Dynamics and geochemical responses of dissolved metals (Mn and Cu) in a subtropical estuary, China.

Abstract

Theresearch delved into the occurrence and dynamics of dissolved metals, specifically manganese (Mn) and copper (Cu), withinthe Jiulong River Estuary, South China,a medium-sized subtropical estuary. Our findingsunveiled a nuancedseasonal and spatial variability of dissolved metals throughout the entire estuarine system.Notably,dissolved Mnconcentrations peaked (~ 3.5μM) in the upper estuary, diminishingsharply along the salinity gradient, with a modest risein the middle estuary and outer Xiamen Bay. In the upper estuary, heightenedconcentrations of dissolved Mn occurred in spring due to augmented terrestrial particle inputs, followed by suboxically reductive releases; conversely, concentrations were low in summer, attributed to dilution fromincreased freshwater discharges and particle scavenging. In contrast, dissolved Cu exhibited differently, with elevated concentrations (29.2-37.5nM) in the upper and middle estuaries, driven byreductive dissolution of Mn particles and chloride-induced ion exchanges, respectively. Concurrently, heightenedinputs of nutrients and metalscorrelated with elevated phytoplankton productivity (indicated by chlorophyll a) in the upper andouter estuaryregions. Our analysis underscored the sensitivity ofdissolved metals to environmental parameters, including temperature, pH, and dissolved oxygen. The integration of compiled historical dataunderscored the dynamic nature of dissolved metals, particularly Cu, in response to geochemical processes.The elevated ion levels indicated intensified ion releasesfrom particles and sediments, attributable to increased anthropogenic perturbation and climatic changes(e. g. oceanwarming).