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In regions of the surface ocean with significant concentrations of unconsumed nitrate (NO3-), such as the Southern Ocean, phytoplankton preferentially assimilate the 14N-bearing form of NO3- during NO3- assimilation. This discrimination against the heavier, 15N-bearing form of NO3- is quantified by the NO3- assimilation isotope effect (15e). While a 15e of 5 per mil is commonly assumed for phytoplankton NO3- assimilation, previous field-based observations of the 15e have ranged from 4 to 11 per mil, and even wider variations in 15e have been observed in culture studies that have subjected phytoplankton to iron and/or light stress. In spite of this prior work, we lack a mechanistic explanation for variations in 15e, yet this information is required for interpreting modern water column NO3- d15N and d18O measurements as well as paleoceanographic d15N records. Here we report 15e estimates from springtime water column NO3- isotope profiles collected across four major zones (Subantarctic, Polar Frontal, Antarctic, and Marginal Ice Zones) in the Atlantic sector of the Southern Ocean on the SCALE cruise (Southern oCean seAsonal Experiment; Oct.-Nov. 2019). Consistent with prior austral summer observations, we generally find higher values of 15e in the Subantarctic compared to the Antarctic; however, variations exist within each zone. These data are interpreted in the context of seasonal mixing (closed vs. open system models), phytoplankton community composition, and physiological markers of iron and light stress.
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Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean
Abstract The Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO3−) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the14N‐ versus the15N‐bearing form of NO3−, quantified as the NO3−assimilation isotope effect (15ε). Previous summertime estimates of the15ε from HNLC regions range from 4 to 11‰. While culture work has shown that the15ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the15ε varies in the field. Here we estimate the15ε from water‐column profile and surface‐water samples collected in the Indian sector of the Southern Ocean—the first leg of the Antarctic Circumnavigation Expedition (December 2016–January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the mid‐to‐late summer Southern Ocean, we estimate a higher15ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower15ε (5.4 ± 0.9‰) at and south of the Subantarctic Front. We interpret our data in the context of coincident measurements of phytoplankton community composition and estimates of iron and light stress. Similar to prior work, we find a significant, negative relationship between the15ε and the average mixed‐layer photosynthetically active radiation flux of 30–100 μmol m−2 s−1, while above 100 μmol m−2 s−1,15ε increases again. In addition, while we observe no robust relationship of the15ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.
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- Award ID(s):
- 1851113
- PAR ID:
- 10640431
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 129
- Issue:
- 7
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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