2015-2016年汉江中下游硅藻水华发生成因分析

Analysis of Factors Influencing Diatom Blooms in the Middle and Lower Hanjiang River

为了探究汉江中下游多年春季易发生硅藻水华的成因,2015年和2016年早春水华发生期在汉江干流布设8个点位(黄家港、襄阳、皇庄、沙洋、兴隆闸、泽口、仙桃、蔡甸),进行密集的浮游植物定性定量监测,同时监测总氮、总磷、硅酸盐、水温、p H和溶解氧等水质理化指标,并搜集水华期汉江中下游干流各水文站的流量数据,分析理化指标与水华细胞密度之间的相关性以及流量与水华细胞密度之间的关系。结果表明,自皇庄以下,水华发生期浮游植物密度高于107个/L,优势种为冠盘藻属(Stephanodiscus),达总生物量的95%以上;各样点浮游植物细胞密度与硅酸盐浓度、硅氮比均呈显著正相关(P<0.01);在考虑上下游点位距离和时间推移的交互作用下,通过逐步向前回归方法拟合多元线性回归模型,发现采样时间和采样点位间距与细胞密度呈极其强烈的显著正相关(P<0.001),流量与细胞密度呈极其强烈的显著负相关(P<0.001)。适宜的早春气候条件、较高的硅氮比、低流量是汉江中下游硅藻水华发生的原因。建议在控源截污的基础上,应注意早春时节对重要站点的流量监控,适时增加下泄流量以防控硅藻水华的发生。

The middle and lower Hanjiang River（ downstream of Danjiangkou Dam） provide drinking water for the people of 24 cities and counties along the river. However,diatom blooms have occurred frequently in winter and spring for nearly ten years,since 2008,and the duration has gradually extended,posing a threat to drinking water safety. To explore the causes of diatom blooms and provide scientific evidence for bloom control and water project operations in Hanjiang River,we carried out intensive monitoring of qualitative and quantitative indices of phytoplankton and physical and chemical indices of water quality（ total nitrogen,total phosphorus,silicate,water temperature,p H and dissolved oxygen） in these sections during the bloom period in the early spring of 2015 and 2016.The correlation between physicochemical indices of water quality and algal cell density and the relationship of diatom cells density to flow discharge were analyzed. In the spring of 2015-2016,the diatom algae bloom occurred from Xiangyang to the lower reaches and lasted 10 days each year. During the bloom,monitoring of phytoplankton and water quality was carried out once a day at eight sampling sites： X1,Huangjiagang; X2,Xiangyang; X3,Huangzhuang; X4,Shayang; X5,Xinglongzha; X6,Zekou; X7,Xiantao; X8,Caidian. The integrated hydrological data was provided by hydrological stations of the middle and lower reaches of Hanjiang River. A total of 31 phytoplankton genera from eight phyla were detected during the investigation,with absolute dominance of Bacillariophyta at all sampling sites; accounting for 29%-48% of the total phytoplankton species at each site. Phytoplankton cell density varied significantly among sampling sites. Phytoplankton cell density was 107 cells/L below the Huangzhuang section（ X3-X8） during the bloom period,and the dominant species was Stephanodiscus,accounting for more than 95% of the biomass at all sampling sites. Phytoplankton cell density was not significantly correlated with DO,temperature,p H,TN and TP,but there was a significant positively correlation with silicate concentration and the silicon to nitrogen ratio（ r = 0. 858 and 0. 941,P〈0. 01）. Because of the interaction of distance between sites and sampling time,the relationships between sampling time,distance of the sampling sites and cell density was analyzed by using stepwise regression and fit to a multiple linear regression model. The results showed a positive correlation between cell density,sampling time and distance between sampling sites that was very highly significant（ P〈 0. 001）,as was the negative correlation with flow discharge（ P 0. 001）. Causative factors for diatom blooms include suitable climatic conditions（ extended periods of relatively high water temperature and strong solar radiation） in early spring,high Si/N,and low discharges. Two suggestions are made to prevent diatom blooms in Hanjiang River：（ 1） Decrease pollution inputs from towns in the Hanjiang River basin,particularly for total nitrogen and silicate,and（ 2） monitor discharge at critical sites（ Shayang,Qianjiang and Xiantao） in early spring. When discharge is less than 500 m3/s,local water conservancy departments should coordinate efforts to increase flow discharge sufficiently to prevent diatom blooms.