Effects of land use on the timing and magnitude of carbon and nitrogen fluxes: an analysis of high-frequency sensor measurements from forested, agricultural, and urban watersheds in the Lake Champlain Basin

Land use/land cover change has been shown to have significant impacts on nutrient loading to aquatic systems, leading to increased nitrogen and phosphorous fluxes from terrestrial ecosystems to streams and rivers. This elevated nutrient loading can have downstream water quality impacts, and has been linked to eutrophication and harmful algal blooms like those observed in the Lake Champlain basin. While it is clear that changes in land use/land cover are associated with changes in aquatic ecosystem function, a mechanistic understanding of how nutrient fluxes from distinct land cover classes respond to hydrologic events on event and seasonal scales remains unknown. Recent advances in the availability of high-frequency water quality sensors provide an opportunity to assess these relationships at a high temporal resolution.

We deployed a network of in-situ spectrophotometers in three sub-watersheds of the Lake Champlain Basin with contrasting land uses: forested, agricultural, and urban. Our study sought to assess how land cover affected the timing and magnitude of fluxes of carbon (C) and nitrogen (N) from watersheds with distinct land uses, and to determine how sensitive these fluxes were to inter-annual climate variability.

We found systematic differences in the timing of C and N fluxes across these different land use classes as well as the total magnitude of C and N exported from these different landscapes. We also found strong inter-annual variability in carbon and nitrogen fluxes in response to inter-annual variability in precipitation and discharge, suggesting a high degree of hydrologic control over nutrient loading. These findings emphasize the potential for climate change, and in particular precipitation variability, to drive strong variation in the magnitude of downstream nutrient flux to receiving lakes and estuaries. Our study emphasizes the pervasive influence of land cover and its effects on water quality in the Lake Champlain watershed, and also highlights the strong signature of anthropogenic land use choices on regional C and N cycling.