The occurrence of alkylated volatile iodide andselenide species was evidenced and investigatedin water, sediments and overlying atmosphere ofthree major European estuaries, such as theGironde (F), the Rhine (NL) and the Scheldt(B/NL), along with the salinity gradient. Foriodine, up to eight volatile species wereobserved as alkyl-iodides in estuarine waters.The major one (ca. 40%) was methyl-iodide(MeI) with average seasonal concentrationsranging from 1 to 100 pmol l-1. Otherspecies observed were found to correspond toseveral halomethane derivatives and lightalkyl-iodide species. For selenium, dimethylselenide (Me2Se) was the main compound(ca. 90%) over three methyl-selenidesencountered in estuarine waters. Me2Seaverage seasonal concentrations were found torange between 0.2 and 100 pmol l-1 in thewater column. The occurrence of methylatediodides and selenides seems to be mainlyrelated to the algae's biomass turnover asindicated by photosynthetic pigment tracers(i.e. chlorophyll a and phaeopigments) andseasonal variation of surface waterconcentrations. The production and release ofgaseous iodide and selenide compounds may thenresult from natural biological pathways leadingto the methylation of their inorganic form.Finally, significant volatilisation rates wereevaluated leading to average seasonal fluxrates for total volatile iodide and selenidecompounds ranging from 4 to 100 and from 1 to75 nmol m-2 d-1, respectively.Estuarine mass balance estimated from MeI andMe2Se distributions indicates thatvolatilisation to the atmosphere represents aprimary sink for MeI and Me2Se fromestuarine surface waters.
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