Apparent biological fractionation between Cd and PO4 in the surface waters of the equatorial Pacific Ocean

Apparent biological fractionation between cadmium (Cd) and phosphate (PO4) in seawater from 10 to 200 m in the western and central equatorial Pacific was examined in January 2003 at nine stations. In this study period, a surface salinity front existed at approximately 175°W, dividing oceanographic conditions into two types: the Western Pacific Warm Pool (WPWP) (westward) and the eastern upwelling zone (eastward), and, at a depth of approximately 100-150 m, a thermocline developed, separating the surface water from the nutrient-rich deep water. Based on the temperature distribution, three types of water masses were defined: within the WPWP (> 29.2 °C), outside the WPWP above the top of the thermocline (the top of the thermocline is the water depth at which the temperature is 1 °C lower than the surface temperature), and below the top of the thermocline, generally corresponding to PO4 concentrations of less than 0.25 μM, from 0.25 μM to 0.50 μM, and over 0.50 μM, respectively. The fractionation coefficient values were determined to be 0.36, 1.09, and 1.68 for the three layers defined, respectively, on the assumption that the uptake and regeneration processes of Cd and PO4 by the biological activity obeyed first-order reaction kinetics. Similarly, Hagino et al. (2000) [Hagino, K., Okada, H., Matsuoka, H. (2000). Spatial dynamics of coccolithophore assemblages in the Equatorial Western-Central Pacific Ocean. Marine Micropaleontology 39, 53-72] have previously found that planktonic coccolithophorid assemblages in these waters also differ between these water masses, presumably discriminating among the three layers when selecting their habitat. By analogy, I speculate that the fractionation coefficient between the two constituents Cd and PO4 varies according to differential uptake/regeneration of the species assembly in these populations of primary producers (and grazers), where sheer biomass and environmental conditions likely also play a role.