基于氮磷比解析太湖苕溪水体营养现状及应对策略

生态化学计量学是评价水体营养状态的重要手段,利用其氮磷比指标探讨了我国太湖主要入湖河流苕溪的营养状态。野外监测结果显示,苕溪水体氮素超标严重,磷素污染轻度,硝酸盐、颗粒态磷为氮磷的主要赋存形态,且氮磷浓度呈现相似的季节变化规律,表明苕溪主要受农业面源污染影响。氮磷比分析表明,苕溪水体春、秋季处于磷素限制状态,夏季适合藻类生长,冬季低温条件下不利于藻类的大量繁殖;苕溪生物量增长受磷素限制,线性拟合亦显示其氮磷比主要受磷素波动的调控;苕溪干流大面积暴发蓝藻水华的风险较部分支流及死水区低,苕溪水入湖后,特别是夏季其暴发风险将显著提高。针对苕溪水体的富营养化现状,提出若干条水质改善应对策略。

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 particulate 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(730),and linear regression showed that N/P ratio was mainly affected by phosphorus fluctuations.In the inlets of Taihu Lake and some other dead water zones,the TN and TP concentrations were higher than 0.80 mg/L and 0.20mg/L,respectively,and the water flow velocity was slow,thus dominant bloom-forming cyanobacteria was not limited by nutrients content.Consequently,the TN/TP ratio,absolute nutrients concentration and water flow status were appropriate to meet the requirements of algal bloom.By researching and analyzing the whole river,it was concluded that the algal bloom risk in main stream of Tiaoxi River was lower than in part tributaries and dead water zones,which would significantly increase the algal bloom risk in the inlets zones of Taihu Lake,especially in summer.Furthermore,some strategies for water quality improvements were proposed based on the eutrophication status of Tiaoxi River.