To study the distribution characteristics and influencing factors of phosphorus in the Dongting Lake at different water periods, water samples were collected from 18 representative monitoring sections located in the Dongting Lake and its upper rivers from January to December 2016. Various forms of phosphorus concentration in the samples were analyzed. The results showed that, overall, the average concentrations (range) of total phosphorus (TP), total dissolved phosphorus (DTP), and phosphate (DPO) in the Dongting Lake were 0.085 mg·L-1(0.030-0.176 mg·L-1), 0.062 mg·L-1(0.017-0.151 mg·L-1), and 0.053 mg·L-1(0.007-0.144 mg·L-1), respectively. These forms of phosphorus concentrations were all characterized by the intermediate season (IS) > the dry season (DS) > the wet season (WS), with obvious seasonal variation characteristics. In the spatial distribution, the phosphorus concentration increased from west to east in DS and IS and decreased from the west to east in WS. In terms of time distribution, various forms of phosphorus were characterized by obvious seasonal distribution in the east, west, and outlet of the Dongting Lake. The phosphorus concentration in WS was higher than that in DS and IS in the West Dongting Lake. However, the concentration was higher during DS and IS than that in WS for the east and outlet of lake. Various forms of phosphorus concentrations in the west were highest in the Dongting Lake during WS, while those in the outlet and east were relatively highest in DS and IS. Therefore, the phosphorus pollution in the West Dongting Lake should receive more attention during WS, and the pollution in the east and outlet requires higher attention during DS and IS. In terms of form composition, according to the monthly data of the sections, the values of ρ(TPO)/ρ(TP) and ρ(DPO)/ρ(TPO) were mainly above 60% in the water body of the Dongting Lake and its upper rivers. DTP was the main form of TP, and DPO was the main form of DTP. It indicated that the phosphorus in the Dongting Lake and its upper rivers was mainly soluble, which is different from that before the impoundment of the Three Gorges Reservoir. The spatio-temporal distribution of suspended particles (SS) and phosphorus in the Dongting Lake had a certain synergistic effect. The concentration of SS and phosphorus in each water period showed a significant or extremely significant correlation. Therefore, SS was considered as an important factor affecting the seasonal variation of phosphorus in the Dongting Lake. In this study, it was concluded that the Nanzui section, the Daxiaoxihu section, and the East Dongting Lake area were clearly affected by surrounding human activities, respectively including domestic, agricultural, and sand mining sources of phosphorus pollution. The water in the West Dongting Lake has even been affected by mild and moderate eutrophication in 2016. These phenomena should be paid more attention.
Keywords: Dongting Lake; different water periods; influencing factors; seasonal variation; suspended particles; various forms of phosphorus.