Surface ocean dissolved organic nitrogen concentration and isotopic composition in the eastern tropical South Pacific

Current models of dissolved organic nitrogen (DON) production and consumption suggest that DON is generated in higher productivity settings such as upwelling environments and is then advected into more oligotrophic gyres where DON may be slowly consumed, in particular if surface DON is exchanged with subsurface microbes. However, evaluation of such models is data limited, and in particular, few measurements of DON concentration and/or nitrogen isotopic composition (“d15N”) exist from the South Pacific. Here we present measurements of the concentration and d15N of DON in surface water samples collected in the eastern tropical South Pacific (ETSP) during the austral summers of 2010 and 2011. Samples were collected from diverse settings, including from regions adjacent to highly productive upwelling systems as well as in the ultra-oligotrophic ETSP gyre. While the concentration and d15N of DON from the ETSP are broadly consistent with prior measurements from the North Pacific, i.e., DON concentrations of 5 +/- 1 uM and DON d15N of 5 +/- 1 per mil, small but potentially significant gradients exist between stations. Here we ask whether: 1) the d15N of newly produced DON is related to the d15N of subsurface NO3, as has been observed elsewhere; 2) there is evidence for the accumulation of DON in surface waters as it is advected from regions of higher to lower productivity; 3) there is evidence for DON loss between surface and sub-euphotic zone waters; and, 4) what the d15N of any “consumed” DON is and how it compares with the d15N of nitrate accumulating in the shallow subsurface.