丹江口水库是我国重要战略水源地,沉积物中营养盐的释放会对水库水质产生影响。为探究不同水期水库沉积物中营养盐的分布情况及污染状况,分别于丰水期、平水期和枯水期开展沉积物采样调查分析。结果表明:2021—2022年丹江口水库沉积物有机质含量丰水期为3.8%~8.6%,平水期为4.3%~11.7%,枯水期为3.7%~11.7%;沉积物总氮含量丰水期为549.6~1 752.0 mg/kg,平水期为571.8~1 144.9 mg/kg,枯水期为496.0~1 948.5 mg/kg;沉积物总磷含量丰水期为395.0~867.0 mg/kg,平水期为360.9~1 379.6 mg/kg,枯水期为496.1~1 053.9 mg/kg;丹江口沉积物中总氮含量高的时段主要集中在丰水期和枯水期, 水库沉积物氮磷含量在不同水期存在一定的波动。与国内其他湖泊相比,丹江口沉积物中氮磷含量处于较低到中等水平,沉积物中有机质的差异主要是受水库水体交换以及水库的调节和储存特性影响,总氮的差异受水库周边农田施肥以及降雨量导致的外源污染与泥沙输入的影响,总磷的差异则主要是受水体交换差异的影响。依据沉积物基准分级,库区氮磷污染总体在安全级至最低级,对丹江口水库水质的影响较小。

[Objective] Danjiangkou Reservoir is a vital strategic water source in China. With the construction and operation of the Middle Route of the South-to-North Water Diversion Project, the heightening of the Danjiangkou Dam has raised the reservoir water level, reduced the flow velocity, and altered the hydrodynamic conditions. Influenced by historical sedimentation processes and socio-economic development in the watershed, the contents of nitrogen and phosphorus in sediments pose potential release risks under the new hydrological regime. This study aims to systematically evaluate the distribution patterns and pollution status of sediment nutrients in Danjiangkou Reservoir across different hydrological periods. [Methods] Sediment samples were collected during the wet, normal, and dry seasons. Following standardized analytical procedures, the contents of organic matter (OM), total nitrogen (TN), and total phosphorus (TP) in the sediments were measured. Based on the obtained data, the concentration levels, spatial heterogeneity, and temporal dynamics of OM, TN, and TP were analyzed, and the associated ecological risks were assessed using established evaluation criteria. [Results] (1) From 2021 to 2022, the OM content in sediments of Danjiangkou Reservoir ranged from 3.7% to 11.7%, with marked variations across hydrological periods. Specifically, the content was 3.8%-8.6% in the wet season, 4.3%-11.7% in the normal season, and 3.7%-11.7% in the dry season. The water exchange capacity and the reservoir’s water regulation and storage regimes were the dominant factors leading to the temporal differences in OM content. (2) The TN content in the sediments ranged from 496.0 to 1 948.5 mg/kg, with seasonal values of 549.6-1 752.0 mg/kg for the wet season, 571.8-1 144.9 mg/kg for the normal season, and 496.0-1 948.5 mg/kg for the dry season. Overall, TN contents were higher in the dry and wet seasons than in the normal season. The elevated TN level in the dry season was closely associated with farmland fertilization, while the increase in the wet season was attributed to intensified rainfall and exogenous pollution inputs. In addition, relatively high TN contents were observed in sediments at the confluences of inflow rivers and in the near-dam area. (3) The TP content in the sediments ranged from 360.9 to 1 379.6 mg/kg, with seasonal ranges of 395.0-867.0 mg/kg in the wet season,360.9-1 379.6 mg/kg in the normal season,and 496.1-1 053.9 mg/kg in the dry season. The spatial and temporal variations of sediment TP were mainly driven by seasonal differences in the reservoir flow velocity, water exchange frequency, and flood discharge activities. (4) The contents of OM, TN, and TP generally decreased with increasing sediment depth. At most sampling sites, the vertical variation trends were not significant, indicating a slight surface accumulation of organic matter, nitrogen, and phosphorus, but no pronounced stratification was observed overall. [Conclusions] Compared with other similar lakes and reservoirs in China, the accumulation levels of nitrogen and phosphorus in Danjiangkou Reservoir sediments are relatively low and exhibit seasonal fluctuations regulated by rainfall-runoff and hydrodynamic conditions. The nitrogen and phosphorus pollution levels in the reservoir fall within the safe to minimally polluted range, demonstrating a favorable background condition for sediment nutrients. In general, the degree of endogenous pollution in the reservoir sediments is low and has limited impact on water quality. Targeted management strategies can be formulated according to the seasonal variation of nitrogen and phosphorus across different hydrological periods.