Distribution and lability of dissolved iron in surface waters of marginal seas in southeastern Asia

More than eighty surface water samples from marginal seas in southeastern Asia, including the South China Sea, East China Sea, Western Philippine Sea, and the Taiwan Strait, were collected in order to assess the sources and transport of iron in this region. Dissolved Fe (Chelex-labile and total dissolved) concentrations were determined. Additionally, samples were collected from near-shore waters along the west coast of Taiwan during two different seasons and analyzed for potential lability of Fe. Although total dissolved Fe concentrations were, in general, replete in coastal waters and decreasing seaward, the proportions of the dissolved labile (Chelex extractable) and non-labile Fe fractions showed substantial spatial variability. Results from dissolved Fe distributions indicate that Fe in estuarine and coastal regimes forms highly stable non-labile complexes, which are removed quickly, resulting in moderate to low dissolved concentrations offshore. Most of the Fe-replete river plumes contain a large proportion of dissolved non-labile Fe species that also show a low potential ‘bio-reactivity’ and seasonally variable concentrations, making biological production in the coastal and adjacent marginal seas temporally dependent on the potential chemical reactivity of the different Fe fractions in the source waters. Regional flow patterns in the study area led to localized supplies of Fe-enriched waters in the northern Taiwan Strait and southern East China Sea waters.

► Higher dissolved Fe in the southern ECS has the potential to enhance phytoplankton bloom.
► Source strength of Fe controlled their bioavailability in marginal seas.
► The characteristics of non-labile Fe are likely Fe oxides and oxyhydroxides.
► Fe-replete water in near-shore waters may not be completely bioavailable.