Variations in the chemical lability of iron in estuarine, coastal and shelf waters and its implications for phytoplankton

The relationship between total and chemically labile Fe has been studied in estuarine, coastal and shelf waters of the Gulf of Maine, U.S.A. Measurements of the labile fraction of total Fe, defined by complexation with 8-hydroxyquinoline in 1 h, correlate with the availability of Fe to marine phytoplankton and therefore can be used to estimate Fe availability in seawater. The results show that the relative lability (=labile/total) of Fe in seawater varied both spatially and temporally from near-zero to 100%. Although particulate Fe (>0.45 μm) was generally less labile than dissolved Fe (<0.45 μm), the particulate fraction often contributed substantially to labile Fe concentrations overall. Conversely, as much as 75% of ‘dissolved’ Fe was non-labile, and therefore was probably not available to phytoplankton. In seawater/river-water mixing experiments, aggregation diminished the relative lability of Fe by 30%, even though much of it remained in the ‘dissolved’ fraction. Considering phytoplankton nutrition, these results demonstrate that equating dissolved Fe concentrations with ‘available’ metal can be misleading. Furthermore, the large variability observed in the labile proportion of total Fe in seawater indicates that Fe availability to phytoplankton cannot be estimated by applying fixed lability-ratios to total Fe concentrations.