Vertical and seasonal variations of dissolved iodine concentration in coastal seawater on the northwestern Pacific coast of central Japan

Abstract

Vertical and seasonal variations in the concentrations of total dissolved iodine (TDI), iodide (I−), iodate (IO3−), and dissolved organic iodine (DOI) were examined for seawaters from the northwestern Pacific coast of central Japan (48 m water depth, ca. 1 km offshore) from 2012 to 2016. The annual averages of TDI and iodate concentrations in the upper layer (water depth < 10 m) were below those from deeper levels. Additionally, the concentrations decreased significantly during summer to autumn. The vertical and seasonal trends in TDI and iodate concentrations were suggested to be originated from the uptake of iodate by settling particles within the water column and partially from the input of river waters. The annual average of iodide concentration in the upper layer was higher than that in deeper levels, and the concentration tended to increase during summer to autumn. Further, the iodide concentration showed a significant positive correlation with the water temperature. The results suggest that the iodide concentration was mainly controlled by the efficiency of vertical mixing in the water column, as deeper seawater generally contains low iodide concentration. The vertical and seasonal variations in DOI concentration were erratic and thus obscured its controlling factors.