Light in Lakes. Light is energy Major energy source to aquatic habitats Productivity controlled by...

Light in LakesLight in Lakes

Light is energyLight is energy

Major energy source Major energy source to aquatic habitatsto aquatic habitats

Productivity controlled Productivity controlled by energy used in by energy used in photosynthesisphotosynthesis

Thermal character of Thermal character of lake determined by lake determined by solar energysolar energy

Light is energyLight is energy

Solar radiationSolar radiation Capacity to do workCapacity to do work Can be transformed Can be transformed

into other energy into other energy formsforms

Light from the sunLight from the sun

Pulsating field of Pulsating field of force, endless series force, endless series of wavesof waves

Packets of energy - Packets of energy - photonsphotons

Energy proportional to Energy proportional to frequency (high-high), frequency (high-high), inversely to inversely to wavelength (high-wavelength (high-short)short)


Mixture of Mixture of wavelengths, wavelengths, energiesenergies

Most (50%) striking Most (50%) striking lake surface is lake surface is infrared, visible infrared, visible (especially red part of (especially red part of spectrum)spectrum)


Amount striking lake Amount striking lake surface dependent surface dependent on:on:

LatitudeLatitude SeasonSeason Time of dayTime of day AltitudeAltitude Meteorological Meteorological

conditionsconditions

Light and atmosphereLight and atmosphere

Light absorbed by Light absorbed by particles in atmosphereparticles in atmosphere

Less atmosphere to pass Less atmosphere to pass through, more light through, more light makes it to earth - angle makes it to earth - angle of incidenceof incidence

Shorter wavelengths Shorter wavelengths selectively absorbed by selectively absorbed by OO22, ozone, H, ozone, H22O vapor, O vapor, COCO22

Red sky at dawn, duskRed sky at dawn, dusk

Indirect LightIndirect Light

Some solar radiation Some solar radiation reaches lake reaches lake indirectlyindirectly

Scattered lightScattered light Light scattered as it Light scattered as it

passes through passes through atmosphere (20%)atmosphere (20%)

Mostly UV and short Mostly UV and short wavelength visible wavelength visible (blue)(blue)

Indirect LightIndirect Light

Importance of indirect Importance of indirect light changes with light changes with angle of incidenceangle of incidence

Contribution of Contribution of indirect small when indirect small when sun directly overheadsun directly overhead

Contribution Contribution significant (~20-40%) significant (~20-40%) when sun low in skywhen sun low in sky

Reflected LightReflected Light

Significant fraction of Significant fraction of light striking lake light striking lake surface may be surface may be reflectedreflected

Amount increases Amount increases with decreased angle with decreased angle of incidenceof incidence

Wave action Wave action increases reflection increases reflection only at low angles of only at low angles of incidenceincidence

Other Losses of LightOther Losses of Light

Reflection comprises ~1/2 Reflection comprises ~1/2 of light lost from waterof light lost from water

Remaining half lost by Remaining half lost by scatteringscattering

Deflection by water Deflection by water molecules, dissolved molecules, dissolved substances, suspended substances, suspended particlesparticles

Varies with depth, Varies with depth, season, particle loadingseason, particle loading

Lake ColorLake Color

Scattering and Scattering and absorption of light give absorption of light give lake part of its lake part of its characteristic colorcharacteristic color

Clean water - blue colorClean water - blue color More and bigger More and bigger

particles scatter longer particles scatter longer wavelengths and wavelengths and absorb shorter absorb shorter wavelengthswavelengths

Blue-green, green, Blue-green, green, yellowyellow

Light AttenuationLight Attenuation

Radiant energy Radiant energy diminished with depthdiminished with depth

Results from both Results from both scattering and scattering and absorptionabsorption

Absorption - loss of Absorption - loss of solar energy with solar energy with depth by its depth by its transformation to heattransformation to heat


In distilled water lake, In distilled water lake, >1/2 of light energy >1/2 of light energy transformed into heat transformed into heat with first 1 meterwith first 1 meter


Absorption not same Absorption not same for all wavelengthsfor all wavelengths

Longer wavelengths Longer wavelengths more readily more readily absorbed than shorter absorbed than shorter wavelengthswavelengths


Few distilled water Few distilled water lakeslakes

Dissolved, suspended Dissolved, suspended stuff affects stuff affects absorptionabsorption

