Life in a Moving Fluid (Steve Vogel)

Two Kinds of Flow:Laminar flow – fluid particles move more or less

parallel to each other in a smooth pathTurbulent flow – fluid particles move in a highly

irregular manner even if the fluid as a whole is moving in a single direction

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FLOW

Which will happen and why?

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http://www.youtube.com/watch?v=KqqtOb30jWs&NR=1

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Reynolds number (Re) describes the nature of the flow regime when a solid

(organism) and liquid encounter one another

Re = σ l v/ μ σ = density of fluid l = length (size) solid

v = velocity of solid μ = viscosity of fluid

Re = FI FI = inertial forces FV FV= viscous forces

Inertia?5

In natureIn nature::

Small = slowSmall = slow Large = fastLarge = fast

< 2000< 2000 > 2000 > 2000

Small creatures live in world Small creatures live in world dominated by dominated by viscous foviscous forcesrces

Large creatures affected most by Large creatures affected most by inertialinertial phenomena phenomena

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Typical Reynolds NumbersTypical Reynolds Numbers

RRee

Large whale swimming at 10 Large whale swimming at 10 m/sm/s 300,000,000300,000,000

Tuna swimming at same Tuna swimming at same speedspeed 30,000,00030,000,000

Duck flying at 20 m/sDuck flying at 20 m/s 300,000300,000

Dragonfly going 7 m/sDragonfly going 7 m/s 30,00030,000

Copepod in a pulse of 20 cm/sCopepod in a pulse of 20 cm/s 300300

Flight of smallest insectsFlight of smallest insects 3030

Invertebrate larva,, 0.3 mm Invertebrate larva,, 0.3 mm long, moving at 1 mm/slong, moving at 1 mm/s 0.30.3

Sea urchin sperm advancing Sea urchin sperm advancing the species at 0.2 mm/sthe species at 0.2 mm/s 0.030.03

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PlanktonPlankton – most have a density that – most have a density that is greater than seawateris greater than seawater

What happens?

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SR = SR = WW11 - W - W22

(R)(V(R)(VWW))

SR = sinking rateSR = sinking rateWW11 = density of organism = density of organism

WW22 = density of seawater = density of seawater

(W(W11-W-W22 = overweight) = overweight)

R = surface of resistanceR = surface of resistanceVVWW = viscosity of water = viscosity of water

Sinking rate equationSinking rate equation

Two solutionsTwo solutionsWW11-W-W22 = organism overweight, reduce density = organism overweight, reduce density

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Reduction of Reduction of overweightoverweight• Make body fluids less dense than Make body fluids less dense than

seawaterseawater• Osmotic problem – Solutions:Osmotic problem – Solutions:

– Replace heavy ions with lighter onesReplace heavy ions with lighter ones– Replace normal body salts with NHReplace normal body salts with NH44ClCl– Replace heavy Cl with SOReplace heavy Cl with SO44

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Reduction of overweight

• Incorporate less dense liquids – oils and fats – in tissues– Copepods – oil droplets– Diatoms - vacuoles

• Decrease overall density by incorporating gas• gas bags• bubbles

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Incorporate gas-filled floats in bodyIncorporate gas-filled floats in bodyPortuguese man-o-war, Portuguese man-o-war, JanthinaJanthina

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BUT – if overweight can’t be reduced ?

SR = SR = WW11 - W - W22

(R)(V(R)(VWW))

SR = sinking rateSR = sinking rateWW11 = density of organism = density of organism

WW22 = density of seawater = density of seawater

(W(W11-W-W22 = overweight) = overweight)

R = surface of resistanceR = surface of resistanceVVWW = viscosity of water = viscosity of water

Sinking rate equationSinking rate equation

Two solutionsTwo solutionsWW11-W-W22 = organism overweight, reduce density = organism overweight, reduce density

#2 - #2 - Increase R - surface of resistanceIncrease R - surface of resistance

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Increase Surface of Resistance

• Why is plankton small?

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Why small is important

• Surface area increases as a square of liner dimension

• Volume increases as a cube

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• Change shape of body - flatten body, feathery projections

• Develop spines, body projections– Lots of surface area, little mass

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Water motion and buoyancy

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Langmuir Cells

• Heating-cooling diurnal cycle, counter –rotating helical cells

• localized vertical movement, scale of meters• Effect – vertical mixing, horizontal differences• Vertically mix zooplankton, horiz. Patches• 2 zones: chaotic regions that help to spread

plankton and locally coherent regions (patches) , that do not mix with the chaotic regions and which persist for long periods of time.

