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Inputs of "New" Nitrogen to the Planktonic Food Web in the North Atlantic: How Far Does the Nitrogen Go?

This study investigates the movement of "new" nitrogen through the planktonic food web in the North Atlantic. It explores the sources of "new" nitrogen, such as N2-fixation, and how it contributes to primary and secondary production. Stable isotopes are used as tracers to calculate the contribution of "new" nitrogen to phytoplankton and zooplankton. The results show that N2-fixation supports both primary and secondary production in the North Atlantic, with diazotrophs contributing a significant amount of nitrogen to the food web.

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Inputs of "New" Nitrogen to the Planktonic Food Web in the North Atlantic: How Far Does the Nitrogen Go?

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  1. Inputs of “New” Nitrogen to the Planktonic Food Web in the North Atlantic: How Far Does the Nitrogen Go? Jason Landrum1 Mark Altabet2 Joseph Montoya1 Georgia Institute of Technology1, UMASS at Dartmouth2 Georgia Tech Biological Oceanography

  2. Marine N Cycle: N limits PP • N generally limits primary production (PP) across large areas of the world’s oceans • Upper ocean requires inputs of “new” N in order to balance N loses (i.e., export) • Thus, “new” N is crucial for sustaining PP, especially in oligotrophic waters Georgia Tech Biological Oceanography

  3. Basin-scale N2-fixation Measurements (Montoya et al., 2007) (Gruber and Sarmiento, 1997) Georgia Tech Biological Oceanography

  4. “New” N Inputs and Oceanic Food Webs • N2-fixation shown as dominant source of “new” N to North Atlantic Georgia Tech Biological Oceanography

  5. “New” N Inputs and Oceanic Food Webs • N2-fixation shown as dominant source of “new” N to North Atlantic • How does “new” N enter and move through food webs? • “Leaky” cells/Viral lysis • Zooplankton consumption Georgia Tech Biological Oceanography

  6. “New” N Inputs and Oceanic Food Webs • N2-fixation shown as dominant source of “new” N to North Atlantic • How does “new” N move through food webs? • “Leaky” cells/Viral lysis • Zooplankton consumption • How does N2-fixation contribute to 2° production? Georgia Tech Biological Oceanography

  7. CO2 N2 Grazing NH4+ Excretion Active Vertical Migration Deep NO3- Microbial Decomposition Grazing Passive Sinking Georgia Tech Biological Oceanography

  8. -2 0 2 4 6 8 10 N2 NH4+ NH4+ NO3- Pycnocline Deepwater NO3- 15N and Food Webs • Stable isotopes used as natural tracers of biologically-mediated transformations of N • Isotope Mass Balance Model • End members: • Subsurface 15NO3- = 4.5‰ • 15NPOMDIAZO = -2‰ • Calculate the contribution of “new” N to both phyto- and zooplankton (adapted from Montoya et al., 2002) Georgia Tech Biological Oceanography

  9. SJ0005 Cruise Track Georgia Tech Biological Oceanography

  10. Nitrate (µM) Fluorometric Measurements Georgia Tech Biological Oceanography

  11. 15N Bulk Particles (‰) Georgia Tech Biological Oceanography

  12. 15N Bulk Particles (‰) Diazotroph N Contribution to Bulk Particles (%) Georgia Tech Biological Oceanography

  13. 15N Zooplankton (‰) >4000µm 2000-4000µm 1000-2000µm 15N Bulk Particles (‰) 500-1000µm 250-500µm Georgia Tech Biological Oceanography

  14. Diazotroph N Contribution to Zooplankton (%) 15N Zooplankton (‰) >4000µm 2000-4000µm 1000-2000µm 500-1000µm 250-500µm Georgia Tech Biological Oceanography

  15. Mean deepwater 15NO3- Georgia Tech Biological Oceanography

  16. Conclusions • N2-fixation supports both 1° and 2° production across North Atlantic Basin • All weighted mean 15N values lower than mean deep water 15NO3- except for easternmost 2 stations • Bulk Particles • Diazotrophs contribute up to 50% of N • Zooplankton • Diazotrophs contribute up to 30% of N Georgia Tech Biological Oceanography

  17. Conclusions • Different diazotroph N contribution to zooplankton between size fractions • Smaller zooplankton consuming particles within the mixed layer • Minimal vertical migration • Large zoops exhibit greater reliance on diazotroph N • Preferential transfer of diazotroph N up the food web • Bioaccumulation • E-W Gradient in zooplankton 15N • Decreasing dependence of zooplankton on diazotrophs • Phytoplankton and/or zooplankton community structure Georgia Tech Biological Oceanography

  18. Acknowledgements Advisors and Collaborators Dr. Joseph Montoya Dr. Mark Altabet Funding National Science Foundation Laboratory and shipboard assistance Officers and crew of the R/V Seward Johnson Chris Payne, Carrie Holl, Rachel Sedlack, Mary Crumley, Poneh Davoodi, Beth van Gessel Georgia Tech Biological Oceanography

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