Less absorption, Less absorption, greater transmittance greater transmittance in clear, unproductive in clear, unproductive lakes than in lakes than in productive, murky productive, murky waterswaters


Blues disappear, Blues disappear, greens penetrate, greens penetrate, reds change with reds change with productivityproductivity

Transmission Transmission drastically affected by drastically affected by cover of cloudy ice, cover of cloudy ice, snowsnow

Shuts down Shuts down photosynthesis, photosynthesis, reduces Oreduces O22 supply supply

Euphotic ZoneEuphotic Zone

Region from surface Region from surface to depth at which 99% to depth at which 99% of the surface light of the surface light has disappearedhas disappeared

Minimum intensity of Minimum intensity of subsurface light that subsurface light that permits permits photosynthesis is photosynthesis is ~1% of incident ~1% of incident surface lightsurface light

Water TransparencyWater Transparency

Measuring light Measuring light penetration before penetration before instrumentation - instrumentation - Secchi diskSecchi disk

Depth at which disk Depth at which disk disappears/reappears disappears/reappears from/to sightfrom/to sight

Water TransparencyWater Transparency

Secchi disk Secchi disk transparency X 3 transparency X 3 used as a “rule of used as a “rule of thumb” estimate of thumb” estimate of depth of euphotic depth of euphotic zonezone

Highly variable (e.g., Highly variable (e.g., Lake Erie 5X)Lake Erie 5X)

Heat & Density LayeringHeat & Density Layering

Light to HeatLight to Heat

Loss of light = gain in Loss of light = gain in heatheat

Should temperature Should temperature profile parallel light profile parallel light profile?profile?

NoNo

Light to HeatLight to Heat

Uniformly mixed layer Uniformly mixed layer of water near surface of water near surface of same temperatureof same temperature

Often extends below Often extends below euphotic zoneeuphotic zone

Mixing of upper layers Mixing of upper layers of water by wind of water by wind distributes heat distributes heat downwarddownward

Direct Thermal StratificationDirect Thermal Stratification

Lighter, warmer layer Lighter, warmer layer overlying denser, overlying denser, cooler layercooler layer

Lake divided vertically Lake divided vertically into 3 regionsinto 3 regions EpilimnionEpilimnion MetalimnionMetalimnion HypolimnionHypolimnion


EpilimnionEpilimnion - uniformly - uniformly warm layer mixed by warm layer mixed by windwind


HypolimnionHypolimnion - - uniformly cool lower uniformly cool lower layer unaffected by layer unaffected by windwind


MetalimnionMetalimnion - - intermediate zone intermediate zone where temperature where temperature drops rapidly with drops rapidly with increasing depthincreasing depth

Also referred to as Also referred to as thermoclinethermocline - plane - plane between two depths between two depths between which between which temperature change temperature change is greatestis greatest

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Temperature (°C)

Dep

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Epilimnion

Hypolimnion

MetalimnionThermocline

A Thermally Stratified LakeA Thermally Stratified Lake

EpilimnionUpper LayerWarmWell mixed

HypolimnionLower layerCooler than epilimnion

Two separate water masses between which there is little mixing

THERMOCLINE

STABILITY OF THERMAL STRATIFICATION

Stability—likelihood that a stratified lake will remain stratified.

This depends on the density differences

between the two layers.

Examples:Epilimnion Hypolimnion Result8°C 4°C Not much density difference22°C 7°C Large density difference,

Strong stratification30°C 28°C Large density difference,

Strong stratification (tropical lakes)

Even a Hurricane Can’t Break StratificationEven a Hurricane Can’t Break Stratification

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Temperature (oC)

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Depth (m)After hurricaneBefore hurricane

Thermal resistance to mixing

(1) Density relationships of water

Why do lakes stratify?

(2) Effect of wind

Less dense water “floats” on deeper water

Molecular diffusion of heat is slow Wind must mix heat to deeper water

How do lakes stratify?

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Example: 10 m deep lake in Lake County, IL

(1) Early Spring

No density difference

No resistance to mixing

Heat absorbed in surface water is distributed throughout

Spring Turnover—time of year when entire water column is mixed by the wind

Duration of spring turnover depends on the surface area to maximum depth

In very deep lakes, the bottom water stays at 4°C, in more shallow lakes, can get up to > 10°C.

Can last a few days or a few weeks.


