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

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

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

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

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

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?

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

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 NH Pycnocline Deepwater NO 3 - N2N2 NO 3 - (adapted from Montoya et al., 2002)

Georgia Tech Biological Oceanography SJ0005 Cruise Track

Georgia Tech Biological Oceanography Nitrate (µM) Fluorometric Measurements

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

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

Georgia Tech Biological Oceanography µm µm µm µm >4000µm  15 N Bulk Particles (‰)  15 N Zooplankton (‰)

Georgia Tech Biological Oceanography µm µm µm >4000µm  15 N Zooplankton (‰) Diazotroph N Contribution to Zooplankton (%) µm

Georgia Tech Biological Oceanography Mean deepwater  15 NO 3 -

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

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

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