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

Anne Willem OmtaBas Kooijman

Theoretical Biology, Vrije Universiteit (Amsterdam)Henk Dijkstra

IMAU, Universiteit Utrecht

www.bio.vu.nl/thb

Grant No. 635.100.009 (Computational Life Sciences)

Project overview

• Organic carbon pump in meso-scale ocean flowsAim: determine effect of (sub)meso-scale flows on

phytoplanktonMethod: computer simulations and theory

developmentSupervisors: Kooijman, Dijkstra, SommeijerPhD’s: Bruggeman & OmtaPostdoc: Van RaaltePeriod: March 2004 – March 2009

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)

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

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

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

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?

Ocean eddy field

Real 3-D ocean eddy

field very complicated: simulate one

single eddy for better

understanding

Flow model

• Non-hydrostatic 3-D model

• Domain 32 km * 32 km * 1 km

• Periodic boundary conditions

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

Initial conditions

• Biomass: – Sinking of organic

nutrient balanced by upward diffusion of inorganic nutrient

• Eddy radius ~8 km, no vertical velocity

Vertical velocity patterns

3.6 days

7.2 days

12 days

Plankton distributions at different light intensities

50 mol/(m2 d) 2 mol/(m2 d)

Two very distinct regimes!

2-D simulations

Again, two regimes show up!

D=0.01m2/sD=0.01m2/s D=1m2/s

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

- Vertical mixing + algal growth

Distinct plankton distributions

- Explanation: critical turbulence

Conclusions

More information: www.bio.vu.nl/thbOmta et al., J. Mar. Res. 66: 61-85 (2008)