长江口及其邻近海域悬浮颗粒物浓度和粒径的时空变化特征

Spatiotemporal variations in concentration and size of suspended particulate matter in the Changjiang(Yangtze River)Estuary and its adjacent sea

长江口是典型的高浊度河口,长江口及其邻近海域悬浮颗粒物(suspended particulate matter,SPM)浓度跨度大,泥沙过程活跃、复杂。2015年7月9-20日(洪季)和2016年3月7-19日(枯季),使用OBS和LISST分别测定了该区域99个和89个站位的SPM浊度、光衰减系数、总体积浓度、平均粒径和粒径谱等参数;同时通过现场过滤测定了各站位表、中、底3层的SPM质量浓度以及典型站位SPM中颗粒有机碳(particulate organic carbon,POC)的!13C、颗粒氮(particulate nitrogen,PN)的!15 N以及POC/PN摩尔比值。结果表明,浊度、光衰减系数、总体积浓度等3个参数均与SPM质量浓度显示出了显著的正相关关系。研究区域SPM平均粒径一般表层大于底层、枯季大于洪季;长江淡水端元输出的SPM粒径枯季也明显大于洪季。具有相似粒径谱特征的SPM可以通过测定!13C和!15 N值来进一步区分其来源和组成。SPM质量浓度和总体积浓度等参数结合还可以计算SPM有效密度,用以了解研究区域SPM的沉降过程。结果表明两个季节SPM有效密度和粒径之间显示出了显著的负相关关系,说明枯季长江输出的SPM由于粒径大、密度小、沉降速度低,加之强烈的垂直混合和口门拦门沙附近的再悬浮,随着环流可能到达研究区域北部的最东端;而洪季长江输出的SPM由于粒径小、密度大、沉降速度高,在口门附近快速沉降。

The Changjiang（Yangtze River）Estuary is a typical high-turbidity estuary,and Suspended Particulate Matter（SPM）in the Changjiang Estuary and its adjacent sea have large variation ranges of mass concentration,and show quite active and complicated behaviors there.SPM at 99 and 89 stations in this area was studied using OBS and LISST instruments in July 9-20,2015 and in March 7-19,2016,respectively,and parameters of turbidity,beam attenuation coefficient,floc total volume,mean size,and size spectrum of SPM were obtained from the two instruments.SPM mass concentration at surface,middle,and bottom layers of all stations were obtained after filtration,and !13 C（‰）values of particulate organic carbon（POC）,!15 N（‰）values of particulate nitrogen（PN）,and molecular ratios of POC/PN in SPM at typical stations were also obtained.The three parameters of turbidity,beam attenuation coefficient,and floc total volume all showed significant positive correlations with SPM mass concentration.Larger SPM mean sizes were generally found at bottom rather than at surface in the study area,and generally found in the dry season rather than in the flood season.The mean sizes of SPM released by the Changjiang freshwater were also much larger in March than in July.The SPM contents with similar size spectra could be further differentiated by their significantly different!13 C and!15 N values.The parameter of SPM mean effective density,calculated by mass concentration divided by floc total volume,can help elucidate the field depositing processes.The two parameters of SPM mean effective density and size showed significant negative correlations during both cruises.All the above results suggest that in March under the influence of strong vertical mixing,the Changjiang-originated SPM（with larger size,lower density and then lower depositing velocity）would be mixed with the resuspended sediments near the Changjiang mouth bar,and then transported as far as the eastern edge of the northern part of the study area.However in July,the Changjiangoriginated SPM with smaller size,higher density and then higher depositing velocity would deposit rapidly near the river mouth.