三峡小江回水区磷素赋存形态季节变化特征及其来源分析

磷被普遍认为是富营养化的限制性因子,但河道型的三峡水库支流回水区在变化的水动力条件下磷的季节变化有其独特性.对2007年3月～2008年3月三峡小江回水区磷素的跟踪观测结果进行了分析.研究期间小江回水区总磷(TP)平均浓度为(61.7±2.7)μg·L~(-1),虽然各采样断面磷浓度差异不大,但其季节变化明显,大体上冬季最高、夏季较高、春季次之、秋季较低.颗粒态磷占TP平均浓度的54.05%,是TP的主要组成部分.结合同期对叶绿素a、悬浮无机颗粒物、悬浮有机颗粒物、河口流量、河口水位等主要环境变量的跟踪观测结果发现,颗粒态磷以吸附于泥沙颗粒表面或同矿质相结合的无机形态为主,并在降雨、径流的作用下进入水体使水中TP含量增加,该现象在低水位运行状态下更加明显.而在高水位运行条件下水动力条件的改变使磷素赋存形态向溶解态形式转变.小江回水区藻类生长对溶解性磷酸盐的生物利用过程十分明显,溶解性磷酸盐浓度同叶绿素a显著负相关.TP亦同叶绿素a呈负相关关系.研究认为,降雨、径流强度的加大及水位的降低虽然带来丰富的营养物,但亦使河道型的回水区水体更新周期缩短;而悬浮颗粒浓度升高导致的水体光学透射性能的下降以及洪水脉冲带来的不稳定的生长环境却阻碍了浮游植物的进一步生长和繁盛,这两方面综合作用的结果可能是小江回水区TP-Chla负相关的原因.

Seasonal Variation of Phosphorus in Xiaojiang Backwater Area, Three Gorges Reservoir

Phosphorus is regarded as the limiting factor for eutrophication in backwater areas of tributaries in Three Gorges Reservoir (TGR). However, distinct seasonal variation patterns and potential ecological impacts of phosphorus are apparent due to the ultimate hydrodynamic condition in the site. Seasonal variation of different forms of phosphorus in Xiaojiang backwater area (XBA) in Three Gorges Reservoir from Mar. 2007 to Mar. 2008 was reported. It was found that average concentration of total phosphorus (TP) was (61.7±2.7) μg·L~(-1). No remarkable difference was found among sampling sites, while significant variation among seasons were detected. Generally, TP was high in summer and winter while low in autumn. Particulate phosphorus (PP) was the major forms of phosphorus in XBA. The co-analysis among phosphorus and key environmental variables, i.e. chlorophyll, inorganic/organic particles, flows and water levels in river estuary, indicated that inorganic forms of PP should dominate in PP, and might transfer into the water body by rainfall and river flows, resulting in the increase of TP in XBA. This phenomenon was more significant when TGR was set at low water level. Soluble reactive phosphorus (SRP) had a significantly negative correlation with chlorophyll a, indicating the evident assimilation process of phosphorus by algae. It was inferred that heavy rainfall and increased river flow shortened the water renewal cycle and brought in amounts of suspended sediments that increased the light attenuation underwater. With the frequent flood impulse, phytoplankton community should suffered the instable living environment that decreased the concentration level of chlorophyll a in XBA. This might be the potential reason for the negative correlation between TP and chlorophyll a.