Seasonal River Export of Nitrogen to Guanting and Baiyangdian Lakes in the Hai He Basin

Eutrophication refers to the nutrient enrichment, leading to blooms of algae. Such blooms in lakes can happen throughout the year because of the changes in nutrient and hydrological cycles. Nutrient export to lakes from rivers is the main cause of eutrophication problems. Seasonal trends in nitrogen (N) export by rivers to lakes are still not well understood. The objective of this study is, therefore, to better understand the seasonal trends in river export of dissolved inorganic N (DIN) to lakes of the Hai He Basin. To this end, we selected Guanting and Baiyangdian as representative lakes, whose drainage areas include various cropping systems. We developed a seasonal version of the MARINA-Lakes (Model to Assess River Inputs of Nutrients to lAkes) model for Guanting and Baiyangdian while assessing N flows from the land to the lakes. The model accounts for the seasonality in human activities (e.g. cropping systems, fertilizer practices), climate and hydrology. The effective seasons are winter (December–February), spring (March–May), summer (June–August), and fall (September–November). The model results for the year 2012 indicate that river export of DIN was highest in winter and lowest in summer. Point sources accounted for over 50% of DIN exports to Guanting and Baiyangdian across seasons. Avoiding direct discharges of animal manure (point source) in winter is needed to reduce future lake pollution. We argue that effective lake pollution control requires accounting for seasonal N cycles. Our study can support effective nutrient management and environmental policies.