Qualitative Dynamics of Suspended Particulate Matter in the Changjiang Estuary from Geostationary Ocean Color Images: An Empirical, Regional Modeling Approach

Abstract

The suspended particulate matter (SPM) in Changjiang Estuary is characterized by a high concentration of significant diurnal dynamics. With a higher temporal resolution (eight images obtained per day), Geostationary Ocean Color Imager (GOCI) was selected as the primary remote sensor for the dynamics monitoring in this paper, instead of other satellite sensor working in polar orbit. Based on the characteristics of the field spectra measured in the estuary, an empirical model was established with the band ratio of Rrs745 divided by Rrs490 and proven effective in Suspended Particulate Matter (SPM) estimation (R2 = 0.9376, RMSE = 89.32 mg/L). While, Validation results showed that the model performed better in coastal turbid waters than offshore clear waters with higher chlorophyll-a concentration, stressing the importance of partitioning SPM into its major components and doing separate analysis. The hourly observations from GOCI showed that the diurnal variation magnitudes exhibited clear regional characteristics, with a maximum in the turbidity belt near the mouth and a minimum in the offshore deeper areas. In addition, comparing the monthly averaged SPM distribution with the amount of sediment discharged into the estuary, the variation in estuarine turbidity maximum zone is more likely contributed by the sediments resuspended from the sea bed that has already accumulated in the estuarine delta.

Variation of Suspended Particles in the Bottom Layer of the East China Sea with Data from Seafloor Observatory

“…In the ECS, tidal currents play an important role in the variation of suspended sediments distribution with the maximum tidal range reaching about 4.5 m [40]. On the inner shelf near the mouth of the estuary, current-induced bottom shear stresses are usually so strong that they could stir the fine-grained sediments accumulated on the seabed and then transport sediments to the upper waters by vertical mixing [21,24,41]. Located near the central area of the shelf with a lower current speed than the inner shelf, tidal influences on sediments at the seafloor observatory may be restricted to layers near the bottom, according to earlier studies based on satellite observations over the region [21,24].…”

“…On the inner shelf near the mouth of the estuary, current-induced bottom shear stresses are usually so strong that they could stir the fine-grained sediments accumulated on the seabed and then transport sediments to the upper waters by vertical mixing [21,24,41]. Located near the central area of the shelf with a lower current speed than the inner shelf, tidal influences on sediments at the seafloor observatory may be restricted to layers near the bottom, according to earlier studies based on satellite observations over the region [21,24]. Since no extreme currents occurred during the two periods according to the measured data (Figure 4), we conducted harmonic analysis with the assumption that variations in the absolute volume concentrations of the suspended particles resulted from tidally-induced resuspension and advection (Figure 6).…”

“…It is seriously influenced by terrestrial sediment input by diluted water from Changjiang River. According to the data measured in the Datong station and published in the East China Sea Marine Environmental Bulletin from 2010 to 2016, the Yangtze River imports about 1.307 × 10 8 tons of sediment into the East China Sea every year [21]. As shown in Figure 1, subtidal currents in the East China Sea are complex and considerably impact the hydrological environment.…”

“…The dispersal pattern of suspended particles in the ECS has been intensively investigated, as it is crucial for describing the distribution of geo-biochemical materials in the coastal and offshore areas. However, most of the studies are limited to the surface layer [21,22] and only few researches provided insight into the sediment transport in the bottom layer [23]. Many studies were conducted to examine sediment resuspension and transportation in an area from a macro perspective [21,24], with few provisions of detail features.…”

“…However, most of the studies are limited to the surface layer [21,22] and only few researches provided insight into the sediment transport in the bottom layer [23]. Many studies were conducted to examine sediment resuspension and transportation in an area from a macro perspective [21,24], with few provisions of detail features. More advanced methods for monitoring and analyzing are needed in current research.…”