太湖西北湖区2003-2012年间氮磷浓度及浮游植物主要类群变化趋势分析

Variations in nitrogen and phosphorous concentrations and major phytoplankton species in the northwestern Lake Taihu between 2003-2012

基于2003—2012年太湖竺山湖和西部沿岸区水体理化指标与浮游植物丰度的逐月监测数据,分析了两个湖区氮磷营养盐状态和浮游植物丰度以及浮游植物主要类群的年际变化趋势及季节变化特征,探讨了浮游植物群落变化与水温及营养盐指标间的关系。结果表明:10年间两个湖区氮磷营养盐浓度总体呈下降趋势,以竺山湖TN、NH_3-N浓度和西部沿岸区NO_3-N浓度下降最为显著;浮游植物丰度总体呈上升趋势,蓝藻在群落结构中日益占据绝对优势;季节变化上,氮营养盐浓度表现为春冬季节高于夏秋季节,TP浓度和浮游植物丰度呈相反的变化趋势。Pearson相关分析显示,水温、NH_3-N浓度和TN/TP是影响蓝藻丰度及其在浮游植物群落中所占比例的主要因素。在温度和营养盐结构的共同作用下,10年间两个湖区蓝藻水华暴发时间逐渐提前,而消退时间逐渐滞后,水华持续时间逐年上升。在全球变暖背景下,太湖水华治理需执行更加严格的氮磷限制阈值,且在重污染的西北湖区控磷依然是关键。

Phytoplankton is one of the major primary producers in aquatic ecosystems,and plays a critical role in the normal functioning of aquatic ecosystems by providing material and energy. Changes in the aquatic ecological environment can affect phytoplankton abundance and community structure,leading to the occurrence of harmful algal blooms. In this study,the trends in temporal variation and seasonal characteristics of nutrient concentrations( including total nitrogen,ammonium, nitrate, and total phosphorus), phytoplankton abundance( including Cyanophyta, Chlorophyta,Bacillariophyta,etc.),as well as phytoplankton community structure were analyzed,using continuous monthly monitoring data of four sampling sites located in Zhushan Bay and West Coast of Lake Taihu between 2003 and 2012. Relationships between phytoplankton abundance and water temperature coupled with nitrogen and phosphorus were evaluatedsimultaneously. The results showed that the concentrations of nitrogen and phosphorous generally decreased during the ten years;this was particularly apparent for the concentrations of total nitrogen( from 7.30 mg/L to 4.25 mg/L) and ammonium( from 4.19 mg/L to 0.81 mg/L) at Zhushan Bay,and nitrate concentration( from 2.01 mg/L to 0.75 mg/L) at West Coast of Lake Taihu. In contrast, phytoplankton abundance generally increased, with the maximum annual phytoplankton abundance of 5293.2 × 10~4cells/L and 3802.8 × 10~4cells/L observed in 2012,at Zhushan Bay and West Coast of Lake Taihu,respectively. Cyanobacteria became the absolutely dominant species,constituting 90. 4% and 87. 5% of the total phytoplankton community in 2012 at Zhushan Bay and West Coast of Lake Taihu,respectively. Additionally,nitrogen concentrations were higher in spring and winter than in the other two seasons,whereas total phosphorous concentrations and phytoplankton abundance showed the opposite seasonal trends. Pearson’ s correlation analysis indicated that cyanobacterial abundance and its percentage in the total phytoplankton community were positively correlated with water temperature( r =0.474,P < 0.01),and negatively correlated with ammonium concentration( r =-0.432,P < 0.01) and TN/TP ratio( r =-0.450,P < 0.01). Under the effects of both temperature and nutrient structure,gradual cyanobacterial blooms occurred earlier but disappeared later,thus,leading to the increased duration of cyanobacterial blooms in Zhushan Bay and West Coast of Lake Taihu during 2003-2012. With the increased deleterious effects of global warming,more stringent nitrogen and phosphorus limiting thresholds are required to control cyanobacterial blooms in Lake Taihu. Furthermore,phosphorus remains the critical nutrient that must be controlled in the heavily polluted northwestern Lake Taihu.