminement des apports acides sous couvert de glace durant la fonte printanière.

Une etude intensive et extensive des eaux de ruisselement, de la colonne d'eau et de la`decharge du lac Laflamme, reserve faunique des Laurentides, Quebec, au printemps 1988, a permis la caracterisation des facteurs determinants dans l'acidification episodique du milieu. Les accroissements d'acidite a la decharge du lac pendant la fonte sont dus en grande partie a la dilution des concentrations des cations par les eaux de fonte. De plus, le premier accroissement d'acidite est relie a l'arrive dans le lac des premieres eaux de fonte fortement chargees en NO3. Par contre, le deuxieme accroissement d'acidite pendant la fonte massive comprend des concentrations minimes de NOa. Nous proposons. que cette baisse de concentration soit le resultat de denitrification dans le sol et! ou une assimilation de NO3 dans la zone des racines, et, lien toute probabilite", l'activite planctonique 'in situ' et sous-jacente au couvert de glace. Afin de valider cette hypothese, nous recommandons qu'une etude extensive et intensive de l'evolution des composes azotes pendant la fonte printaniere soit entreprise. L'etude devrait comprendre une approche experimentale; elle sera extensive de nature mais restreinte spatialement sur l'evolution physico-chimique de la colonne d'eau peu profonde et a l'interaction eaux de fonte - sol sur la rive connexe. Abstract An intensive and extensive study of the chemical composition of stream /lm- and runoff waters, the water column and the lake discharge of Lac Laflamme, Reserve faunique des Laurentides, Quebec, in the spring of 1988, led to the characterisation of the main factors for the episodic acidification of lake sub-ice waters. Increase in the acidity of the waters at the lake discharge during the melt is due in large part to the dilution of base cation concentrations by meltwaters. In addition, the first pulse of acidity du ring the early melt is related to the entry into the lake of snow meltwaters with a relatively high concentration of NO3. Conversely, the second pulse of acidity during the main melt is accompanied by waters with a very low concentration of NO3. We suggest that this removal of NO3 may be due to denitrification in the soil and NO3 assimilation in the rootzone during meltwater infiltration, and to plankton activity in the upper sub-ice waters. In arder to validate these hypotheses we recommend that an extensive study of nitrogen dynamics in meltwaters, infiltrating meltwaters in the vegetative zone and upper soil horizons, sediment-water interactions and the water column be undertaken during the spring melt period. The study should be restricted to a small area in the shallow inshore waters so that process studies may be facilitated by controlling the experimental conditions during the different phases of the melt season.