Physicochemical speciation of bioactive trace metals (Cd, Cu, Fe, Ni) in the oligotrophic South China Sea
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Bioactive trace elements Cd, Cu, Fe, and Ni in the oligotrophic South China Sea (SCS) waters exhibit distinct chemical affinities and physical size distributions when using cross flow ultrafiltration and ion exchange techniques, even though they display similar nutrient-type distributions. Dissolved Cu ( 0.4 m) concentrations increased from 0.9 nM in surface waters to 3 nM at depths below 500 m, while those of dissolved Ni increased from 2 to 9 nM; those of dissolved Cd, from 0.01 to 0.9 nM; and those of dissolved Fe, from 0.1 to 0.7 nM. The majority of the dissolved Cd ( 100%) and Ni (> 80%) concentrations were found in the 1 kDa size fraction and as cationic labile forms. More than 50% of the total dissolved Cu in surface waters was in the 1 kDa cationic labile form, while the non-exchangeable fraction that cannot be adsorbed by either cation or anion exchange resins increased from 28% at the surface to 50% below 500 m depth. Similarly, about 60% of total dissolved Fe in surface waters was in the 1 kDa fraction; the colloidal (1 kDa-0.4 m) form of Fe was relatively constant throughout the water column, and amounted to 40% at the surface and 20% in deeper waters. Some fractions of the total dissolved Cu and Fe could be adsorbed by both cation and anion exchange resins, suggesting binding to "zwitterionic" molecules with both anionic and cationic binding sites. The cationic labile form of Fe tightly correlates with phosphate and nitrate, total dissolved Ni significantly correlates with silicate, and total dissolved Cd with phosphate, with different slopes in the upper 100 m than below. These correlations suggest a tight coupling of the cycling of these trace metals with those of individual nutrient elements. Biogeochemical and biophysical interactions between metals and organisms are not only through surface complexation to organic molecules of different molecular weights but they are also tightly coupled to colloidal aggregation due to their surfactant activity, properties that should be considered in future studies of oceanic elemental cycles. 2006 Elsevier B.V. All rights reserved.
