[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) microg x 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, indicate 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.