Dynamics of trace metals with different size species in the Pearl River Estuary, Southern China

The Pearl River Estuary (PRE), the largest estuary in Southern China, regulates the fluxes of riverine trace metals into the South China Sea. However, the geochemical behavior of trace metals in this estuary is still ambiguous. In this study, we investigated the dynamics of trace metals in different phases (i.e., particulate, colloidal and truly dissolved) in the PRE in both wet and dry seasons characteristic of the region. Transformations of trace metals between particulate (>0.45 μm), colloidal (1 kDa to 0.45 μm) and truly dissolved (<1 kDa) phases were observed during the estuarine mixing. Colloidal metals (except for Pb) showed non-conservative 'removal' behavior in the low-salinity zone (S < 10‰), suggesting the coagulation of colloidal metals and subsequent deposition to bed sediments being an important sink of dissolved trace metals. By contrast, truly dissolved metals exhibited a mid-salinity maximum distribution (i.e., Mn, Ni, Cu and Cd) or little variation along the salinity gradient (i.e., Fe). The increase of truly dissolved metal concentration was accompanied by the decrease of particulate metal concentration, indicating that the desorption of suspended particles was an important source of dissolved Mn and Cd in the PRE. Metal released from the suspended particles increased in the dry season due to the high suspended particulate matter concentration. Dissolved Mn concentration in the bottom water in wet season was higher than that in dry season, implying that benthic Mn input increased as the bottom water became hypoxic. Abnormally high concentrations of particulate and dissolved Pb were observed at the lower PRE, implying the presence of a potential point source pollution. A flux model predicted that total dissolved Fe, Ni, Cu and Zn underwent net removal while dissolved Mn and Cd had net inputs during the estuarine mixing. This study raveled contrasting geochemical behaviors of trace metals with different size phases and the different sources and sinks of dissolved metals in the PRE.