Real-Time Redox Speciation of Iron in Estuarine and Coastal Surface Waters

An automated, shipboard-use system was developed for real-time speciation of iron in coastal surface waters. It comprised a towed Fish underway sampler and a modified reverse flow injection analysis system with a liquid waveguide capillary flow cell spectrophotometric detection device. The detection was based on the reaction between ferrozine and Fe(II). The detection limits of 0.3 and 0.7 nM were achieved for Fe(II) and Fe(II+III), together with their respective dynamic linear ranges of 0.5-250 and 0.9-250 nM. The system was successfully deployed and run consecutively for about 1 week during a cruise in August 2009 to the East China Sea off the Changjiang Estuary. The distribution of operationally defined field dissolvable Fe(II) and Fe(II+III) (expressed as Fe-a(II) and Fe-a(II+III)) in these areas was obtained, which showed that both Fe-a(II) and Fe-a(II+III) concentrations decreased with salinity when there were relatively high Fe-a(II) concentrations (up to about 120 nM) near shore. A distinct distribution of Fe-a(II) to Fe-a(II+III) ratios was also revealed, with a ratio of 0.58 in the water off Changjiang Estuary and 0.19 in the open ocean.