Dynamics of trace metals during the subarctic Pacific iron experiment for ecosystem dynamics study (SEEDS2001)

During the first iron-enrichment experiment in the northwest subarctic Pacific (SEEDS2001), the dynamics of dissolved and acid dissolvable trace metals were studied. Seawater samples were collected from the upper water column (5-70 m) of In- and Out-patch stations. Immediately after the collection, an aliquot of seawater for dissolved species was filtered through an 0.2 μm filter and acidified to pH 2.2. An aliquot for acid dissolvable species was acidified without filtration and stored for 3 years at an ambient temperature. Before the Fe enrichment, the dissolved Fe concentration in the surface mixed layer was <0.13 nM (the detection limit) and the acid dissolvable Fe concentration was 4.7 nM. The difference was a particulate fraction. Since the photochemical quantum efficiency of algal photosystem II was low, Fe in this fraction was not easily available to phytoplankton. After the Fe enrichment, acid dissolvable Fe in the patch decreased from 9.6 nM on day 2 to 4.6 nM on day 13, which was still ∼3 nM higher than that at the Out-patch station. Dissolved Fe was 1.4 nM on day 2 and decreased exponentially to <0.13 nM on day 11. The concentration ratio of the acid dissolvable fraction to the dissolved fraction for the other trace metals was lower than that for Fe. The acid dissolvable concentrations for Mn, Co, Ni, Cu, Zn and Cd did not show significant change during the observation. The dissolved concentrations for Co, Ni, Cu, Zn and Cd in the surface layer on day 2 were 0.039, 5.0, 1.7, 2.2 and 0.27 nM, respectively. They decreased exponentially to 0.013, 4.3, 1.2, 0.87 and 0.21 nM on day 13, respectively. The mole ratio of the concentration difference between days 2 and 13 was similar to the elemental ratio reported for phytoplankton. These are the first data showing that mesoscale iron fertilization alters the dynamics of dissolved Co, Ni, Cu and Zn.