Identifying spatiotemporal alteration of nitrogen to phosphorus ratio of Lake Dianchi and its driving forces during 1988-2018

Abstract

氮、磷浓度是制约湖泊营养状态和生产力水平的重要环境因子，而氮磷化学计量比是湖泊生态系统的主要指标，因此，判识氮磷比变化趋势及其驱动力对湖泊生态恢复具有重要意义.研究基于1988—2018年连续观测数据，分析了滇池氮磷浓度和氮磷摩尔比（简称氮磷比）的时空分布演变特征；采用多元线性回归模型分别对滇池草海和外海氮磷比驱动效应进行定量解析，筛选出影响湖体氮磷比变化的潜在驱动因子.结果表明：①1988—2018年滇池氮磷比呈现显著的线性上升趋势，其中草海和外海氮磷比分别上升1.3和0.7 a^-1.②草海和外海分别在2008年和2004年发生了氮磷比上升突变，突变前上升归因于总氮浓度快速增加，突变后则是由于总磷浓度下降较快.③滇池的氮磷浓度变化主要是受流域氮磷输入负荷、跨流域调水、流域氮磷削减、风速和水位的综合影响，但受控因子在不同区域可能存在差异.④气温是滇池氮磷比变化的主要驱动因子，流域人为氮磷输入差异是滇池氮磷比变化的次要驱动因子.;Nitrogen and phosphorus are two key elements controlling lake eutrophic level and primary productivity. As a substantial stoichiometry indicator of lake ecosystem, identifying change of nitrogen to phosphorus ratio (N/P molar ratio) and its driving forces is fundamental to lake management. Nitrogen and phosphorus concentrations and N/P ratio from 1988 to 2018 were assessed by examining their temporal trend and the time of abrupt change. Controlling factors and their relative contributions to N/P ratio variability were quantified by an established multiple linear regression model. Results show that: linear significantly growing trends of N/P ratios were observed in both Lake Caohai and Lake Waihai over the study period, increasing 1.3 and 0.7 a^-1 for Lake Caohai and Lake Waihai, respectively. Abrupt changes of N/P ratios were observed in 2008 for Lake Caohai and 2004 for Lake Waihai. The increasing N/P ratios in both sub lakes were due to the faster increasing rate of total nitrogen (TN) concentration to total phosphorus (TP) before the abrupt point and the slower falling rate of TN concentration to TP after the abrupt point. TN and TP concentrations were influenced by watershed anthropogenic nutrient inputs, trans-boundary water diversion, watershed nutrient mitigation, wind speed and water level with varying impact on Lake Caohai and Lake Waihai. Driving factors varies between concentrations and N/P ratios. Air temperature contributed mostly to the variation of N/P ratios in Lake Dianchi while watershed anthropogenic nutrient inputs and anthropogenic nitrogen inputs act as the second-largest contributors.