IRON

INTERNATIONAL POLAR YEAR – The Netherlands

IPY-GEOTRACES	IRON

Physical and Chemical Speciation of Dissolved Fe in the Polar Oceans
NIOZ Project Leader: - Dr. L.J.A. (Loes) Gerringa Other (NIOZ) contributors: - Prof. dr. H.J.W. (Hein) de Baar (Activity leader of IPY-GEOTRACES) - P. (Patrick) Laan (Technician) - C.-E. (Charles-Edouard) Thuroczy (PhD student) - M. (Maarten) Klunder (PhD student) Abstract: Dissolved (<0.2 micron) Fe in seawater in fact consists of several sizes, from fine colloidal Fe next to an operationally defined soluble (<smallest size cutoff ultra filtration) pool. Moreover organic Fe (III)-complexes exist within both the colloid pool(s) and the soluble pool. Implications are twofold. Firstly within surface waters, the colloid fractions cannot be assimilated unless first dissolved or photo reduced into a bio-available Fe form. Similarly not all Fe (III)-organic complexes are available, where dissociation or photo-reduction serve to make Fe more suitable for plankton uptake. Secondly dissolved Fe in the deep ocean appears controlled by competition between two pools. Global ocean Fe models postulate the soluble Fe (III)-organic maintaining Fe in solution, while the colloids are removed towards the seafloor. The more or less equilibrium distribution of dissolved Fe between colloids and organic complexes will be measured along two sections in the Southern Ocean, and at stations in the Arctic Ocean. Moreover it is realized this quasi-equilibrium description alone is inadequate to fully understand and quantify the reactivity of dissolved Fe for either uptake by plankton in surface waters, or adsorptive scavenging removal in deep waters. Towards more realistic and complete understanding it is essential to combine the above with a kinetic approach for quantification of the rates of transformation between various Fe pools in seawater. Latter transformation rates likely are the key rate-limiting factor(s) for Fe uptake by biota or Fe scavenging rates in the deep ocean. These kinetic experiments will be done in pristine natural seawater, and combined with plankton growth studies as the ultimate proof of biological availability of dissolved Fe in polar ocean waters.