Nutrient deposition over the past 60 years in a reservoir within a medium-sized agricultural catchment.

Widespread dam construction has reduced the hydrological connectivity of catchments of various sizes. Nutrients lost from upper catchments can be intercepted by dams and deposited in reservoir sediments, and this reduces downstream nutrient transportation. Few previous studies have assessed historical nutrient deposition rates (NDRs) in man-made reservoirs. Using Cs and Pb deposition chronologies, this paper examines the total nitrogen (TN), total phosphate (TP), and organic matter (OM) concentrations in six sediment cores recovered from Xujiaya reservoir (catchment area 580 km), provides estimates of historical NDRs by referencing the original capacity curve, and explores temporal changes to the NDR since dam construction. The results show that anthropogenic sources resulted in the increase in nutrient concentrations in the upper parts of the cores, whereas natural sources were the main contributors to nutrient deposition across the whole reservoir. In addition, sediment supply from the catchment was the main source of the nutrients deposited in the reservoir, and the changing patterns of NDRs were overall regulated by sediment yields. The TN, TP, and OM profiles in the six cores, together with the historical NDRs, reflect the pattern of nutrient deposition in the reservoir derived from this agricultural catchment over the past 60 years. The results provide new insight into the effect of dam construction on nutrient deposition at a medium-sized catchment scale against a background of recent environmental change influenced by human activity.