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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. Critical turbulence revisited: The impact of submesoscale vertical transports on plankton patchiness.

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Critical turbulence revisited: The impact of submesoscale vertical transports on plankton patchiness

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  1. Anne Willem Omta Bas Kooijman Theoretical Biology, Vrije Universiteit (Amsterdam) Henk Dijkstra IMAU, Universiteit Utrecht www.bio.vu.nl/thb Critical turbulence revisited: The impact of submesoscale vertical transports on plankton patchiness 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. Real 3-D ocean eddy field very complicated: simulate one single eddy for better understanding Ocean eddy field

  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. 7.2 days 3.6 days 12 days Vertical velocity patterns

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

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

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

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

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