鄱阳湖浮游植物时空变化特征及其对极端洪枯事件的响应

Spatial and temporal characteristics of phytoplankton in Lake Poyang and its response to extreme flood and drying events

鄱阳湖作为中国最大的淡水湖泊,其水生态健康状态始终是人们关注的热点。近些年,鄱阳湖极端洪旱灾害频发,浮游植物生长受极端洪旱的影响发生了明显变化。为分析浮游植物时空变化特征、探究环境因子对鄱阳湖浮游植物影响机制以及极端洪枯事件对浮游植物的影响,利用结构方程模型(SEM)构建浮游植物与环境因子的影响路径模型,定量分析环境因子对浮游植物的影响程度。结果表明,鄱阳湖浮游植物以蓝藻、绿藻为主且有明显的季节特征,在7月丰水期浮游植物密度达到最高;由结构方程模型(SEM)可知,影响浮游植物密度最关键的因子为物理因子(水温>pH>透明度>溶解氧),其次为营养物质(总氮>硝态氮>总磷>磷酸盐),浮游植物对高温、高营养和高pH较偏好。2020年极端洪水和2022年极端高温干旱,浮游植物密度主要受水温、溶解氧、透明度等物理因子的影响;在影响较小的营养物质中,主要的限制性因素分别为磷和氮。与正常年份相比,极端洪水年鄱阳湖受入湖来水及降雨的增多,湖区水量急剧增加,“稀释”作用超过水温和透明度对浮游植物生长的促进作用造成浮游植物密度和生物量有所下降;在营养物质中,磷成为主要限制性因素。而极端高温干旱年受入湖来水的减少及湖水的快速蒸发,“浓缩”作用超过水温和透明度对浮游植物生长的抑制作用造成浮游植物密度和生物量显著增加,同时,营养物质对浮游植物的作用更加凸显。研究结果表明极端洪枯事件导致鄱阳湖浮游植物变化明显,确定其对浮游植物的影响机制,可以为极端洪枯事件下浮游植物监测、管理提供一定的理论基础。

As the largest freshwater lake in China,the water ecological health status of Lake Poyang has been highly concerned.In recent years,extreme floods and droughts occurred frequently in Lake Poyang.Phytoplankton growth has changed significantly under the influence of extreme floods and droughts.In order to investigate the spatial and temporal dynamics of phytoplankton,and to explore the influence mechanism of environmental factors on phytoplankton and the influence of extreme flood and drought events on phytoplankton in Lake Poyang,structural equation modeling was used to develop the influence path model between phytoplankton and environmental factors,and to quantify the extent of the influence of environmental factors on phytoplankton.The results showed that the phytoplankton in Lake Poyang was dominated by cyanobacteria and green algae with seasonal variation.The most critical factors affecting the abundance of phytoplankton were physical factors(water temperature>pH>secchi depth>dissolved oxygen),followed by nutrients(total nitrogen>nitrate nitrogen>total phosphorus>phosphate).Phytoplankton preferred high temperature,nutrients and pH.In the extreme flood in 2020 and the extreme high temperature and drought in 2022,the density of phytoplankton were mainly affected by physical factors such as water temperature,dissolved oxygen and secchi depth.For nutrients,the main limiting factors were phosphorus and nitrogen.Compared with normal years,in the extreme flood year,Lake Poyang suffered from the increase of incoming water,rainfall and lake water volume.The dilution effect exceeded the promotion of phytoplankton growth by water temperature,dissolved oxygen and secchi depth,resulting in the decrease of phytoplankton density and biomass.Among the nutrients,phosphorus was the main limiting factor.In extreme high temperature and drought years,due to the reduction of incoming water and the rapid evaporation of lake water,the concentration effect exceeded the inhibitory effect of water temperature and secchi depth on phytoplankton growth,resulting in a significant increase in phytoplankton density and biomass.Meanwhile,the impacts of nutrients on phytoplankton were enhanced.This study showed the significant impacts of the extreme flood and drought events on phytoplankton in Lake Poyang.Determining the mechanism of their impacts on phytoplankton can provide scientific support for phytoplankton monitoring and management under extreme meteorological events.