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 (p < 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）的湖泊受气候变化与高强度人为活动胁迫，面临严峻的富营养化风险。作为富营养化状态的核心指示指标，本研究首次系统探究了NPLR湖区叶绿素a（chlorophyll-a，Chl-a）浓度的动态变化规律。
本研究构建了基于支持向量回归（support vector regression，SVR）的反演模型，结合中等分辨率成像光谱仪（MERIS，2003—2011年）与海洋陆地颜色成像仪（OLCI，2016—2019年）的遥感观测数据，为33个NPLR湖泊重建了长时序Chl-a浓度序列。
研究区内33个湖泊的气候学年均Chl-a浓度为12.3 mg·m⁻³，单个湖泊的浓度区间为6.8~18.6 mg·m⁻³。在研究时段内，11个湖泊的Chl-a浓度呈现统计学显著上升趋势（p<0.05），另有3个湖泊呈现显著下降趋势。空间分布特征显示，平原低地湖泊的Chl-a浓度显著高于山地湖泊。
本研究量化了7个解释因子对各湖泊Chl-a年际变化的相对贡献权重。结果表明，农业化肥（影响10个湖泊）、生活污水（影响4个湖泊）、径流量（影响7个湖泊）与风速（影响5个湖泊）对Chl-a年际变异存在统计学显著影响。Chl-a浓度的下降主要源于畜禽粪便排放量的减少。相较于水文气候因子，人为活动压力（即农业化肥施用、畜禽粪便排放与污水排放）对Chl-a年际变化的影响更为显著，其对18个湖泊的贡献占比超过50%。
本研究深化了学界对NPLR湖泊长时序Chl-a动态变化及其对水文气候因子与人为强迫响应机制的认知。上述研究结果对流域尺度的水环境保护与可持续发展具有重要的指导与参考价值。