Labile forms of iron in coastal seawater: Otago Harbour, New Zealand

Abstract

The horizontal distribution and speciation of iron in the Otago Harbour, New Zealand, a shallow, vertically well mixed estuary, was investigated on six occasions from April to November 1993. Iron speciation was determined by a suite of analytical methods focussing on the chemical reactivity of iron. The dominant source of iron into the water column in these waters was resuspension of bottom sediments, particularly under windy conditions. The only important freshwater source of iron to the harbour, the Water of Leith, appeared to be a major source of iron to Otago Harbour only under conditions of high stream flow. However, the influence of the Water of Leith was often detectable down harbour by the presence of more reactive/labile iron phases. Prolonged periods of strong winds also appeared to supply large concentrations of reactive iron to the water column, possibly by mixing and resuspension of sediment material from below the redox boundary. Experiments on natural samples from Otago Harbour revealed that at pH 4.0 ascorbic acid was able to reduce Fe(III) oxyhydroxides more effectively than was hydroxylamine hydrochloride. This difference may reflect either different mechanisms for the reduction of iron oxyhydroxides or differences in the crystallinity of these oxyhydroxides.