Interbasin isotopic correspondence between upper-ocean bulk DON and subsurface nitrate and its implications for marine nitrogen cycling

Abstract

Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/14N that is substantially higher than the 15N/14N of suspended particulate organic nitrogen (PNsusp) found in the same surface waters (with δ15N of ∼4 to 5‰ and ∼−1 to 1‰, respectively). Moreover, the concentration and 15N/14N of DON are much less dynamic than those of PNsusp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and 15N/14N of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 μM, but the average δ15N of DON is significantly different, 3.9‰ versus air in the North Atlantic and 4.7‰ in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the δ15N of nitrate is 2.6‰, while it is 4.0‰ in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the 15N/14N of DON relative to surface ocean PNsusp as well as the interbasin difference in the 15N/14N of DON. In this model, DON is produced from PNsusp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PNsusp pool, as an integral part of the ammonium‐centered cycle that lowers the 15N/14N of PNsusp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the 15N/14N elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N2 fixation rates and DON concentration and 15N/14N on short time scales.

The Origin and Role of Organic Matrix in Coral Calcification: Insights From Comparing Coral Skeleton and Abiogenic Aragonite

“…Most (∼75%) of the N appears to be in a loosely bound organic form with a δ 15 N of 5.6‰ (Table 2). Although we did not measure the nitrogen isotopic composition of the Vineyard Sound seawater from which the aragonites precipitated, 5.6‰ is within the range (albeit on the high end) of dissolved organic nitrogen (DON) δ 15 N in the Atlantic Ocean (∼3-6‰) (Knapp et al, 2011). Thus, our results may indicate that DON is either adsorbed or weakly trapped by aragonite crystals precipitating from seawater, as previously hypothesized (Benzerara et al, 2011).…”

Elemental and isotopic characterization of dissolved and particulate organic matter in a unique California upwelling system: Importance of size and composition in the export of labile material

“…A key advantage to isolating ultrafiltered (i.e., HMW) DOM is that it allows for the direct measurement of stable isotopes (d 13 C and d 15 N), which are extremely difficult to make for total DOM when significant quantities of DIN are present (Meador et al 2007;Knapp et al 2011). Average HMW DOM d 13 C (220.4% 6 0.8%) from all periods ( Fig.…”

Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

“…15 NO 3 value of 6.2‰ below the permanent thermocline near SEATS Station [Wong et al, 2002] [Altabet, 1988;Saino and Hattori, 1987;Knapp et al, 2011].…”

“…A significant flux of low d 15 N-DON derived from N fixation could export out of surface waters [Mulholland et al, 2006], although, many studies argued that a large fraction of DON is recalcitrant [Knapp et al, 2005] and not responsive to the sporadic events of N fixation [Knapp et al, 2011]. On the other hand, we prescribed a steady state for this model which might not be realistic; accumulation of low d 15 N-DON in surface might cause uncertainties.…”