Controls on Dissolved Organic Carbon Concentrations in Streams, Southern Québec

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Abstract
Dissolved organic carbon (DOC) concentrations were measured in streams draining 42 small (0.6–37.2 km2) catchments located in the Appalachian Uplands and St. Lawrence Lowlands. Weekly sampling from mid-April to late-November in four catchments containing < 1% wetland revealed DOC concentrations in streams averaging 3.5–7.2 mg∙L−1 with significant (r2 = 0.28–0.66, P < 0.01) positive relationships to discharge. In four catchments containing 15–69% wetland, average DOC concentrations in streams ranged from 14.5 to 40.0 mg∙L−1 and there was no significant relationship to discharge. Based on five sampling dates from May to November, DOC concentrations in streams draining the 42 catchments showed consistent relationships of varying strength (r2 = 0.26–0.67, P < 0.01) with the variable percent wetland in the catchment. Soil drainage rating, percent forest, mean catchment slope, and catchment area provided little improvement in the regression model. Separating the catchments into two regions (Uplands and Lowlands) improved the predictive power of the regression model for the upland catchments. The poor relationships between DOC and percent wetland in the lowland catchments are attributed to the dry summer during sampling and extensive modification of land use, such as the drainage of wetlands and agriculture. These results indicate that stream DOC concentrations may be predicted from easily-obtained catchment variables, such as percent wetland.