Ecosystem and Seasonal Control of Stream Dissolved Organic Carbon Along a Gradient of Land Use

Abstract

We examined the influence of watershed land use and morphology on dissolved organic carbon (DOC) concentration in 32 south-central Ontario streams having varying agricultural land-use intensities in their catchments. For streams in this region, both univariate and multivariate regression models identify the proportion of the watershed with poorly drained soils (r2 up to 0.67) as a better predictor of stream DOC concentrations than any other landscape characteristic, including the proportion of the watershed as wetland. Agricultural land use did not strongly influence DOC concentrations in our study area; however, we do show that land-use changes could significantly alter the delivery of DOC to streams in the region. We also identify how landscape–DOC relationships change over a 2-year time period, as related to season, regional climatic conditions, soil moisture, and hydrology. Our results indicate that the relationships between landscape predictors and stream DOC concentrations are temporally dynamic. Strong temporal trends are shown seasonally and in association with climate, through its control of modelled soil moisture conditions. During periods of positive and negative deviation from normal soil moisture conditions, the relationships of DOC concentrations with landscape characteristics become less predictable. We show that these dominant patterns are likely a function of varying flow paths and that anthropogenic changes that affect soil moisture conditions or flow path will in turn strongly influence DOC dynamics.