Sources and transport of dissolved Mn, Fe, Cu, and Co in a semi-enclosed bay (Ariake Sea) in Japan

Abstract

This study investigates the origins and transport mechanism of dissolved trace metals (d-Mn, d-Fe, d-Cu, and d-Co) and macronutrients in Ariake Sea, a shallow and semi-enclosed inlet bay during early summer. Concentrations of macronutrients and dissolved trace metals (d-Mn, d-Fe, d-Cu, and d-Co) were the highest in the surface waters of the innermost station in Ariake Sea and decreased gradually from the nearshore regions to the East China Sea. During estuarine mixing, d-Fe, d-Mn, and d-Cu showed non-conservative patterns, while d-Co exhibited a conservative pattern. This variability reflects the combined effects of water circulation and geochemically driven cycles in shaping the distribution of the metals. D-Mn and d-Fe were removed during estuarine mixing. Meanwhile, d-Cu pattern indicated input via intra-estuarine processes. The internal flux of d-Cu (1.6 × 1013 nmol/month) calculated from our data was slightly higher than its river fluxes (1.1 × 1013 nmol/month). We proposed sedimentary fluxes (2.1 × 1011–1.8 × 1013 nmol/month) supplied more d-Cu than other possible sources such as photodissolution of SPM, groundwater discharge, and atmospheric deposition. The conservative pattern of d-Co reflected a balance between input and removal during estuarine mixing.