Upload
priscilla-merritt
View
216
Download
0
Tags:
Embed Size (px)
Citation preview
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)
Light from the sunLight from the sun
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)
Light from the sunLight from the sun
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
Light AttenuationLight Attenuation
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
Light AttenuationLight Attenuation
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
Light AttenuationLight Attenuation
Light AttenuationLight Attenuation
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
Light AttenuationLight Attenuation
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
Direct Thermal StratificationDirect Thermal Stratification
EpilimnionEpilimnion - uniformly - uniformly warm layer mixed by warm layer mixed by windwind
Direct Thermal StratificationDirect Thermal Stratification
HypolimnionHypolimnion - - uniformly cool lower uniformly cool lower layer unaffected by layer unaffected by windwind
Direct Thermal StratificationDirect Thermal Stratification
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
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
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
6 8 10 12 14 16 18 20
Temperature (oC)
15
13
10
8
5
3
0
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?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
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.
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(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.
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(3) Late Spring
With the density difference established, the epilimnion “floats” on the colder hypolimnion
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(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
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(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
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(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
How do lakes stratify?
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Temperature (°C)
Dep
th (
m)
(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
14121086420
Depth (m)
2
3
4
4
5 6 7
7
8
8
9
10 10 11
11
13
14
15 16
17 18
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar
ice
0 5 10 15 20Temperature ( 0C)
Summer stratification Winter stratification
0 5 10 15 200 5 10 15 20
14
12
10
8
6
4
2
0
Depth (m)
Spring mixing Fall mixing
0 5 10 15 20
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
Holomictic: lakes mix completely
Dimictic—two periods of mixing and two periods of stratification
Freeze in winter (inverse stratification)Thermally stratify in summer
Wetzel 2001
Holomictic: lakes mix completely
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
Holomictic: lakes mix completely
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
Holomictic: lakes mix completely
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
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
1. Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.
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
0
1
2
3
4
0 10 20 30
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
0
2
4
6
8
10
12
0 5 10 15 20 25 30
Temperature (C)
Depth (m)