Soluble and Colloidal Iron in Southern Ocean Waters During Spring

Historical measurements of "dissolved" iron in open-ocean seawater have been operationally defined as the fraction of iron which passes through a 0.4 μm (or 0.2 μm) pore membrane filter. As such, these measurements are likely to include iron(III) and iron(II), both free and complexed by a variety of organic and inorganic ligands, as well as colloidal organic and inorganic iron-containing species. While this "dissolved" fraction may be a useful indicator for the biological availability of iron in the field, recent work suggests that perhaps only soluble iron in the <0.03 μm size range is directly available to phytoplankton. The spatial and temporal changes in the vertical distribution of soluble iron (<0.03 μm), colloidal iron in two size-fractions (0.03-0.2 μm and 0.2-0.4 μm) and acid-soluble particulate iron (total dissolvable minus "dissolved") were studied in the open Southern Ocean and seasonal sea-ice zone during austral spring along a transect from Hobart (Australia) to the Antarctic continent, with analysis by flow injection chemiluminescence. Our results show that the majority (59 % on average) of the "dissolved" pool was in the smallest size-fraction (i.e. soluble species). However, colloidal iron accounted for a significant fraction (up to 67 %) of the "dissolved" pool in the upper water column at stations near the sea-ice zone and in ice-free waters over the Antarctic shelf. Clear differences in iron distributions were observed between the <0.2 μm and <0.4 μm size-fractions at all stations, which has important implications for the choice of filter size used by different investigators to define the "dissolved" pool. Total dissolvable iron concentrations were ~41 % greater than "dissolved" (<0.4 μm) iron values, implying the presence of a significant proportion of acid-soluble particulate iron. These observations will be compared to other ocean regions and discussed in terms of the bioavailability of different iron fractions to the Antarctic biomass.