Abstract:

Since 1990s,〖WTBX〗Stephanodiscus hantzschii〖WTBZ〗 bloom occurred frequently in the middledown reaches of the Hanjiang River during the early spring.From January 1st to March 6th 2010,investigaitons were performed to study phytoplankton spatiotemporal distribution in the Hanjiang River basin.During the study period,we found that phytoplankton density at upstream of Xiangfan City were significantly lower than that at downstream,and the density of phytoplankton in mainstream of the Hanjiang River were significantly lower than that of its branch:Tangbai River.On January 7th,the highest density of total phytoplankton reached 5050×106 ind./L,at site of the Tangbai River,of which the 〖WTBX〗Stephanodiscus hantzschii〖WTBZ〗 occupied 9609%.From January 27th,algal bloom appeared in the mainstream,with average phytoplankton density of 1442×106 ind./L,and increased to 2466 ×106 ind./L on February 23,then declined to 66×106 ind./L on March 3rd.Correlation analysis indicated that phytoplankton density had siginificant positive correlations with dissolved oxygen(DO)(Pearson,r=063 p〖WTBZ〗＜005),soluble total nitrogen(STN)(Pearson,〖WTBX〗r=085,p＜001)and soluble total phosphorus(STP)(Pearson,〖WTBX〗r=077,p〖WTBZ〗＜001),but extremely significant negative correlation with water transparence(SD)(Pearson,〖WTBX〗r=- 044,p〖WTBZ〗＜001),and significant negative with soluble silicate (SSi)(Pearson,〖WTBX〗r=- 035,p〖WTBZ〗＜005).Based on the spatiotemporal pattern of phytoplankton,it was proposed that the higher density of 〖WTBX〗Stephanodiscus hantzschii〖WTBZ〗 in the Tangbai River may serve as a crucial seed bank for the outbreak of 〖WTBX〗Stephanodiscus hantzschii〖WTBZ〗 bloom in the middledown reaches of the Hanjing River.The duration of the earlyspring 〖WTBX〗Stephanodiscus hantzschii〖WTBZ〗 bloom was also significantly influenced by the weather condition of the Hanjiang River basin