Three Years of Observations Show a Sharp Boundary for Diazotrophs and N2 Fixation Rates near the Northern Subtropical Front of the North Pacific Ocean

Wingkwan Mak1, Mary Rose Gradoville1, Hanna Farnelid2, Angelicque E White3, Kendra A Turk-Kubo1, Brittany Stewart4, Francois Ribalet5, Sara Ferrón6, Paulina Pinedo-Gonzalez7, Virginia Armbrust5, David M Karl3, Seth John8 and Jonathan P Zehr1, (1)University of California Santa Cruz, Ocean Sciences, Santa Cruz, CA, United States, (2)Linnaeus University, Department of Biology and Environmental Science, Kalmar, Sweden, (3)University of Hawaii at Manoa, Department of Oceanography, Honolulu, HI, United States, (4)University of Southern California, Los Angeles, United States, (5)University of Washington, School of Oceanography, Seattle, WA, United States, (6)University of Hawaii, Department of Oceanography, Honolulu, HI, United States, (7)Lamont -Doherty Earth Observatory, Geochemistry, Palisades, NY, United States, (8)University of Southern California, Earth Sciences, Los Angeles, United States
Abstract:
Recent studies have documented the presence of N2-fixing microorganisms (diazotrophs) in unexpected environments, including temperate and high-latitude waters. One region which has been particularly under-sampled for diazotrophs is the North Pacific Transition Zone (NPTZ), which spans the Pacific Ocean between the subtropical and subarctic gyres. Here, we explore patterns in diazotrophic abundance and N2 fixation rates in the North Pacific Subtropical Gyre (NPSG), a habitat for diazotrophs, and in the NPTZ, where colder water temperatures and higher nutrient concentrations could be expected to exclude diazotrophs. Samples were collected during three springtime meridional cruises along ~158°W, spanning from ~22°N to ~42°N. We observed strong patterns in diazotrophic abundances and N2 fixation rates on all three cruises. In the NPSG, the most abundant diazotroph was the symbiotic cyanobacterium UCYN-A1 (2.8 x 103 to 1.0 x 106 nifH gene copies L–1). In the NPTZ, diazotroph abundances and rates of N2 fixation were low or undetected during all three cruises. However, the latitudinal boundary for diazotrophs and its coincidence with other biogeochemical features differed among the three years. In 2016 and 2017, abundances of UCYN-A1 decreased sharply at or near the subtropical frontal zone (~31°N-34°N), which was coincident with a sharp increase in Synechococcus abundance. In contrast, UCYN-A1 abundances decreased sharply in the northern section of the NPSG (~29°N) in 2019; this decrease was south of the subtropical frontal zone and was not coincident with a change in Synechococcus abundance. The sharp decrease in UCYN-A1 abundances we observed between ~29°N and ~34°N was not coincident with measurable changes in macronutrient concentrations during any of the three cruises. We will explore patterns in diazotroph abundance and N2 fixation rates in the context of local hydrographic conditions to discuss the possible controls for diazotrophy in the NPTZ.
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