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 (NO3-) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14N- versus the 15N-bearing form of NO3-, quantified as the NO3- assimilation isotope effect (15 epsilon). Previous summertime estimates of the 15 epsilon from HNLC regions range from 4 to 11 parts per thousand. While culture work has shown that the 15 epsilon varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15 epsilon varies in the field. Here we estimate the 15 epsilon 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 epsilon (8.9 +/- 0.6 parts per thousand) for the northern Subantarctic Zone and a lower 15 epsilon (5.4 +/- 0.9 parts per thousand) 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 epsilon and the average mixed-layer photosynthetically active radiation flux of 30-100 mu mol m-2 s-1, while above 100 mu mol m-2 s-1, 15 epsilon increases again. In addition, while we observe no robust relationship of the 15 epsilon to iron availability or phytoplankton community, mixed-layer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude. The Southern Ocean, and the Subantarctic in particular, is an import region for global carbon cycling that is largely driven by microscopic algae, phytoplankton. Regarded as a high-nutrient and low-chlorophyll region, the Subantarctic surface nutrient essential for phytoplankton growth, nitrate, remains in high concentrations over the summertime growing season. During the consumption of nitrate, phytoplankton assimilates nitrate isotopes at varying ratios, likely due to environmental factors, and imparting isotopic signatures. During the Antarctic Circumnavigation Expedition and Crossroads transect, we collected samples from the Indian Sector of the Subantarctic during the mid-to-late summer. Here we use measurements of nitrate and its isotope ratios to better constrain these signatures and identify potential environmental drivers. While phytoplankton community composition and iron availability from coincident measurements may influence the nitrate isotopic assimilation ratios, we observe a strong, significant correlation between light stress across the upper ocean and isotopic signatures. This suggests that the influence of light availability for phytoplankton growth is the leading driver of nitrate isotopic ratios in the mid-to-late summer season of the Indian Sector of the Subantarctic. Nitrate assimilation isotope effects are estimated for the Indian sector of the Subantarctic and Antarctic regions of the Southern Ocean Nitrate assimilation isotope effects increase with decreasing average mixed layer PAR flux within an optimal range Dual nitrate isotopic composition profiles showed evidence of significant summertime nitrification on the Kerguelen Plateau