PLANKTONPATCHINESS

Physical processes implicated in patchiness

Diffusion-related processesPatchesFilamentsTuring MechanismPlankton waves

Lateral stirringEarly observations of phytoplankton spectraPhysical turbulenceExplaining phytoplankton spectraZooplankton and spectraPitfalls of spectral analysisBiological forcing at intermediate scales

Vertical-horizontal coupling

Platt, 1972 Powell et al., 1975

K.L.Denman, 1976

Limited range where Chl and “passive” T are correlated

~10-100m to ~100-1000km

To what extent are spectra controlled by stirring and mixing?

A theory is required to underpin the analysis and comparisonof plankton and “inert” tracer spectra.

Such a theory needs to relate structure in biological fieldsand velocity fields at all scales.

A pre-requisite of this is a suitable model of turbulent flow.

Several user-friendly options have been used.

All rely on cascades – the conservative transfer of a property from one scale to the next,

from large scale to small scale.

Cascading properties scale – spectral density ~ kp

Commonly used turbulence models

Range: >1kmCascading property: enstrophy

(square of vorticity)Spectral slope: -1

Two-dimensional turbulence

Three-dimensional turbulence

Range: <1kmCascading property: energySpectral slope: -5/3

Motivation:- simplest option- “cascade” paradigm easy to apply to tracers-“agreement” with observations- if know what inert tracer should do, easy to diagnose when biology is dominant effect

Drawbacks:- far from clear that any are accurate models- unlikely that cascading range exists in practice- forcing only at large scale

Critical scale

Numerous independent derivationsCorrsin, 1961Denman and Platt, 1976Denman et al., 1977Powell and Okubo, 1994

Different formulation for critical scaleFasham, 1978

kc=() KiSS scale

kc~0.2-20kmPhysically controlled at smaller scalesBiologically controlled at larger scales

Spectra can vary with time

“…the fact that the spectra of these twovariables [chlorophyll and temperature]have the same slope cannot be used toinfer that the chlorophyll distribution iscontrolled physically rather than biologically.”

M.J.R.Fasham (1978)

Antony von Leeuwenhoek, 1632-1723

“Passing just lately over thislake…I took up a little of it in a glass phial; and examiningthis water next day, I found…very many little animalcules. These animalcules had diverscolours…and the motion of most of these animalcules was so swift, and so various upwards, downwards and round about that ‘twas wonderful to see.”

7 September 1674

Hardy, 1955

“A marked patchiness was demonstrated for all animalsoccurring in sufficient numbers.”

“I once heard an eminent planktologist say thatit did not do to arrange your stations too closetogether because it made it almost impossible to use contour methods when charting the results; I don’t think he realized the significanceof what he was saying.”

Mackas and Boyd, 1979

Tsuda et al., 1993

Theories for zooplankton having a whiter spectrum than phytoplankton

SwarmingMackas and Boyd, 1985; Levin, 1994

- accumulation in small groups increases structure at small scales

Non-linear interactionsSteele and Henderson, 1992; Abraham, 1998

- non-linear transfer of structure modifies spectral slope

Zooplankton and phytoplanktonspectra the same.

Powell and Okubo, 1994

Slope relative topassive tracer isdependent on theturbulence model.

“Do any generalities emerge from our studies ofthe modifications that biological processes make to the patchiness imposed by solely physical processes?

“…one might also expect that several biologicalmechanisms might lead to the same (or very similar) spatial patterns in the plankton.”

Powell and Okubo, 1994

We answer no.”

Denman and Platt, 1975

Star and Cullen, 1981

SPECTRA ARE TOO BLUNT A TOOL

Majority of patchiness models assume physicalforcing of biology is restricted to large scale.

Convenient as this assumption is common to theparadigm of cascading in the turbulence models they use.

Numerous observations indicate that physicalforcing of biology occurs at all scales, however.

- from 100km to 1km- upwelling very strong at fronts and eddies

Martin, Richards and Fasham (2001)

“…clear evidence of avariance input atwavelengths between15 and 35km…”

Martin and Richards (2001)

Horizontal velocity Vertical velocity

Early attempts to include mesoscale forcing

Fasham, 1978white noise forcing on growthP/ t = P + 2P/ x2 + E(x,t)

Powell and Okubo, 1994assumed spectrum for forcingpower spectra ~ k-1

Neither explicitly link forcing to flow

Need for explicit models(?)

Smith et al., 1996

Explicit biological forcingthrough…

Light - depth of mixed layer

Nutrients - mixed layer deepening

20km resolution

29% inc.

208% inc.139% inc.

Physical processes implicated in patchiness

Diffusion-related processesPatchesFilamentsTuring MechanismPlankton waves

Lateral stirringEarly observations of phytoplankton spectraPhysical turbulenceExplaining phytoplankton spectraZooplankton and spectraPitfalls of spectral analysisBiological forcing at intermediate scales

Vertical-horizontal coupling

dept

hEffective horizontal diffusivity

is a function ofvertical diffusivity and shear

Vertical diffusivity is a function of

stratification

KiSS length may vary with location

Taylor Dispersion

ZOOPLANKTON DIEL VERTICAL MIGRATION

Range: up to 500m

Rate: 50-200m/day

Causes: predator evasion,ultraviolet light evasion

Riley, 1976

Requires: - depth-varying velocities - zoo spend longer at depth - initial patchiness in zoo

Evans, 1978

Rovinsky et al., 1997

SUMMARY

Huge range of ways stirring and mixing caninteract with biological processes to produce

plankton patchiness.

Still no general, multiscale theory of patchiness,or concensus on whether one exists.

Need for better observations to constrainmodels.

Need for better techniques to analyse observations.