Summer and winter distributions of dissolved iodine in the Skagerrak

The distribution of dissolved iodate, iodide and total iodine at 12 stations across the Skagerrak between Denmark and Norway in August 2000 and February 2001 is described. Overall, close agreement exists between the graphs of either iodate or total iodine versus salinity, despite hydrographic and nutrient observations demonstrating strong seasonal change in this temperate zone shelfsea system. The study, therefore, demonstrates for a third time that iodine interconversion in temperate shelf-seas is not related simply to either phytoplankton growth or photochemistry. Iodine–salinity graphs consist of a steep part between salinities 35.2 and 34.5, concerned with mixing between surface, intermediate and deep Atlantic water, and a near horizontal part between salinities of 23 and 34.5 concerned with the mixing of surface water from the North Sea and the Baltic Sea, and river run-off in the Skagerrak/ Kattegat area. It is concluded that the iodine system in the Skagerrak is controlled principally by advection in and mixing of waters of pre-formed iodine chemistries. A mechanism in which iodate is reduced during irrigation of coastal anoxic sediment is proposed to account for the lack of seasonal behaviour yet sustained iodate reduction in the water column. The problem of confusing actual chemical reduction of iodate with changes imposed by advection is discussed.