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(2) Mid Spring

Longer and warmer days mean more heat is transferred to the surface water on a daily basis

Surface waters are heated more quickly than the heat can be distributed by mixing

This increase in surface waters relative to the rest of the water column often occurs during a warm, calm period

Now have resistance to mixing.

Hypolimnion water temperature will not change much for the rest of the year.


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(3) Late Spring

With the density difference established, the epilimnion “floats” on the colder hypolimnion


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(4) Late Summer

The epilimnion has continued to warm

Strong thermal stratification

In very clear lakes, can get direct hypolimnetic heating

The decomposition of dead plankton may result in loss of oxygen from the hypolimnion


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(5) Early Autumn

Thermocline deepens and epilimnion temperature is reduced

Heat is lost from the surface water at night

Cool water sinks and causes convective mixing


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(5) Mid-late Autumn

As epilimnion cools, reduce density difference between layers

Eventually, get “Fall Turnover”

Turnover returns oxygen to the deep water and nutrients to the surface water


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(7) Winter

Surface water falls below 4°C and “floats” on 4°C water

Get “inverse stratification”

Ice blocks the wind from mixing the cooler water deeper

Seasonal Stratification in a Temperate LakeSeasonal Stratification in a Temperate Lake

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Depth (m)

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ice

0 5 10 15 20Temperature ( 0C)

Summer stratification Winter stratification

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Depth (m)

Spring mixing Fall mixing

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InverseDirect

Dimictic LakesDimictic Lakes

Complete circulations Complete circulations (turnovers) in spring and (turnovers) in spring and fall separated by summer fall separated by summer thermal stratification and thermal stratification and winter inverse winter inverse stratificationstratification

Very common in Very common in temperate regionstemperate regions

Many other types based Many other types based on circulation patternson circulation patterns

1. Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.

2.Holomictic—lakes mix completely (top to bottom)

3.Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.

Mixing Patterns

Holomictic: lakes are classified by the frequency of mixing

Monomictic lakes: one period of mixing- Cold- Warm

Dimictic lakes: two periods of mixing and two periods of stratification

Polymictic lakes: mix many times a year- Cold- Warm

Cold monomictic lakes — one period of mixing

Frozen all winter (reverse stratification) Mix briefly at cold temperatures in summer Arctic and mountain lakes

Kalff 2002

Meretta Lake, CA

Holomictic: lakes mix completely

Warm monomictic lakes — one period of mixing

Thermal stratification in summer

Does not freeze, so mixes all winter

Lake Kinneret

Kalff 2002


Dimictic—two periods of mixing and two periods of stratification

Freeze in winter (inverse stratification)Thermally stratify in summer

Wetzel 2001


Ice covered in winter, ice free in summer

May stratify for brief periods during the summer, but stratification is frequently interrupted

Shallow temperate lakes (< ~20 m) with large surface area

mountain or arctic lakes

Cold polymictic lakes — mix many times a year


Never ice covered

Tropical lakes

Warm polymictic lakes — mix many times a year

May stratify for days or weeks at a time, but mixes more than once a year



2.Holomictic—lakes mix completely (top to bottom)

3.Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.

Mixing Patterns

Meromictic: lakes are chemically stratified

Monimolimnion

Mix

olim

nion

Thermocline

Chemocline

Recall that salinity increases density

The water in the monimolimnion does not mix with the upper water

The mixolimnion can have any mixing pattern (e.g., dimitic, monomictic)

Meromictic: lakes are chemically stratified

Recall salinity increases density

Can get interesting thermal profiles

Warmer water below colder water above 4ºC


2. Holomictic—lakes mix completely (top to bottom)Monomictic lakes: Cold / WarmDimictic lakes: Polymictic lakes: Cold / Warm

3. Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.

Mixing Patterns

Geographic Distribution

What is meant by “shallow” and “deep enough” is determined by the fetch and depth

All of these classification patterns are for lakes that are deep enough to form a hypolimnion

“Shallow” lakes do not form a hypolimnion and are therefore unstratified.

They have similar temperatures top to bottom.

A lake with a maximum depth of 4m can stratify if it is in a protected basin

Bullhead PondSurface Area = 0.02 km2

Maximum fetch < 300 m

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Temperature (C)

A lake with a maximum depth of 12m can be

unstratified if the fetch is long enough

Oneida Lake, NYSurface Area = 207 km2

Maximum fetch = 33 km

22 August 1993

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Depth (m)

Light in Lakes. Light is energy Major energy source to aquatic habitats Productivity controlled by...

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