Spatial, Temporal, and Human-Induced Variations in Suspended Sediment Concentration in the Surface Waters of the Yangtze Estuary and Adjacent Coastal Areas

P Li,Y P Chen,K H Xu,B W Shi,C S Wu,J D Milliman,W H Qin,S L Yang

doi:10.1007/s12237-012-9523-x

Abstract

To delineate temporal and spatial variations in suspended sediment concentration (SSC) in the Yangtze (Changjiang) Estuary and adjacent coastal waters, surface-water samples were taken twice daily from 10 stations over periods ranging from 2 to 12 years (total number of samples >28,000). Synoptic measurements in 2009 showed an increase in surface SSC from 0.058 g/l in the upper sections of the estuary to ∼0.6 g/l at the Yangtze River turbidity maximum at the river mouth, decreasing seaward to 0.057 g/l. Annual periodicities reflect variations in the Yangtze discharge, which affect the horizontal distribution and transport of SSC, and seasonal winds, which result in vertical resuspension and mixing. Over the past 10–20 years, annual surface SSC in the lower Yangtze River and the upper estuary has decreased by 55%, due mainly to dam construction in the upper and middle reaches of the river. The 20–30% decrease in mean surface SSC in the lower estuary and adjacent coastal waters over the same period presumably reflects sediment resuspension, in part due to erosion of the subaqueous Yangtze Delta. SSCs in the estuary and adjacent coastal waters are expected to continue to decline as new dams are constructed in the Yangtze basin and as erosion of the subaqueous delta slows in coming decades.

Highlights

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We focus on the Yangtze River (Fig. 1), one of the largest rivers in the world, third in length (6,300 km), fourth in sediment load, and fifth in water discharge (∼900 km3/year) (Zhao et al 2000; Milliman et al 2010)
We address spatial and temporal variations in suspended sediment concentration (SSC) in the surface waters of Yangtze estuary and adjacent coastal waters based on multiple-year samplings at 10 sites

Summary

Methods

Surface water was sampled with 600-ml bottles at nine stations in the Yangtze estuary and surrounding waters (Fig. 1b) twice daily, generally 8AM and 2PM, between 1998 and 2009, and daily at Luchaogang at high tides between June 2002 and May 2010 (Table 1) This sampling arrangement allowed us to define SSC variations along both cross-shore and along-shore transects (Fig. 1b). We analyzed only 20 samples (collected at the Luchaogang Station in July 2003; SSC ranging from 0.2 to 1.4 g/l) for particulate carbon, both calcium carbonate and organic matter. These samples were treated with hydrochloric acid (30%) to remove carbonate, dried at 45°C for 48 h, and combusted at 950°C.

Results

Discussion

Conclusion