Effects of the disappearance of seasonal thermal stratification on the mobilization of iron and phosphorus in reservoirs, southern China—A case of Tianbao Reservoir, Nanning City

Abstract

为了探究我国南方地区水库季节性热分层消亡前后沉积物铁和磷的迁移规律，于2019年11月-2020年1月对天雹水库多点位（浅水区和深水区）水体理化指标进行原位监测，并利用薄膜扩散梯度技术（DGT）高分辨率获取沉积物有效态铁和磷的浓度分布.结果表明：（1）天雹水库沉积物有效态Fe和P浓度分别为4.67~18.72和0.003~0.073 mg/L，其中有效态Fe浓度较太湖、鄱阳湖和洞庭湖高出一个数量级，且浅水区沉积物中有效态Fe浓度较深水区高；（2）热分层消亡过程，沉积物有效态Fe和P浓度的时空差异大，浅水区有效态Fe浓度表现为Day14>Day1>Day32>Day54，有效态P浓度表现为Day1>Day14>Day32>Day54，而深水区有效态Fe和P浓度变幅较小甚至呈增长趋势，归因于浅水区热分层结构对气象因子扰动的响应速率快，水体垂向混合加剧了沉积物中Fe和P的释放；（3）沉积物-水界面处有效态Fe和P的交换通量分别为6.58~31.59和0.008~0.086 mg/（m²·d），均自沉积物向上覆水中不断迁移，由于浅水区的热分层消亡时间早于深水区，浅水区内源Fe²⁺和PO₄^3-的释放对冬季水质恶化的贡献较大，应密切关注水库浅水区沉积物内源释放对整个库区水质的影响.;To investigate the mobilization of iron and phosphorus in the sediments of reservoirs in southern China during the disappearance of seasonal thermal stratification, physicochemical indexes were in-situ monitored from November 2019 to January 2020 in shallow and deep area of Tianbao Reservoir. Simultaneously, diffusive gradients in thin films (DGT) technique were used to acquire the concentration distributions of labile Fe and P in the sediments. The results show: (1) The concentrations of labile Fe ranged from 4.67 to 18.72 mg/L, which far exceeded that of Lake Taihu, Lake Poyang and Lake Dongting, while the concentrations of labile P ranged from 0.003 to 0.073 mg/L. The concentrations of labile Fe in sediments of the shallow area were higher than those of the deep area. (2) The space-time differences between labile Fe and P in the sediments were obvious during the disappearance of thermal stratification. In the shallow area, the concentrations of labile Fe presented decline order: Day14>Day1>Day32>Day54, and labile P was Day1>Day14>Day32>Day54, while the concentrations of labile Fe and P increased in the sediments of the deep area. It was attributed to the quick response of thermal stratification in shallow area to the sudden drop in temperature, consequently increasing the release of Fe and P at the sediment-water interface. (3) The diffusion flux of labile Fe at the sediment-water interface ranged from 6.58 to 31.59 mg/(m²·d), and that of labile P ranged from 0.008 to 0.086 mg/(m²·d), which meant Fe and P diffused from the sediments to the overlying water. The mixing time in the shallow area was earlier than the deep area, resulting in the significant contribution of internal Fe and P release to deterioration of water quality in winter. Therefore, the internal release of sediments in the shallow area should be paid more attention in reservoirs of southern China.