Contribution of living and degrading kelp to coastal iodine fluxes

Laminaria digitata forms locally extensive and ecologically important kelp beds in northern temperate regions. Its sporophytes retain large quantities of iodine which, when released into seawater (as iodide, I⁻) and into air (as molecular iodine, I₂), have the potential to contribute significantly to coastal iodine biogeochemical processes. Here, standing crop of a living L. digitata bed and biomass of degrading kelp deposited near the high tide mark were quantified in western Ireland (53°09′25″N, 09°06′58″W) in summer 2011. Measurements revealed an average of 17 living individuals m⁻², accounting for 0.6 (kg dw) m⁻², and the biomass of degrading kelp averaged 0.05 (kg dw) m⁻². We further investigated the iodine-retention capacity of L. digitata and the effect of degradation. Under controlled conditions, I⁻ release into seawater and I₂ emission into air depended on the physiological state of L. digitata. Fresh and “healthy” phylloids absorbed I⁻ from seawater with I₂ emission rates reaching 27 nmol (g dw)⁻¹ h⁻¹. After 4 days of degradation, I₂ emission rates were enhanced by a factor of 3.1. Over 29 days of submersed degradation, ~4.7 μmol I⁻ (g dw)⁻¹ were released into seawater, a process which could trigger iodine recycling in coastal areas. Combined field and laboratory data suggest that living L. digitata sporophytes possess an I₂ emission capacity of ~16 μmol m⁻² h⁻¹, whilst emersed degrading kelp contributes 1–5 μmol m⁻² h⁻¹ to coastal atmospheric I₂ during summer. This study provides evidence that not only living but also degrading kelp acts as driver of coastal iodine fluxes.