1. Georgia Tech Biological Oceanography Inputs of “New” Nitrogen to the Planktonic Food Web in the North Atlantic: How Far Does the Nitrogen Go? Jason Landrum 1 Mark Altabet 2 Joseph Montoya 1 Georgia Institute of Technology 1, UMASS at Dartmouth 2

2. Georgia Tech Biological Oceanography 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

3. Georgia Tech Biological Oceanography Basin-scale N 2 -fixation Measurements (Montoya et al., 2007)(Gruber and Sarmiento, 1997)

4. Georgia Tech Biological Oceanography “New” N Inputs and Oceanic Food Webs N 2 -fixation shown as dominant source of “new” N to North Atlantic

5. Georgia Tech Biological Oceanography “New” N Inputs and Oceanic Food Webs N 2 -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

6. Georgia Tech Biological Oceanography “New” N Inputs and Oceanic Food Webs N 2 -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 N 2 -fixation contribute to 2 ° production?

7. Georgia Tech Biological Oceanography Grazing NH 4 + Excretion Microbial Decomposition Grazing Active Vertical Migration CO 2 N 2 Passive Sinking Deep NO 3 -

8. Georgia Tech Biological Oceanography  15 N and Food Webs Stable isotopes used as natural tracers of biologically-mediated transformations of N Isotope Mass Balance Model –End members: Subsurface  15 NO 3 - = 4.5‰  15 N POMDIAZO = -2‰ Calculate the contribution of “new” N to both phyto- and zooplankton -286420 NH 4 + 10 Pycnocline Deepwater NO 3 - N2N2 NO 3 - (adapted from Montoya et al., 2002)

9. Georgia Tech Biological Oceanography SJ0005 Cruise Track

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

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

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

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

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

15. Georgia Tech Biological Oceanography Mean deepwater  15 NO 3 -

16. Georgia Tech Biological Oceanography Conclusions N 2 -fixation supports both 1 ° and 2 ° production across North Atlantic Basin –All weighted mean  15 N values lower than mean deep water  15 NO 3 - except for easternmost 2 stations –Bulk Particles Diazotrophs contribute up to 50% of N –Zooplankton Diazotrophs contribute up to 30% of N

17. Georgia Tech Biological Oceanography 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  15 N Decreasing dependence of zooplankton on diazotrophs Phytoplankton and/or zooplankton community structure

18. Georgia Tech Biological Oceanography 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