Trace metal concentrations in the bulk and non-sinking suspended particulate matter of the Gaoping River plume after heavy rainfall events

Abstract

Between May and September, monsoon rains contribute 99.8 % of the water flowing down the Gaoping River annually, and an even greater percentage of the suspended particulate matter (SPM) discharged to the coast. The Gaoping River links vast amounts of highly erodible sedimentary material to the marine environment, creating turbid plumes at the freshwater-marine interface. Climate warming is projected to increase the intensity of flood events and the amount of erosion in the river catchment, yet the export processes delivering this material to the offshore environment are poorly quantified. In this study, particulate organic carbon (POC) and trace metals were measured in the SPM collected in surface waters of the river plume after moderate, strong and very strong pulses of freshwater discharge. Both the bulk and non-sinking portions of the SPM had a terrigenous origin and were not distinguishable from each other based on their trace metal contents. Particulate metal contents primarily depended on SPM concentration and increased continuously with decreasing SPM, i.e. when progressing towards the open sea. A striking result was that the non-sinking SPM discharged to the coast exhibited only a 3-fold increase in response to a 10-fold increase in river discharge and 400-fold increase in total SPM. This may reflect the limited supply of organic compounds responsible for imparting a stabilising surface charge on these mineral particles. Despite representing a very small percentage of the total SPM, the non-sinking SPM was shown to be a potentially important and long-lived transport vehicle for river-derived metals in the surface circulation of the South China Sea.