The current water trophic status in Tiaoxi River of Taihu Lake watershed and corresponding coping strategy based on N/P ratio analysis

Ecological stoichiometry is an important measurement to evaluate water trophic status. In this study, the ratio of nitrogen to phosphorus (N/P, an index of stoichiometry) was used to assess the nutrient conditions of Tiaoxi River which is the main inflow river of Taihu Lake. The results from field monitoring data showed that nitrogen contents in Tiaoxi River were much higher than surface water quality standards in China but phosphorus pollution was relatively light. The average concentration of total nitrogen (TN) was up to 3.5 mg/L, which was worse than Grade V (2mg/L) of national surface water quality standards in China, while the average concentration of total phosphorus (TP) was 0.18 mg/L, which met the water quality requirement of Level III (0.2 mg/L). In addition, nitrate and paniculate phosphate were the main pollutants in Tiaoxi River. The average content of nitrate was up to 2.1 mg/L, indicating that Tiaoxi River was in a high oxidation environment and its self-purification ability was strong, whereas the mean value of dissolved phosphate (DP) was low (0.07 mg/L). Nutrients concentrations presented the similarly seasonal variations. The maximum concentration of TN was in winter, and the minimum level was in spring and autumn. The dynamic changes of TP concentration were roughly similar to TN, but TP presented a sharp decline in spring and fell to the lowest level throughout the year. Furthermore, correlation analysis also showed certain correlations among different nutrients index. TN and TP, DP were significantly positively correlated, indicating that the ways for nutrients inputs to Tiaoxi River were substantially the same, which suggested that Tiaoxi River was mainly influenced by agricultural non-point source pollution. TN/TP ratio represents the nutrients structure in Tiaoxi River for phytoplankton growth. According to the previous studies, TN/TP < 7 and TN/TP > 30 were used to categorize the different trophic status of nitrogen and phosphorus limitation in aquatic ecosystem, respectively. The N/P ratios showed that Tiaoxi River in spring and autumn (TN/TP > 30) were under the condition of phosphorus limitation. It was demonstrated that the water was suitable for algae growth in summer (7