ABSTRACT

The vertical distributions of seven dissolved trace metals (Mn, Fe, Co, Ni, Cu, Zn and Cd) in seawater were investigated in Mikawa Bay of central Japan. We collected seawater samples two times in summer, 2012, when a hypoxic layer developed at the bottom, and analyzed them by an inductively coupled plasma mass spectrometer (ICP-MS) following filtration through a 0.45-μm membrane filter and pre-concentration using a chelating resin column. Different distribution patterns are observed in the vertical profiles of the analyzed metals in spite of the shallow water depth up to 14 m. Firstly, Mn, Fe and Co showed high concentrations at the lower hypoxic layer. It is very likely that Mn and Fe oxides in sediments dissolved into seawater under reducing environment. At the same time, Co adsorbed onto Mn/Fe oxides may also have been released into water column. The maximum Mn concentration reached 565 nmol/kg at the hypoxic layer. This value exceeded predicted no-effect concentration of Mn²⁺ for marine biota (496 nmol/kg) defined by Ministry of the Environment, Japan. Secondly, Cu, Cd and Zn concentrations decreased with increasing water depth like scavenged-type distribution although they are normally observed as recycled-type in open ocean and Suruga Bay, another bay in central Japan. These metals had negative correlations with salinity, suggesting that their distributions were regulated by dilution with open seawater. Moreover, sediments might uptake these metals under reducing condition. Surprisingly, Cd depleted in all the layers; its concentration was quite low (0.030 nmol/kg on average) compared with other bays and North Pacific Ocean. Lastly, Ni showed more complex pattern similar both to Mn and Cu depending on sampling stations and times. In this research, we could make it clear that DO in seawater and inflowing river water are important factors to control the distribution of the seven bioactive trace elements. Given that whole of the area is photic in shallow Mikawa Bay, where phytoplankton grows from the surface to the bottom, hypoxia here will directly influence primary production and marine biota in terms of rich in or lack of the bioactive trace elements as well as a shortage of oxygen.