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Superoxide, formed by the single electron reduction of oxygen, is important due to its ability to act as a mild reductant under typical environmental conditions. Using chemiluminescence detection with the reagent methyl Cypridina luciferin analog (MCLA), we have measured superoxide at picomolar concentrations in the water column of the Costa Rica Dome (CRD) seasonal upwelling region of the eastern equatorial Pacific Ocean. After 0.45 μm filtration, superoxide decayed in a pseudofirst order manner but more slowly than expected (rate constants varied from -4 to 9.7 × 10-3 s -1), which we hypothesize is due to complexation of metals that could otherwise scavenge superoxide, and the noncatalytic nature of superoxide decay at subnanomolar concentrations. Depth profiles revealed a particle-associated and nonphotochemically generated source of superoxide, implicating biological processes in its production. Superoxide in surface waters exhibited a diel cycle but concentrations were comparable to those in samples stored in the dark for at least 30 min, suggesting that the nonphotochemical source may dominate over abiotic photochemical superoxide production in the CRD. Elevated concentrations of superoxide in localized zones of the ocean may promote the reduction of several biologically important trace metals, thereby increasing their solubility and potential bioavailability. © 2008 American Chemical Society.