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

abstract

  • AbstractThe Southern Ocean is a highnutrient, lowchlorophyll (HNLC) region characterized by incomplete nitrate (NO3) consumption by phytoplankton in surface waters. During this incomplete consumption, phytoplankton preferentially assimilate the 14N versus the 15Nbearing form of NO3, quantified as the NO3 assimilation isotope effect (15). Previous summertime estimates of the 15 from HNLC regions range from 4 to 11. While culture work has shown that the 15 varies among phytoplankton species, as well as with light and iron stress, we lack a systematic understanding of how and why the 15 varies in the field. Here we estimate the 15 from watercolumn profile and surfacewater samples collected in the Indian sector of the Southern Oceanthe first leg of the Antarctic Circumnavigation Expedition (December 2016January 2017) and the Crossroads transect (April 2016). Consistent with prior work in the midtolate summer Southern Ocean, we estimate a higher 15 (8.90.6) for the northern Subantarctic Zone and a lower 15 (5.40.9) 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 and the average mixedlayer photosynthetically active radiation flux of 30100molm2s1, while above 100molm2s1, 15 increases again. In addition, while we observe no robust relationship of the 15 to iron availability or phytoplankton community, mixedlayer nitrification over the Kerguelen Plateau appears to strongly influence its magnitude.

published proceedings

  • Journal of Geophysical Research - Oceans

author list (cited authors)

  • Thomas, R. K., Fawcett, S. E., Forrer, H. J., Robinson, C. M., & Knapp, A. N.

complete list of authors

  • Thomas, RK||Fawcett, SE||Forrer, HJ||Robinson, CM||Knapp, AN