Rivers are a major source of trace metals to the ocean, influencing biogeochemical cycling and ecological functioning in shelf environments. Trace metals delivered by rivers can function as essential micronutrients or toxic contaminants, depending on their concentration and speciation. In recent decades, increasing anthropogenic pressures, including sewage discharge, urbanization, and agricultural runoff, have likely altered the concentrations of trace metals in river systems, thereby raising questions about their possible downstream impacts on coastal environments.
Despite the major role of rivers as trace metal sources to the ocean, there are few systematic surveys of riverine trace metal distributions. Current understanding remains disproportionately focused on a limited number of large, well-monitored systems. In southern China, metal concentrations in the Pearl River Estuary have been extensively surveyed; however, the extent to which these distributions are representative of other regional systems remains unclear.The majority of smaller and medium-sized rivers remain insufficiently characterized, leading to large uncertainties in regional flux estimates and coastal metal budgets.
To address these limitations, we conducted a systematic survey of 136 river outlets along the southern coast of China, providing a comprehensive assessment of spatial variability in dissolved trace metals alongside other key biogeochemical flux terms including nutrients and carbonate chemistry. Nutrient concentrations clearly show our survey captured a broad range of systems from catchments with limited nutrient inputs, to heavily polluted rivers (ranges 1-1244 µM nitrate, 0.05-31 µM phosphate, and 2-383 µM dissolves silica). A subset of 17 rivers was further investigated during both wet and dry seasons to evaluate seasonal dynamics. Riverine trace metal fluxes were quantified by integrating concentration measurements with discharge estimates, enabling a first-order constraint on the magnitude and variability of trace metal inputs from river systems to the northern South China Sea.