Factors regulating the concentration of particulate iodine in coastal seawater

To investigate the factors regulating the concentration of particulate iodine (PI) in seawater, it was measured in two Pacific coastal areas adjacent to Japan (offshore Aomori and offshore Fukushima) in two seasons (May and October) along with parameters such as particulate organic carbon (POC) and nitrogen (PON), dissolved iodine (DI), and phytoplankton pigments. The observed PI concentrations ranged from 0.0068 to 2.2 nmol L-1, with uncertain seasonal and regional differences. The atomic ratio of PI to POC (I/C) ranged from 4.4 x 10(-7) to 1.1 x 10(-4), in line with previous studies on marine phytoplankton, although the lower part of the range of the I/C in this study was found to be somewhat lower compared to the previous studies. The dataset was divided in three groups namely, lower (Group A), average (Group B), and higher (Group C) I/C because the I/C is an indicator of the accumulation efficiency of PI in particles in seawater and POC is the main carrier of PI. Based on the observed POC concentrations, the atomic ratio of POC to PON (C/N), and the ratio of carbon derived from chlorophyll a to POC, the three groups were characterized by phytoplanktonic physiological states as highly productive, steady, and senescent states for Groups A, B, and C, respectively. Based on the finding that PI production is associated with the phytoplankton physiological state, the seasonal and regional differences in PI concentration and I/C in the observation areas were consistently explained. Finally, we suggest that the phytoplanktonic physiological state is one of the vital factors regulating the PI concentration in seawater.

Automated summary

To investigate the factors regulating particulate iodine concentration in seawater, measurements were taken in two Pacific coastal areas off Japan (Aomori and Fukushima) in two seasons (May and October) along with various parameters. The dataset was divided into three groups based on the atomic ratio of iodine to particulate organic carbon, which is an indicator of iodine accumulation efficiency. These groups were characterized by different phytoplanktonic physiological states, explaining the observed seasonal and regional differences in iodine concentration. The study suggests that the phytoplanktonic physiological state is a vital factor regulating iodine concentration in seawater.