Seasonal changes in speciation of dissolved chromium in the surface Sargasso Sea

The concentrations of Cr (III) and Cr (VI) were determined in the upper 500 m of the Sargasso Sea at different times of the year. Total Cr concentrations ranged between 2.5 and 4.5 nM, and the predominant species of Cr was the thermodynamically favoured Cr (VI). Cr (III) concentrations were greatest during periods of high biological activity, as indicated by strong correlations between the ratio of Cr (III):Cr (VI) and both bacterial biomass and primary productivity. Whilst the oxidation state varied substantially over the seasonal cycle, the upper water column inventory of Cr was relatively constant. The long residence time of Cr in the surface Sargasso Sea estimated with respect to atmospheric inputs (1410–2240 years) is indicative of the largely conservative behaviour of this element. Deep particle fluxes of Cr are similar or somewhat greater than atmospheric inputs, and this downward flux of Cr is well correlated with organic carbon fluxes from the upper Sargasso Sea. The long residence time coupled with the seasonal changes in oxidation state of Cr implies rapid seasonal recycling of Cr between redox states, with only slow removal via particles to the deep ocean at this site.