Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean

The Southern Ocean is a high‐nutrient, low‐chlorophyll (HNLC) region characterized by incomplete nitrate (NO 3 − ) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14 N‐ versus the 15 N‐bearing form of NO 3 − , quantified as the NO 3 − assimilation isotope effect ( 15 ε). Previous summertime estimates of the 15 ε from HNLC regions range from 4 to 11‰. While culture work has shown that the 15 ε varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15 ε varies in the field. Here we estimate the 15 ε 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 higher 15 ε (8.9 ± 0.6‰) for the northern Subantarctic Zone and a lower 15 ε (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 the 15 ε 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 , 15 ε increases again. In addition, while we observe no robust relationship of the 15 ε to iron availability or phytoplankton community, mixed‐layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.