Dissolved organic nitrogen in the global surface ocean: Distribution and fate

Abstract

The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean's oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface‐accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone.

Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives

“…In the oxic surface waters of the 506 modern ocean, any NH 4 + released by breakdown of organic matter is immediately and 507 typically quantitatively nitrified via sequential oxidation to NO 2 -and NO 3 -, in most cases 508 erasing any isotope fractionation associated with nitrification. In rare cases where the 509 oxidation is incomplete, the residual NH 4 + is enriched in 15 N (e.g., Granger et al, 2011), 510 owing to the first oxidation step to nitrite (NO 2 -), which involves a large fractionation (ε NO2- The distribution and fate of DON in the modern ocean, in particular its possible assimilation 527 by primary producers, remain poorly constrained (Bronk et al, 2007;Letscher et al, 2013). 528…”

A multitrophic model to quantify the effects of marine viruses on microbial food webs and ecosystem processes

“…Total virus counts are centered in the range of 10 8 -10 11 particles l À 1 (Danovaro et al, 2011;Breitbart, 2012) and are set here to be 10-fold higher than their corresponding microbial hosts (Suttle, 2007). Dissolved inorganic nitrogen is depleted in the ocean surface, with concentrations of o1 mmol l À 1 typical in the subtropics (Williams and Follows, 2011), in contrast to dissolved organic nitrogen that is enriched in the ocean surface with concentrations in the range of 2 to 7 mmol l À 1 (Letscher et al, 2013). Population densities and nutrient concentrations vary spatiotemporally, often deviating from the current set of target densities; other values could also be used in the targeted optimization procedure.…”

Variable C : N : P stoichiometry of dissolved organic matter cycling in the Community Earth System Model

“…We have also tested other hypotheses for DOM cycling formulations such as non-variable C : N : P cycling stoichiometry (i.e., DOM cycling occurs at the Redfield ratio) as well as more rapid turnover of DOM in the EZ compared to the MZ (the DOM OPT simulation contains more rapid turnover of DOM in the MZ, following the work of Carlson et al, 2004;Letscher et al, 2013a). To test these hypotheses, we performed two additional BEC simulations termed RED-FIELD and EZRAPID using a coarser-resolution version of the BEC model with a nominal 3 • horizontal resolution.…”