Trophic Assessment and Water-Quality Spatial Distribution of Luoma Lake During Summertime

Luoma Lake is a suburban lake with the characteristics of water carrying in the northern Jiangsu Province, China. The information of physicochemical parameters in Luoma Lake will help to assess the trophic state and well understand the application values of suburban lakes, and further provide scientific basis for protecting and exploiting the water resources. In this study, the current water-quality characterization and trophic state at 20 uniform distributed sites in Luoma Lake were investigated monthly from June to August, 2017. The spatial distributions of the physicochemical parameters were assessed by the piper plot and isoline map. Results show that total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD_Mn) were significantly increased and the dominant ions were transformed from Ca²⁺+HCO₃^- to Na⁺(K⁺)+Cl^-, while the secchi depth, dissolved oxygen and pH were not changed compared to the years from 1991 to 2014. The concentrations of ρ(TN), ρ(TP) and COD_Mn varied in the range of 2.540-3.246, 0.173-0.531 and 9.90-14.29 mg·L^-1, respectively. According to the Environmental Quality Standards for Surface Water, Luoma Lake fell into the categories of Ⅴ in water quality. The comprehensive trophic level index indicate that most sites of the Lake were reached the state of mesotrophication, some sites even reached the state of hypertrophication. The spatial distributions of pH, HCO₃^-, Ca²⁺, Mg²⁺, total alkalinity and total hardness were significant different among sampling sites (P < 0.05). The spatial distributions of water-quality parameters were complicated, Ca²⁺, Mg²⁺, total alkalinity and total hardness were higher in the northeast region than that in the southwest region, while ρ(TN), ρ(TP), COD_Mn and ρ(chlorophyll a) were higher in the north region than in the east region in Luoma Lake. The differences of spatial distribution among these physicochemical parameters were attributed to the contaminants contained in the inflowing rivers, the human activities and the self-purification functions in the Luoma Lake, which indicate that the disturbances in suburban lakes caused by human activity should not be ignored.