Abstract
Estuary plays an essential role in the transformation processes and fate of riverine substances, such as trace metals. Therefore, it is important to reveal the distributions of trace metals in estuarine to understand their biogeochemical cycle. In this study, we determined the concentration of dissolved trace metals; manganese (d-Mn), iron (d-Fe), copper (d-Cu), and cobalt (d-Co), and macronutrients in Ariake Sea, a shallow semi-enclosed inlet bay in western Japan. This study aims to investigate possible sources and transport of these metals.The seawater samples were taken by clean sampling method on board Kakuyo-maru (Nagasaki Univ.) in May during 2018 cruise. By using a single preconcentration system with chelating resin column and inductively couple plasma mass spectrometry (ICP-MS), we determined d-Mn, d-Fe, and d-Cu concentration in the samples. On the other hand, d-Co concentration in seawater was measured by cathodic stripping voltammetry (CSV) with nioxime.The concentration of all analyzed trace metals and macronutrients showed higher values in river waters compared to those in seawaters. The highest concentration of trace metals and macronutrients were detected in the surface waters of the innermost station of Ariake Sea and decreased gradually from the nearshore region to the East China Sea (ECS).During estuarine mixing, all trace metals showed non-conservative patterns, suggesting the distributions of all analyzed trace metals were not simply controlled by physical mixing between rivers and ECS waters. The deviation of trace metals concentration relative to the mixing line indicated either removal or addition. The metal removal was pronounced for d-Fe due to the flocculation; d-Mn was also scavenged from the water column during estuarine mixing. D-Cu and d-Co exhibited a similar pattern in our study, implying that a similar mechanism governs their distributions. Moreover, the upward deviations of d-Cu and d-Co relative to the mixing line highlighted the importance of their benthic input from the seafloor sediments within the embayment system. By applying the estuary mixing model, we estimated their internal fluxes were slightly higher than those of riverine fluxes.
