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

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 SPM was obtained from the two instruments. SPM mass concentration at surface, middle, and bottom layers of all stations were obtained after filtration, and δ¹³ C(%) values of particulate organic carbon (POC), δ¹⁵ N(%) The 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 SP M mass concentration. Larger SPM mean sizes were generally found at bottom rather than surface in the study area, and generally found in the dry season rather than the flood season. The mean sizes of SPM released by the Changjiang freshwater were also greater In March than in July. The SPM contents with similar size spectra could be further differentiated by their significant different δ¹³ C and δ¹⁵ N values. The parameter of SPM mean effective density, calculated by mass concentration divided by floc total volume, can help 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 f In as the eastern edge of the northern part of the study area. July, the Changjiang-originated SPM with smaller size, higher density and then higher depositing velocity would deposit rapidly near the river mouth.