1. Critical turbulence revisited: The impact of submesoscale vertical transports on plankton patchiness Anne Willem Omta Bas Kooijman Theoretical Biology, Vrije Universiteit (Amsterdam) Henk Dijkstra IMAU, Universiteit Utrecht www.bio.vu.nl/thb Grant No. 635.100.009 (Computational Life Sciences)

2. Project overview Organic carbon pump in meso-scale ocean flows Aim: determine effect of (sub)meso-scale flows on phytoplankton Method: computer simulations and theory development Supervisors: Kooijman, Dijkstra, Sommeijer PhD’s: Bruggeman & Omta Postdoc: Van Raalte Period: March 2004 – March 2009

3. My PhD Feedback mechanisms between climate and Redfield ratio (GRL 33, L14613, 2006) Impact of submesoscale eddies on organic carbon pump (JGR 112, C11006, 2007) Critical turbulence revisited (JMR 66, 61- 85, 2008) How to interpret satellite chlorophyll observations (submitted to DSR)

4. Feedback mechanisms between climate and Redfield ratio With plankton physiological model, I investigated impact of mixed-layer depth and temperature on C:N ratio Increase of C:N ratio with decreasing mixed-layer depth and temperature: possible implications for glacial cycles

5. Impact of submesoscale eddies on organic carbon pump 3D-simulation of phytoplankton in baroclinically unstable submesoscale eddy Vertical transports lead to upward N transport and plankton bloom Effect on distribution and net transport of carbon very modest: enhanced upward transport of DIC, enhanced downward transport of organic carbon

6. How to interpret satellite chlorophyll observations Looked at seasonal Chl cycle in Mozambique Channel Tried to reproduce cycle with various plankton population models Modeled Chl/N ratio gave best correlation with observed Chl: suggests that cycle represents variation in Chl/N rather than in plankton

7. Critical turbulence Huisman et al. (1999): if downward transport of plankton is faster than growth, then plankton goes extinct Critical turbulence = 1-D concept: How does it work out in 3-D?

8. Ocean eddy field Real 3-D ocean eddy field very complicated: simulate one single eddy for better understanding

9. Flow model Non-hydrostatic 3-D model Domain 32 km * 32 km * 1 km Periodic boundary conditions

10. SU-based Internal Transformation Yield (SITY) model - Three state variables (nutrient, algal biomass, detritus), only six parameters - Uptake according to SU-kinetics: organisms can be limited by light and nutrients - Detritus sinks

11. Initial conditions Biomass: –Sinking of organic nutrient balanced by upward diffusion of inorganic nutrient Eddy radius ~8 km, no vertical velocity

12. Vertical velocity patterns 3.6 days 7.2 days 12 days

13. Plankton distributions at different light intensities 50 mol/(m2 d) 2 mol/(m2 d) Two very distinct regimes!

14. 2-D simulations Again, two regimes show up! D=0.01m2/s D=1m2/s

15. Explanation of regimes Eddy region optimal for plankton (high nutrients) Vertical exchange subcritical everywhere Vertical exchange supercritical in eddy region Adjacent regions optimal for plankton (relatively high nutrients and low vertical exchange) 1-D simulations consistent with explanation

16. - Vertical mixing + algal growth Distinct plankton distributions - Explanation: critical turbulence Conclusions More information: www.bio.vu.nl/thb Omta et al., J. Mar. Res. 66: 61-85 (2008)