The Nitrogen Isotopic Composition of Tissue and Shell‐Bound Organic Matter of Planktic Foraminifera in Southern Ocean Surface Waters

Abstract

We present the first nitrogen isotope (δ15N) measurements of planktic foraminifera, paleoceanographically important zooplankton, from the nutrient‐rich waters of the modern Southern Ocean. Foraminifera were collected from net tows in the Subantarctic and Polar Frontal Zones (SAZ and PFZ, respectively) south of Africa during winter 2015 and late summer 2016. In late summer, consistent with preferential uptake of 14N‐nitrate and the progressive, northward depletion of nitrate by phytoplankton across the Southern Ocean, foraminifer tissue and shell‐bound δ15N rise equatorward along with nitrate δ15N. However, foraminifer δ15N is ~3‰ lower than expected for heterotrophs relying on photosynthetic biomass generated directly from nitrate. This discrepancy appears to originate with the particulate organic N (PON) in late‐summer surface waters, the δ15N of which is lowered by ammonium recycling. In winter, when overall productivity and foraminifer production are reduced, foraminifer δ15N is higher (by 4.6 ± 0.8‰ for tissue and by 4.0 ± 1.5‰ for shell‐bound N compared to late summer) and exhibits no clear north‐south trend. These characteristics can also be explained by the feeding‐driven connection of foraminifera to PON, which is elevated in δ15N by net degradation in winter. Therefore, foraminifer δ15N is more closely tied to PON δ15N than to nitrate δ15N in the Southern Ocean mixed layer. Combining our isotope data with previously reported sediment trap fluxes from the western Pacific SAZ/PFZ suggests that, under modern conditions, the late‐summer ammonium recycling signal outweighs that of wintertime decomposition on the annually integrated δ15N of sinking foraminifera.

The Seasonal Cycle of Nitrogen Uptake and Nitrification in the Atlantic Sector of the Southern Ocean

“…In the summer by contrast, light inhibition and rapidly growing phytoplankton allow for rates of surface nitrification that are insignificant relative to both wintertime nitrification and summertime autotrophic NO 3 − consumption. This seasonal pattern is also evident in the available Southern Ocean NO 3 − isotope data that show assimilation to be the major biological process acting on the surface NO 3 − pool in summer (DiFiore et al 2009;Fripiat et al, 2019;Kemeny et al, 2016) with nitrification dominating in winter (Smart et al, 2015).…”

“…This, coupled with the relatively high ambient NH 4 + concentrations that appear to typify the winter mixed layer, may favor bacterial NH 4 + consumption. For the AZ south of Africa, Smart et al (2015) hypothesized that autumn/early winter surface waters are characterized by intense N recycling. Heterotrophic bacteria must play a central role in such recycling, possibly in both the production and consumption of NH 4 + .…”

“…− isotope ratios, Smart et al (2015) concluded that the AZ winter mixed layer is conducive to nitrification because of the highly seasonal nature of these polar waters. Our data set extends this characterization to include the PFZ and SAZ.…”

“…Indeed, it is the strong seasonal cycle-a discrete summertime period of mixed-layer NO 3 − drawdown followed by a low-light winter period characterized by high rates of mixed-layer nitrification and little NO 3 uptake-that appears to render the new production paradigm appropriate for estimating carbon export potential in summer. However, this is complicated in the AZ by the fact that the winter mixed layer, in the form of the T min , is an important NO 3 − source to surface waters in summer (Difiore et al, 2010;Smart et al, 2015). While the T min continuously exchanges NO 3 − with underlying deep waters (i.e., CDW, with NO 3 − originating from this deep water mass being truly "new" to the surface layer), our data and those of Smart et al (2015) suggest that a significant fraction of the T min NO 3 − pool is produced in the winter mixed layer.…”

“…In the winter, a role for surface water nitrification in replenishing nutrients has been hypothesized (Sanders et al, 2007) given the low light levels and deep mixed layers (Buongiorno Nardelli et al 2017;Mitchell & Holm-Hansen, 1991;Sverdrup, 1953). NO 3 − isotope measurements from the polar Southern Ocean support this hypothesis, suggesting significant rates of mixed-layer nitrification in winter (Smart et al, 2015). In addition, most Southern Ocean N uptake studies were conducted during spring and summer (e.g., Cavagna et al, 2015;Joubert et al, 2011;Lucas et al 2007;Mengesha et al, 1998;Sambrotto & Mace, 2000;Savoye et al, 2004;Thomalla et al, 2011;Tripathy et al, 2017;Waldron et al, 1995) with very few in winter (Cota et al, 1992;Philibert et al, 2015).…”