低通时序滤波轨线法在太湖营养过程识别中的应用

环境背景条件变化会导致湖泊ρ（Chla）与环境因子响应关系发生变化.采用低通时序滤波轨线方法可以方便地识别ρ（Chla）与环境因子响应关系的时间转折点，将长时间序列数据进行分段，从而建立分段回归函数，为研究环境因子与湖泊ρ（Chla）的因果关系提供了一种新的思路.以太湖为研究对象，采用低通时序滤波轨线方法，评估了2001—2018年太湖的ρ（Chla）与营养盐〔ρ（TN）、ρ（TP）〕以及氮磷比〔ρ（TN）/ρ（TP）〕的变化过程，研究了年均气温、滞留时间对产藻效率〔ρ（Chla）/ρ（TP）〕的影响过程.结果表明:①2006年、2011年为太湖营养过程轨线的两个时间转折点，将太湖的营养过程轨线分为3段.第1段为污染阶段（2001—2006年），太湖的ρ（TN）、ρ（TP）、ρ（Chla）同步升高，于2006年达到第一个峰值；第2段为修复阶段（2006—2011年），太湖的ρ（TN）、ρ（TP）、ρ（Chla）同步降低，于2011年达到谷值；第3段为富营养化加剧阶段（2011—2018年），太湖的ρ（TN）呈下降趋势，ρ（TP）与ρ（Chla）同步升高，至今未出现转折点.②太湖藻类生长的限值因子为ρ（TP），2011年之后氮磷比进入浮游藻类适宜生长区，为蓝藻暴发提供了条件.③2011—2018年产藻效率增长了51%，且目前仍在升高未出现转折点，气温升高可能是主要原因.④依据2011—2018年的滤波值建立ρ（Chla）-ρ（TP）的函数预测，为控制蓝藻暴发〔ρ（Chla） < 10 mg/m3〕，太湖的ρ（TP）需要控制在52 μg/L以下.⑤2006年后，太湖的滞留时间呈现缩短趋势，对藻类的繁殖形成抑制，但滞留时间不是影响产藻效率的关键因子.研究显示:自2006年太湖流域实施一系列生态修复工程后，湖泊氮浓度明显降低，但由于流域氮磷排放量较大而且湖体沉积物中累积磷含量较高，致使水体营养盐水平仍未降到能显著抑制蓝藻生长的水平；目前气温升高趋势仍在持续，太湖的控藻形势严峻，为摆脱气候变暖对蓝藻水华趋势的决定作用，应当在控氮基础上加大控磷的力度，同时更多考虑水文调节、生物修复、加强打捞等措施. 

Application of Low-Pass Sequential Filtering Trajectory Method in Identification of Eutrophication Process in Taihu Lake

Environmental condition changes will alter the response relationship between ρ(Chla) and the environmental factors in lakes. With the low-pass sequential filtering trajectory method, the turning point of the response relationship between ρ(Chla) and environmental factors with time can be easily identified, and the long time series data can be segmented to establish the piecewise regression function, which provides a new perspective for studying the causal effect relationship between the environmental factors and ρ(Chla) in lakes. Taking Taihu Lake as an example, the change process of ρ(Chla) and nutrients ρ(TN), ρ(TP) and the ratio of nitrogen to phosphorus (ρ(TN)/ρ(TP)) in Taihu Lake from 2001 to 2018 was evaluated by using using a low-pass sequential filtering trajectory method. The effects of annual average temperature, nitrogen to phosphorus ratio and retention time on algae production efficiency (ρ(Chla)/ρ(TP)) were studied. The results showed that: (1) 2006 and 2011 were the two turning points of the nutrient process trajectory in Taihu Lake, and the eutrophication process trajectory of Taihu Lake was divided into three stages: the first stage was pollution stage (2001-2006), and ρ(TN), ρ(TP) and ρ(Chla) increased synchronously, reaching the first peak in 2006; the second stage was the restoration stage (2006-2011), and the concentrations of ρ(TN), ρ(TP) and ρ(Chla) in Taihu Lake were concentrated; the third stage was the stage of eutrophication aggravation (2011-2018), while the ρ(TP) and ρ(Chla) increased synchronously, and there is no turning point so far. (2) The limiting nutrient for algae growth in Taihu Lake was ρ(TP). After 2011, the nitrogen to phosphorus ratio became suitable for algae growth, which provided conditions for the outbreak of cyanobacteria. (3) From 2011 to 2018, the efficiency of algae production increased by 51%, and it is still increasing with no sign of leveling off. The increased air temperature maybe the main reason for the increase of algae production efficiency. (4) According to the prediction of ρ(Chla)-ρ(TP) function based on the filter values from 2011 to 2018, the ρ(TP) in Taihu Lake should be controlled below 52 μg/L to control the outbreak of algae bloom. (5) Since 2006, the residence time of Taihu Lake presented a decreasing trend, which inhibited the algae growth. However, the retention time was not the key factor affecting the algae production efficiency. Research showed that since 2006, a series of ecological restoration projects have been carried out in the Taihu Lake Basin, and the lake nitrogen concentration has been significantly reduced. However, due to the large amounts of nitrogen and phosphorus discharges from the basin and high concentration of phosphorus accumulated in lake sediments, the nutrient level in the water column has not been reduced to the level that can significantly inhibit algal growth. Presently, the air temperature increase trend still continues, and the algae control is still in a severe situation. In order to get rid of the decisive effect of climate warming on cyanobacteria blooms, phosphorus control should be strengthened on the basis of nitrogen control, and various measures such as hydrological regulation, bioremediation, and algae salvage should be given more considerations.