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.

富营养化（Eutrophication）是指水体营养物质富集，进而引发藻类水华暴发。受营养物质与水文循环变化的影响，湖泊中的此类水华可全年发生。河流向湖泊输入的营养物质是引发富营养化问题的主要成因。目前，关于河流向湖泊输运氮（N）的季节变化规律仍未得到充分阐明。因此，本研究的核心目标是深入解析海河流域（Hai He Basin）内河流向湖泊输运溶解态无机氮（dissolved inorganic N, DIN）的季节变化特征。为此，本研究选取官厅湖（Guanting）与白洋淀（Baiyangdian）作为典型研究湖泊，其汇水区涵盖多种种植制度。本研究针对官厅湖与白洋淀构建了季节版MARINA-Lakes模型（Model to Assess River Inputs of Nutrients to lAkes），同时模拟了陆地向湖泊的氮素迁移过程。该模型充分考虑了人类活动（如种植制度、施肥方式）、气候与水文的季节变异特征。研究将季节划分为冬季（12月至次年2月）、春季（3月至5月）、夏季（6月至8月）与秋季（9月至11月）。2012年的模型模拟结果显示，河流向湖泊输运的DIN通量在冬季达到全年最高值，夏季则为全年最低。在各季节中，点源（point source）贡献了官厅湖与白洋淀总DIN输入量的50%以上。为降低未来湖泊污染风险，需避免冬季直接排放畜禽粪便类点源。本研究认为，高效的湖泊污染防控策略需充分纳入氮素循环的季节维度。本研究成果可为精准的营养物质管理与环境政策制定提供科学支撑。