The lability of iron in seawater and its relationship to phytoplankton

A technique was developed to measure chemically labile Fe in seawater with the complexing agent 8-hydroxyquinoline (oxine) and then used to test the hypothesis that Fe lability controls the availability of Fe to phytoplankton. Three colloidal ferrihydrites (X-ray amorphous FeOOH) having different labilities were tested for their ability to support phytoplankton growth. Freshly-precipitated ferrihydrite was approximately 65% oxine-reactive and proved to be an excellent source of available Fe but limited heating caused sharp decreases in lability and in the algal growth response, demonstrating a strong link between Fe lability and availability.; The relationship between total and labile Fe was studied in estuarine, coastal and shelf waters of the Gulf of Maine. Fe lability varied both spatially and temporally from near zero to 100% of the total Fe, indicating that Fe availability cannot be estimated by applying fixed lability ratios to total Fe concentrations in seawater. Furthermore, particulate fractions ({dollar}>{dollar}0.4{dollar}mu{dollar}m) often contributed significant amounts of labile Fe; assumptions that equate "dissolved" Fe with "available" metal are therefore likely misleading when considering phytoplankton nutrition.; The labile Fe requirement for growth of the toxic dinoflagellate Alexandrium tamarense (Balech) was determined in batch culture and compared with labile Fe concentrations in coastal waters. Red tide outbreaks did not correlate with changes in either total or labile Fe concentrations. The organism did not appear to be Fe-limited in nearshore coastal waters or over Georges Bank, but may have been Fe-limited in central basin waters, consistent with the distribution of A. tamarense in the Gulf of Maine.; Irradiating synthetic crystalline and non-crystalline colloidal Fe oxyhydroxides in pH 8 seawater with various light sources increases their chemical lability. The lability of a hypothesized photoproduct decreased rapidly in the dark and similar decreases in Fe lability occur when sunlit surface seawater is placed in the dark. The results suggest that Fe lability in seawater is partly a function of dynamic photoredox cycling of Fe. It is calculated that Fe photoconversions may be a significant, if not major, source of biologically available Fe in surface seawaters.