Fe-binding ligands, TAN1504 and TAN1605

The governance of organic ligands in Fe availability for marine microbes has simultaneous effect on the biogeochemical cycling of trace metals and major nutrients, i.e. C and N. Investigating organic Fe speciation is of fundamental important to identify the processes regulating the dynamics of key ligand groups. Here we present organic Fe speciation along a transect covering the inner and outer shelf to open ocean, from the Hauraki Gulf in north-eastern of the North Island of New Zealand, which is sampled in two consecutive year during Autumn. The electrochemical approach, competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE-AdCSV) was applied. In the inner shelf, total ligand concentration ([Lt]) was ranged from 2.08 to 5.23 nM eq. Fe for both voyages, and explained by the various ligand sources in the inner shelf including primary production, the sediment flux and remineralization. At the shelf break, [Lt] was still relatively high ranged from 1.00 ??? 5.65 nM eq. Fe, with indication of post-bloom ligand sources in surface water, i.e. secondary production after a phytoplankton bloom. From the shelf-break further to open ocean, surface [Lt] was decreased (~0.6 ??? 2.7 nM eq. Fe) along with the decreased in surface primary production. The passive generation of ligands via bacterial remineralization of organic matter appears to be a ligand sources in the deep water >500m from the shelf break toward the open ocean. Our result showed that the most variable excess ligands concentration ([L??], ~0.5 ??? 5 nM eq. Fe) was observed in surface water despite its oligotrophic condition. In the deeper water, [L??] was fairly static at 0.5 to 1 nM eq. Fe, with a higher complexation capacity relative to the surface water. This results could have further implication that complexation capacity of natural ligands determine the threshold of DFe concentration in the water column, especially at depth.