1. 15 N in marine plants Alison Collins

2. Major Inputs of Nitrogen in the Ocean 1. Deep Water Nitrate 2.Atmospheric deposition Largest in areas near continental land masses 3.Nearshore and continental shelf waters Terrigenous runoff may be a large source

3. Outline Broad processes Seasonal processes

4. Nitrogen Cycle phytoplankton NO 2 - NO 3 - NH 4 + N2N2 NO 2 - N2ON2O nitrification ammonification denitrification nitrogen fixation nitrate assimilation ammonia assimilation +5 ~35 0 / 00 0 0 0 / 00 -3 ~20 0 / 00 ~15 0 / 00

5. Deep Water Circulation global 15N values of deep water Global Average  15 N ~4‰ - 5‰

7. Sources N 2 fixation: δ 15 N ~0‰ for phytoplankton - BUT…low value could also be an indication of recycled NH 4 + being used in oligotrophic waters NO 3 : available through upwelling and convection - δ 15 N depends on regional processes - δ 15 N >0 NH 4 + : available from urea - typically lighter than the global ocean average - δ 15 N is low

8. Differential 15 NO 3 - in Surface Waters Nitrogen fixation – waters ~0 ‰ Denitrification – waters isotopically heavy Riverine input – depends on inputs in watershed; heavy if fecal material, light if agricultural input, soil signature if relatively pristine Atmospheric deposition - waters ~0 ‰

9. ** 15 N value of phytoplankton will be very close to the average of the 15 N of the new nitrogen as long as all the nitrogen is utilized by phytoplankton regenerated production new production Atmosphere Reassimilation export production vertical mixing (Upwelling) nitrogen fixation

10. Nutrient Profiles – Monterey Bay Inverse relationship between [NO 3 - ] and  15 NO 3 -  15 NO 3 - decreases with depth due to remineralization of sinking particles  15 NO 3 - higher than oceanic values (4‰ - 5‰) – probably due to infusion of California undercurrent waters

11. Nutrient Profiles – Gulf of California Surface waters are enriched compared to Monterey profile Increase in  15 N at the surface is most likely due to uptake by phytoplankton Nitrate drawdown (by denitrifying bacteria) within OMZ corresponds with increase in d 15 N

12. Outline Broad processes Seasonal processes

13. i Upwelling nutrient rich water from depth equatorward winds surface waters

14. Variation in  15 N during upwelling  15 N at minima shortly after upwelled waters come to surface then increase until all nitrate is taken up time  15 NO3 upwelling begins [NO3]  15 NO3 [NO3]

15. Nitrogen fixation in oligotrophic central gyres

16. Nitrogen Fixation Lower than average  15 N values  15 N of sediment increases with depth Isotopically light sinking organic matter lowers the  15 N of the subsurface pool below the global deep water average Subsurface pool  15 N is a mix between particle flux from the surface and vertical mixing of deep water

17. Trichodesmium abundance and  15 N of zooplankton  15 N values lowest with highest abundance of Trichodesmium  15 N values highest in areas with low abundance of Trichodesmium

18. HNLC areas and nutrient limitation

19. Shows that  15 N is inversely related to N utilization, with lower values where waters still have [NO 3 ] available

20. C i,δ i Inside leaf C a,δ a C a,δ a C f,δ f φ 1,δ 1,ε t φ 3,δ 3,ε t φ 2,δ 2,ε f -12.4‰ -35‰ -27‰ Plant δ 13 C (ifatm=-8‰) ε p = ε t = +4.4‰ ε p = ε f = +27‰ ε f 00.51.0 Fraction C leaked (φ 3 /φ 1 ?C i /C a ) δ i δ f δ 1 Available nitrate similar to leakiness in land plants available nitrate

21. Bottom Line…  15 N of phytoplankton depends on: denitrification nitrogen fixation upwelling and currents