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Kelp (Laminaria digitata) uses iodide as a unique inorganic antioxidant to protect its surface and apoplastic space against reactive oxygen species such as ozone, hydrogen peroxide and superoxide, with implications for atmospheric and marine chemistry as well as regional climatic processes. If kelp is covered by seawater, this results in iodide leaching into surrounding sea water. In this study, the influence of the kelps Laminaria digitata , L. hyperborea , L. ochroleuca and Saccharina latissima on iodine speciation chemistry was explored at two sites in Oban (Argyll, Scotland) and Roscoff (Brittany, France) based on diver-operated in situ sampling. Seawater samples were subsequently analysed voltammetrically, accompanied by determination of extractable iodine concentrations in the tissues of the thalli surveyed by ICP-MS. The main result is that iodide concentrations in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate concentrations are decreased in comparison to open coastal water and open ocean concentrations. Using diver-operated in situ sampling with seawater samples subsequently analysed voltammetrically, this study finds that iodide levels in the vicinity of kelp thalli are strongly enhanced, especially at low tide, while iodate levels are decreased in comparison to open coastal water and open ocean concentrations. [Display omitted] • This study uses a new diver-operated in situ syringe sampling technique. • Samples analysed voltammetrically. • Iodide levels around kelp thalli are strongly enhanced, especially at low tide. • In contrast, iodate levels are decreased in comparison to open waters. [ABSTRACT FROM AUTHOR]