湖泊蓝藻水华对连通河道水质的影响

Effects of Cyanobacterial Blooms in Eutrophic Lakes on Water Quality of Connected Rivers

为认知湖泊蓝藻水华灾害对连通河道的影响强度及其变化因素,以太湖梅梁湾连通河道梁溪河为例,在夏季水华期开展河道蓝藻颗粒物及氮磷和溶解氧等水质指标的逐日监测,结合河道水质指标空间变化调查,以及同期流量、温度和风场特征等水文气象因子数据,分析湖泊蓝藻水华物质对连通河道水质的影响特征.结果发现,夏季蓝藻水华暴发期间,水华颗粒物能够随水流大量进入连通的河道,进入河道的藻颗粒通量总量可观,夏季3个月梅梁湾进入梁溪河的蓝藻水华颗粒物总量达到9 733 t,与当年全湖的工程打捞量相近;由湖泊进入连通河道的水华颗粒物通量日变化很大,夏季调查期间进入梁溪河的藻颗粒通量介于75～496 t·d-1,平均值为105 t·d-1,通量大小主要受水文气象条件控制,水量、温度、风向是最主要的影响因素;携带大量蓝藻水华颗粒物的湖水进入河道后,在显著改善城市河道水体溶解氧、氨氮等指标的同时,也显著增加了河道颗粒态氮、磷等营养盐含量,对溶解态氮磷的影响较小;空间上,因蓝藻颗粒物带来的氮、磷浓度随离湖距离增加而下降,水体叶绿素浓度也迅速下降,汇入7 km之后的运河后,蓝藻颗粒物基本分解,水体颗粒态藻体叶绿素a含量由刚入河的152. 93μg·L^(-1)下降到1. 99μg·L^(-1).结果表明,受蓝藻水华问题困扰的湖泊对周边河道水质影响也很大,河湖连通过程尽管能有效缓解湖泊的湖泛灾害,解决城市河道黑臭的问题,但是对河道的营养盐等指标会产生较大影响;管理上应因河道和湖泊不同的保护目标,充分考虑湖泊水华情势及水文气象因素,制定机动灵活的调水方案,优化湖泊及河道的水生态服务功能.

The location at which the Liangxi River meets Meiliang Bay of Lake Taihu was selected to explore the influence and controlling factors of algal blooms on the connected rivers. The cyanobacterial particulates and water quality parameters including nitrogen,phosphorus,and dissolved oxygen were monitored daily during the summer bloom season. The spatial variation in water quality parameters along the river were also surveyed,and data from hydrological and meteorological factors such as discharge,temperature,and wind field characteristics over the same period were collected to help investigate the effects of cyanobacterial blooms on the water quality of connected rivers. The results showed that the total fresh biomass of algal blooms entering Liangxi River for three months during the summer cyanobacterial bloom period in Meiliang Bay was 9 733 t,which was similar to the amount harvested from the entire lake throughout the year. The flux of water bloom particles to the river varied widely,ranging from 75-496 t·d-1,with an average of 105 t·d-1. The flux was primarily controlled by hydrological and meteorological conditions. Water volume,temperature,and wind direction were the most important influencing factors. Lake water with a large amount of cyanobacterial particles entering the river can significantly improve the dissolved oxygen,ammonia-nitrogen,and other qualities of the river water in the city. It also significantly increased the concentrations of nutrients such as particulate nitrogen and phosphorus in the river,while it had a relatively weak effect on dissolved nitrogen and phosphorus. Spatially,the chlorophyll a concentration rapidly decreased with increasing distance from the lake,and the concentration of nitrogen and phosphorus exist as particulate cyanobacteria also declined. Upon being discharged into the canal 7 km away,the cyanobacterial particles were largely decomposed,and chlorophyll a content from the phytoplankton collected from the river by a 67 μm net decreased from 152. 93 μg·L-1 to 1. 99 μg·L-1. The results indicated that lakes plagued with cyanobacterial blooms have a great impact on the water quality of surrounding rivers. Although the connection between rivers and lakes can effectively relieve black spots and solve the black and odorous phenomenon in urban rivers,it had a great impact on nutrientsconcentration in the rivers. Depending on the different protection targets of rivers and lakes, lake water blooms and hydrometeorological factors should be considered during water transfer management to optimize the water ecological services of lakes and rivers.