Tracking the multidecadal variability of the surface turbidity maximum zone in Hangzhou Bay, China

Abstract

Surface suspended sediment concentration (SSC) retrieved from the satellite data was used to study the multidecadal (1984–2015) dynamics of the surface turbidity maximum zone (TMZ) in Hangzhou Bay (HZB), China. A correlation algorithm was calibrated to estimate the surface SSC using the remote sensing data. The averaged deviation of the retrieved SSC from the field data was 15.6%. The impacts of tidal amplitude and phase on remote sensing data were specifically considered and accurately removed. Five periods were organized to analyze the multidecadal variability of surface SSC: (a) 1984–1988, (b) 1990–1994, (c) 1997–2001, (d) 2002–2006, and (e) 2013–2015. Results showed that the areas of low turbidity zone (<200 mg L–1) increased from period a to e, indicated by the trend ratio (Ts) of 2.58%. The area of TMZ (>700 mg L–1) decreased significantly from 2205 km2 in the period a to 647 km2 in period e, with Ts = −4.57%. The distribution pattern of the SSC was more skewed in period e (Sk = 2.03) than that in the period a (Sk = 0.63). Tidal flat reclamation around HZB contributed to the multidecadal reduction of the surface SSC from 1984 to 2015, through changing the hydrodynamics and reducing sediment source. The decreased sediment load from the Changjiang River contributed to the decline in the surface SSC in HZB, by reducing the sediment source. Flooding of the Qiantang River during wet seasons played an important role in increasing the surface SSC at the bay head, as the deposited sediment during wet seasons reduced the water depth and increased the resuspension. This study helps the estuarine morphology and environment management worldwide.