Seasonal metal fluxes derived by the interaction of surface water and groundwater in an aquaculture estuary

Submarine groundwater discharge (SGD) plays a major role as a conveyor of metals to coastal waters. However, the seasonal change of metal fluxes derived through SGD is unclear. Here, we evaluated the behaviours and fluxes of trace metals (Mn, Fe, Ba, Pb, U, Cr, Zn, Cu) in an estuary under different seasonal conditions. The behaviours of trace metals revealed that SGD was the source of Mn (3.51 mmol/(m²·d)), Fe (0.174 mmol/(m²·d)) and Ba (0.024 mmol/(m²·d)), but the Cu sink (−0.55 μmol/(m²·d)) and other metals exhibited a seasonal source-sink conversion. The seasonal variation of dissolved organic matter and the fresh groundwater proportion in subterranean estuaries may have an important effect on metals fluxes especially for the Fe, Mn and Ba. Our result shows that the single seasonal metal fluxes estimation applied to the annual scale will cause a large deviation, up to 3.6 times for Fe, 5.5 times for Mn, and 15 times for Ba. Therefore, the influence of seasonal fluctuations on SGD-derived metal fluxes cannot be ignored, and our findings will be important for comprehending the metal budget and cycle in nearshore environment.