VARIABILITY IN THE SPECIATION OF IRON IN THE NORTHERN NORTH-SEA

Variations in the speciation of iron in the northern North Sea were in vestigated 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 react ive iron (Fe-R) and total iron (Fe-T) in unfiltered samples, while dis solved 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 complexi ng ligands and values for conditional stability constants, were determ ined 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 a rea investigated, as a result of the increased influence of continenta l run off. Fe-R concentrations were found to be enhanced below the nut ricline (below similar to 40 m) as a result of the remineralisation of organic material. Enhanced levels of Fe-T were observed in some surfa ce samples and in samples collected below 30 m at stations in the sout h of the area studied, thought to be a result of high concentrations o f biogenic particulate material and the resuspended sediments respecti vely, 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 distr ibution 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 li gands, which could possibly include a contribution from adsorption sit es on particulate material, increased slightly in the bloom area and i n North Sea waters. Iron was found to be fully (99.9%) complexed by th e 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 t hat the ligand concentration could limit the concentration of iron occ urring as dissolved species. (C) 1998 Elsevier Science B.V.