Variability in the speciation of iron in the northern North Sea

Variations in the speciation of iron in the northern North Sea were investigated in an area covering at least two different water masses and an algal bloom, using a combination of techniques. Catalytic cathodic stripping voltammetry was used to measure the concentrations of reactive iron (Fe-R) and total iron (Fe-T) in unfiltered samples, while dissolved iron (Fe-D) was measured by GFAAS after extraction of filtered sea water. Fe-R was defined by the amount of iron that complexed with 20 mu M 1-nitroso-2-napthol (NN) at pH 6.9. Fe-T was determined after W-digestion at pH 2.4. Concentrations of natural organic iron complexing ligands and values for conditional stability constants, were determined in unfiltered samples by titration. Mean concentrations of 1.3 nM for Fe-R, 10.0 nM for Fe-T and 1.7 nM for Fe-D were obtained for the area sampled. Fe-R concentrations increased towards the south of the area investigated, as a result of the increased influence of continental run off. Fe-R concentrations were found to be enhanced below the nutricline (below similar to 40 m) as a result of the remineralisation of organic material. Enhanced levels of Fe-T were observed in some surface samples and in samples collected below 30 m at stations in the south of the area studied, thought to be a result of high concentrations of biogenic particulate material and the resuspended sediments respectively, Fe-D concentrations varied between values similar to those of Fe-T in samples from the north of the area to values similar to those of Fe-R in the south. The bloom was thought to have influenced the distribution of both Fe-R and Fe-T, but less evidence was observed for any influence on Fe-R and Fe-D. The concentration of organic complexing ligands, which could possibly include a contribution from adsorption sites on particulate material, increased slightly in the bloom area and in North Sea waters. Iron was found to be fully (99.9%) complexed by the organic complexing ligands at a pH of 6.9 and largely complexed (82-96%) at pH 8. The Ligands were almost saturated with iron suggesting that the ligand concentration could limit the concentration of iron occurring as dissolved species. (C) 1998 Elsevier Science B.V.