EFFECTS OF LAND-USE CHANGE ON WATERSHED EROSION AND NITROGEN CYCLING IN THE NORTHEASTERN UNITED STATES

Land-use changes can result in a multitude of direct and indirect consequences to surface processes and downstream waterbodies. This study examines sedimentary archives from a network of 17 lakes across the northeastern United States. The watersheds of these lakes span a range of topographic and surficial geologic characteristics, and have land-use histories with differing types, timing, intensity, and duration of anthropogenic disturbance. Sediment cores from all lakes were analyzed to identify distinct event deposits and changes in clastic sediment input indicative of landscape disturbances. Watershed sensitivity to erosional disturbances was greatest in higher relief, mountainous watersheds with abundant glacial-age sediment. A subset of 3 lakes representing different patterns of land use disturbance and erosional responses were further analyzed for geochemical indicators (C and N, δ ¹³C; δ ¹⁵N) of changes in nutrient cycling and aquatic productivity. The three watersheds associated with these lakes are distinguished by differing land use histories described as 1) limited anthropogenic impact until the onset of lumbering ~1900 CE and ongoing timber harvest, 2) early land clearance and conversion to agriculture in the 18-19^th centuries followed by some reforestation and rural development, and 3) a period of timber harvest in 19^th century and ending early in the 20^th century, followed by strict conservation and complete reforestation. In the watershed subject to ongoing timber harvest, sediment yield increased markedly while δ ¹⁵N remains near pre-disturbance levels. Only in the watershed with strict conservation does lake sediment δ ¹⁵N show depletion consistent with global trends in atmospheric in N deposition. Where forested land was converted for agriculture use and rural residential development, terrestrial sediment yield and δ ¹⁵N have increased and remained enriched.