Monitoring and understanding chlorophyll-a concentration changes in lakes in northeastern China using MERIS and OLCI satellite data

Lakes in the Northeast Plains-Mountain Lake Region (NPLR) of China face severe risks of eutrophication due to climate change and intensive anthropogenic pressures. As a vital indicator for eutrophication status, the dynamics of chlorophyll-a (Chl-a) concentrations in NPLR lakes were, for the first time, comprehensively investigated in this study. A support vector regression (SVR)-based model was established and applied to the MERIS (2003–2011) and OLCI (2016–2019) observations to derive a long-term Chl-a record for 33 NPLR lakes. The NPLR lakes exhibited a climatological annual mean Chl-a of 12.3 mg m⁻³, ranging from 6.8 to 18.6 mg m⁻³ among the 33 studied lakes. During the study period, 11 lakes exhibited statistically significant increases in Chl-a concentrations (<i>p</i> &lt; 0.05), and 3 lakes showed significantly decreasing trends. Spatially, lakes in lowland regions had higher Chl-a than those in mountainous regions. This study quantified the relative importance of seven explanatory factors in influencing interannual Chl-a changes for each lake. Results showed statistically significant impacts of agricultural fertilizer (10 lakes), wastewater (4 lakes), runoff (7 lakes), and wind (5 lakes) in affecting the interannual variations of Chl-a. The decreases in Chl-a were primarily attributed to the reduced livestock excrement. Compared to hydro-climatic factors, anthropogenic pressures (i.e. agriculture fertilizer, livestock excrement, and wastewater discharge) had more significant impacts on the interannual variations of Chl-a, accounting for more than 50% of 18 lakes. This study enhances our understanding of the long-term Chl-a dynamics in NPLR lakes and their responses to hydro-climatic factors and anthropogenic forcing. These findings are valuable for basin-scale water environment protection and sustainable development.

中国东北平原-山地湖区（Northeast Plains-Mountain Lake Region, NPLR）的湖泊面临着由气候变化与高强度人为活动带来的严重富营养化风险。作为富营养化状态的关键指示指标，本研究首次全面探究了NPLR湖泊叶绿素a（chlorophyll-a, Chl-a）浓度的动态变化特征。本研究构建了基于支持向量回归（support vector regression, SVR）的模型，将其应用于MERIS（2003–2011年）与OLCI（2016–2019年）遥感观测数据，为33个NPLR湖泊生成了长期叶绿素a浓度序列。33个研究湖泊的气候学年平均叶绿素a浓度为12.3 mg·m⁻³，区间范围为6.8至18.6 mg·m⁻³。研究期间，11个湖泊的叶绿素a浓度呈现统计学意义上的显著上升趋势（p<0.05），另有3个湖泊呈现显著下降趋势。空间分布上，低地湖泊的叶绿素a浓度高于山地湖泊。本研究量化了7个解释因子对各湖泊叶绿素a年际变化的相对重要性。结果显示，农业化肥（影响10个湖泊）、废水排放（4个湖泊）、径流量（7个湖泊）与风速（5个湖泊）对叶绿素a的年际变化存在显著影响。叶绿素a浓度下降主要归因于畜禽粪便排放量的减少。相较于水文气候因子，人为压力（即农业化肥、畜禽粪便与废水排放）对叶绿素a年际变化的影响更为显著，在18个湖泊中该类因子的贡献占比超过50%。本研究加深了学界对NPLR湖泊长期叶绿素a动态变化及其对水文气候因子与人为强迫响应机制的理解，相关研究成果对流域尺度水环境管护与可持续发展具有重要参考价值。