长江口外海域夏末温跃层与底层水低氧现象研究

基于长江口外海域2009年夏末调查的温盐和溶解氧(DO)资料,采用垂向梯度法对夏末长江口外海域温度垂向结构类型进行划分,计算了温跃层深度、强度和温跃层处DO垂向梯度等参数。长江口外海域夏末水温垂向结构类型及其温跃层强度的分布表现为长江冲淡水、黄海沿岸流、台湾暖流表层暖水和台湾暖流深层冷水交汇、混合的态势。相关性分析表明,温跃层深度与温跃层处DO垂向梯度之间为负相关,温跃层深度与垂向最小DO浓度之间为正相关,温跃层强度与温跃层处DO垂向梯度之间为正相关。最接近DO供给源的上温跃层强度与温跃层处DO垂向梯度具有较强的相关性(r=0.69)。温跃层越浅、强度越大,对底层水低氧现象的影响越大。温跃层、特别是上温跃层引起的水体层化阻碍DO向底层输运,起到了物理隔氧的作用,与低氧现象具有紧密的联系,是低氧现象在长江口外海域从春末到秋季发生、发展、维持和消亡过程的重要环境控制因素。

Analysis of thermocline and hypoxia off the Changjiang Estuary in late summer

Based on the data of temperature,salinity and DO observed in the Changjiang Estuary covering 54 stations in late summer,2009,vertical structure types of water temperature were classified by the method of vertical gradient,and the parameters including depth and intensity of thermocline and vertical gradient of DO on the water layer of thermocline were calculated.In late summer,the distribution of temperature vertical structure and intensity of thermocline off the Changjiang Estuary was a situation of confluence and mixing by the dilute water of Changjiang,the coastal current of Yellow Sea,the warm upper water of Taiwan Warm Current and the cold deep water of Taiwan Warm Current.Correlation analysis showed that,there was a negative correlation between the depth of thermocline and the vertical gradient of DO on the layer of thermocline,there was a positive correlation between the thermocline depth and the vertical minimum DO,and also a positive correlation between the thermocline intensity and vertical gradient of DO on the layer of thermocline.The thermocline intensity,especially the intensity of upper thermocline which was near sea surface,the origin of DO,has a strong correlation(r=0.69)with the DO vertical gradient on the layer of thermocline.The shallower and the stronger of the thermocline,the greater effect of the thermocline on the hypoxia of deep water.Thermocline,in particular,the upper thermocline hindered the surface water with saturated DO mixed with the deep water,played a role of physical barrier of oxygen vertical transportation.The thermocline was an important environmental control factor of the hypoxia off the Changjiang Estuary from late spring to autumn,covering a period of occurrence,development,maintenance and decay of the hypoxia.