Dissolved organic carbon as an indicator of the scale of watershed influence on lakes and rivers

Land use and land cover can have a significant impact on water chemistry, but the spatial scales at which landscape attributes exert a detectable influence on aquatic systems are not well known. This study quantifies the extent of the landscape influence using the proportion of wetlands in the watershed measured at different distances to predict dissolved organic carbon (DOC) concentrations in Wisconsin lakes and rivers, and to determine whether the watershed influence varies with season or hydrologic type of lake. The proportion of wetlands in the total watershed often explained the most variability of DOC in lakes when stepwise regression was used. However, best-model techniques revealed that, for lakes, r(2) values often only differed 1-3% between models using the proportion of wetlands in the total watershed and models using only the proportion of wetlands in nearshore riparian areas (25-100 m). In rivers, the proportion of wetlands in the watershed always explained considerably more of the variability in DOC than did the proportion of wetlands in the nearshore riparian zone. The watershed influence also varied seasonally in rivers, as the proportion of the watershed covered by wetlands explained more of the variability in DOC in the fall than in the spring. Overall, the proportion of wetlands in the landscape explained much more of the variability of DOC concentrations in rivers than in lakes.