Coupling iodine speciation to primary, regenerated or “new” production: a re-evaluation

In two recent studies (Tian et al., Deep-Sea Res. I 43 (1996) 723–738; Campos et al., Deep-Sea Res. II 43 (1996) 455–466), in order to explain the observed temporal variations in the distributions of the dissolved iodine species at the time-series stations in the Mediterranean Sea, the North Atlantic and the North Pacific, diverse assumptions were invoked on the relationships between changes in the speciation of dissolved iodine in the surface oceans and biological production such that the surface enrichment of I− was linked to both regenerated production and primary production while the surface depletion of [IO3−+I−] was linked to “new” production. However, while some of the major conclusions in these studies are critically dependent on these assumptions, the validity of the assumptions has yet to be verified with experimental evidence. On the other hand, while there are still significant unknowns in the understanding of the cycling among dissolved iodine species in the surface oceans, presently available data from laboratory cultures and field observations are consistent with an alternative conceptual model in which IO3− and NO3− are taken up at some ratio to each other during NO3− uptake and almost all of the IO3− taken up is exuded as I−. Thus, the depletion of IO3− and the enrichment of I− in the surface water are linked to NO3− uptake. This alternative model is also consistent with the data sets presented by Tian et al. (1996) and Campos et al. (1996). By linking the surface depletion of IO3− to NO3− uptake, significantly different biogeochemical behavior of the marine dissolved iodine system may be inferred. The extent to which I− may be oxidized to IO3− within the euphotic zone during the residence time of the water in the surface ocean is still an open question.