洞庭湖区沉积物氮磷累积时空分布及其水沙响应机制

Distribution and spatiotemporal accumulation of nitrogen and phosphorus in sediments of Dongting Lake and their water-sediment response mechanism

河、湖岸带随泥沙沉积的氮、磷等营养盐是维系河流廊道生态系统的重要资源,其沉积特征与水沙情势密切相关.在洞庭湖区,受长江上游筑坝影响,下游河湖水沙输送在近20年来发生显著变化,必然影响岸带营养盐沉积.本研究在洞庭湖洲滩及长江分流入湖的松滋、虎渡、藕池河尾闾岸滩采集沉积物柱状样,测定分层营养盐含量,通过放射性核素测年,结合历史水沙数据,阐明沉积物营养盐时空分布特征,揭示营养盐沉积对水沙条件变化的响应.结果表明:长江三口-洞庭湖沉积物总磷、总氮含量分别为285~781、339~1041 mg·kg^(-1),平均值分别为640、658 mg·kg^(-1),垂向上大多点位氮、磷含量随深度增加呈先增大后减小或减小-增大-减小的“S”形变化趋势,峰值出现在沉积柱中上段;空间分布上,三口沉积物营养盐含量与其分泄水沙量及上游来水氮、磷浓度表现一致,由大到小依次为松滋、藕池、虎渡,河道沉积物营养盐含量与沿程冲淤强度有关,三口尾闾冲刷强度与口门段相比较小,营养盐含量相对较高,洞庭湖洲滩营养盐含量受面源污染、泥沙淤积及入湖径流影响表现为东洞庭湖>西洞庭湖>南洞庭湖;时间分布上,沉积物营养盐含量以葛洲坝(1980年)和三峡(2003年)建成运行为节点呈稳定波动-上升-下降的三阶段变化,三峡水库蓄水后三口分沙量锐减、湖区由淤转冲是沉积物氮、磷含量减少的关键因素.现阶段洞庭湖区沉积物TN、TP总体呈轻度到中度污染水平,其中,磷污染需重点关注.

Nutrients,such as nitrogen and phosphorus,that deposit with sediment in river and lake riparian zones are vital resources for maintaining river corridor ecosystems.The features of nutrient deposition are closely related to water and sediment conditions.In the Dongting Lake area,dam construction in the upper Yangtze River has significantly altered the transport of water and sediment in downstream rivers and lakes in the past two decades,thereby impacting the deposition of nutrients in the riparian zone.This study involved the collection of sediment columns from the beaches of Dongting Lake and the outlets of the Songzi,Hudu,and Ouchi rivers,which serve as the three main sources supplying the lake with water and sediment from the Yangtze River.Nitrogen and phosphorus concentrations were measured and the age of the deposited sediment was determined using radioactive isotopes.In combination with historical hydrological data,the vertical distribution characteristics of sediment nutrients as well as the differences among the sampling sites were revealed;the response of nutrient dynamics to changes in water and sediment conditions were discussed.Results indicated that the total phosphorus and nitrogen content in the sediment of Songzi,Hudu,Ouchi River,and Dongting Lake ranged from 285~781 mg·kg^(-1)(mean value 640 mg·kg^(-1)) and 339~1041 mg·kg^(-1)(mean value 658 mg·kg^(-1)),respectively.Sediment nutrient content in the vertical profile generally showed a trend of S-shape(decrease-increase-decrease) or single-peak(increase-decrease),and the peaks mostly appeared in the middle or upper sediment layers.Spatially,the nutrient content in the sediment of the three outlets correlates with their water and sediment discharge,as well as the nitrogen and phosphorus concentrations in the upstream inflow.In descending order,this trend is observed as follows:Songzi,Ouchi,and Hudu.The nutrient content in river sediment is related to erosion and deposition intensity along the channel.The scouring intensity in the outlets was relatively moderate compared to the inlets,resulting in relatively higher nutrient content.Under the influence of non-point source pollution,sediment deposition,and water inflow,nutrient content in the beach of Dongting Lake followed the order of East Dongting>West Dongting>South Dongting.Temporally,dynamics in nutrient content could be divided into three stages:maintained stable before Gezhouba Dam construction(1980),increased in 1980—2002,and decreased after the Three Gorges Reservoir operation(2003).The sharp reduction in sediment discharge,the change from deposition to scouring,and the decrease in water level under the same water discharge were the key factors to the reduction after 2003.Currently,the sediment environment of Dongting Lake is generally under slight or moderate pollution,with particular attention needed for phosphorus pollution.