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http://www.youtube.com/watch?v=sWYXY7S6yGs

Nekton – also have adaptations for buoyancy

• Make body fluids less dense - • Fish – swim bladders• Lay down lipid layer – in enlarged liver

(sharks), throughout body (mackerel), subcutaneous lipid layer (marine mammals)

• Pectoral fins, flippers, heterocercal tail

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Drag – loss of momentum in movement through a fluid

• Skin friction – results from stickiness of water– Proportionally more important at low Re

• Pressure drag – results because dynamic pressure to separate flow at front end is not counter balanced by an opposite pressure at rear end– Proportionally more important at high Re

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Adaptations to reduce drag

Change body shape

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• Reduce frontal area and streamline shape – to minimize skin friction

• Long tapering tail- reduces pressure drag by recapturing energy as fluid closes in rear

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What is “streamlined?”

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Speedo LZR Racer•Reduces skin drag•Compresses body

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Adaptations to reduce drag

Minor adaptations:

• Eliminate protuberances – eyes, pectoral fins, etc. recessed into depressions

• Surface roughness – overcome viscosity at high velocities

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Mucus, compliant skins, scales, riblets and roughness

• polymers found in mucus decrease the pressure gradient, channel water molecules in direction of flow

• Compliant surfaces equalising and distributing pressure pulses

• Small longitudinal ridges on rows of scales on fish can reduce shear stress in the boundary

• a rough surface increases the shear stress in the boundary layer and makes it thinner.

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• The function of the roughness on the sword of a swordfish is probably to reduce the total drag by generating premature turbulence and by boundary layer thinning, despite an increased friction over the surface of the sword.

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Phytoplankton Dynamics

Primary Productivity (g C/m2/yr)

Gross (total) production = total C fixedNet production = C remaining after respirationStanding crop = biomass present at a point in

time

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Factors Affecting Primary Production

1. Light2. Nutrients3. Loss out of the photic zone due to sinking or

mixing4. Grazing

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1. Light

Light penetration – affected by– Angle of incidence– Surface reflection– Suspended particles– Adsorption by the water itself

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Water absorption of light

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DDCC = = Compensation Compensation depthdepth

At DAt DCC ---- ----

primary production primary production (P) = respiration (P) = respiration (R) in each algal (R) in each algal cell cell

IIC C = Compensation = Compensation light intensitylight intensity

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2. Nutrients

• Nutrients = essential elements needed for cell maintenance and growth

N, P, Si, Ca, K, etc.• N and P in ocean water about 10,000 x less

than on land (“ocean desert”)• Redfield ratio C:N:P 105:16:1

same in seawater and pp cell

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• Body form of pp adapted for nutrient uptake• Rate of uptake is concentration dependent

low nutrient species – very efficient at nutrient uptake in low concentrations, but rate saturates out at high concentrations

high nutrient species – less efficient uptake at low concentrations, but can exploit high concentrations

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3. Loss out of the photic zone

• Sinking• Water turbulence mixes plankton deeper into

the water column

DM = mixing depth

Vertical mixing can take plankton below the compensation depth

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DDCC = compensation depth, = compensation depth,

(P = R in each pp cell)(P = R in each pp cell)

DDCR CR = critical depth, = critical depth,

(P = R in whole pp (P = R in whole pp population)population)

DDMM < D < DCRCR P PWW > R > RWW

DDMM > D > DCRCR P PWW < R < RWW

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4. Grazing

• Copepods can have extremely high grazing rates

GR < PW

GR = PW

GR > PW

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WinterWinter

SpringSpring

SummerSummer

FallFall

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DDCRCR moves up in the water column moves up in the water column – photic zone smaller– photic zone smaller

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What happens in late What happens in late spring/early summer?spring/early summer?

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Zooplankton – grow and Zooplankton – grow and reproduce in spring, GR??reproduce in spring, GR??

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Spring Spring Summer Summer

FallFall

Magnitude of fall Magnitude of fall bloombloom

-- timing, when -- timing, when DDMM falls below falls below

DDCRCR

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Idealized Chart

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Why are the Why are the polar oceans polar oceans different?different?

Light in Light in springspring

No No thermoclinethermocline

Grazing?Grazing?

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Why are the Why are the tropical tropical oceans oceans different?different?

Permanent Permanent thermoclinethermocline

Nutrients Nutrients always lowalways low

Blips?Blips?55

What’s What’s different different between the between the Atlantic and Atlantic and Pacific? Pacific?

Copepod life Copepod life history – history –

lag timelag time

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Coast vs. Open Ocean

• Coast receives nutrients from land• Upwelling • Shallow water depth – bottom shallower

than the DCR

• Thermocline not as well developed or persistent

• Turbidity of water can counteract other factors

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