Light in Lakes
Light is energy

Major energy source
to aquatic habitats
 Productivity controlled
by energy used in
photosynthesis
 Thermal character of
lake determined by
solar energy
Light is energy

Solar radiation
 Capacity to do work
 Can be transformed
into other energy
forms
Light from the sun

Pulsating field of
force, endless series
of waves
 Packets of energy photons
 Energy proportional to
frequency (high-high),
inversely to
wavelength (highshort)
Light from the sun

Mixture of
wavelengths,
energies
 Most (50%) striking
lake surface is
infrared, visible
(especially red part of
spectrum)
Light from the sun






Amount striking lake
surface dependent
on:
Latitude
Season
Time of day
Altitude
Meteorological
conditions
Light and atmosphere




Light absorbed by
particles in atmosphere
Less atmosphere to pass
through, more light
makes it to earth - angle
of incidence
Shorter wavelengths
selectively absorbed by
O2, ozone, H2O vapor,
CO2
Red sky at dawn, dusk
Indirect Light

Some solar radiation
reaches lake
indirectly
 Scattered light
 Light scattered as it
passes through
atmosphere (20%)
 Mostly UV and short
wavelength visible
(blue)
Indirect Light

Importance of indirect
light changes with
angle of incidence
 Contribution of
indirect small when
sun directly overhead
 Contribution
significant (~20-40%)
when sun low in sky
Reflected Light

Significant fraction of
light striking lake
surface may be
reflected
 Amount increases
with decreased angle
of incidence
 Wave action
increases reflection
only at low angles of
incidence
Other Losses of Light

Reflection comprises
~1/2 of light lost from
water
 Remaining half lost by
scattering
 Deflection by water
molecules, dissolved
substances, suspended
particles
 Varies with depth,
season, particle loading
Lake Color




Scattering and
absorption of light give
lake part of its
characteristic color
Clean water - blue color
More and bigger
particles scatter longer
wavelengths and
absorb shorter
wavelengths
Blue-green, green,
yellow
Light Attenuation

Radiant energy
diminished with depth
 Results from both
scattering and
absorption
 Absorption - loss of
solar energy with
depth by its
transformation to heat
Light Attenuation

In distilled water lake,
>1/2 of light energy
transformed into heat
with first 1 meter
Light Attenuation

Absorption not same
for all wavelengths
 Longer wavelengths
more readily
absorbed than shorter
wavelengths
Light Attenuation
Light Attenuation

Few distilled water
lakes
 Dissolved, suspended
stuff affects
absorption
 Less absorption,
greater transmittance
in clear, unproductive
lakes than in
productive, murky
waters
Light Attenuation

Blues disappear,
greens penetrate,
reds change with
productivity
 Transmission
drastically affected by
cover of cloudy ice,
snow
 Shuts down
photosynthesis,
reduces O2 supply
Euphotic Zone

Region from surface
to depth at which 99%
of the surface light
has disappeared
 Minimum intensity of
subsurface light that
permits
photosynthesis is
~1% of incident
surface light
Water Transparency

Measuring light
penetration before
instrumentation Secchi disk
 Depth at which disk
disappears/reappears
from/to sight
Water Transparency

Secchi disk
transparency X 3
used as a “rule of
thumb” estimate of
depth of euphotic
zone
 Highly variable (e.g.,
Lake Erie 5X)
Heat & Density Layering
Light to Heat

Loss of light = gain in
heat
 Should temperature
profile parallel light
profile?
 No
Light to Heat

Uniformly mixed layer
of water near surface
of same temperature
 Often extends below
euphotic zone
 Mixing of upper layers
of water by wind
distributes heat
downward
Direct Thermal Stratification

Lighter, warmer layer
overlying denser,
cooler layer
 Lake divided vertically
into 3 regions



Epilimnion
Metalimnion
Hypolimnion
Direct Thermal Stratification

Epilimnion - uniformly
warm layer mixed by
wind
Direct Thermal Stratification

Hypolimnion uniformly cool lower
layer unaffected by
wind
Direct Thermal Stratification

Metalimnion intermediate zone
where temperature
drops rapidly with
increasing depth
 Also referred to as
thermocline - plane
between two depths
between which
temperature change
is greatest
A Thermally Stratified Lake
Temperature (°C)
5
1
10
15
20
25
30
Epilimnion
2
Depth (m)
3
4
5
Metalimnion
Thermocline
6
7
8
9
10
Hypolimnion
Two separate water masses between which there is
little mixing
Epilimnion
Upper Layer
Warm
Well mixed
THERMOCLINE
Hypolimnion
Lower layer
Cooler than epilimnion
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
8°C
22°C
Hypolimnion
4°C
7°C
30°C
28°C
Result
Not much density difference
Large density difference,
Strong stratification
Large density difference,
Strong stratification
(tropical lakes)
Even a Hurricane Can’t Break Stratification
Temperature (oC)
0
6
8
10
12
14
16
18
20
3
Depth (m)
5
8
10
13
15
After hurricane
Before hurricane
Thermal resistance to mixing
Why do lakes stratify?
(1) Density relationships
of water
Less dense water
“floats” on deeper
water
(2) Effect of wind
Molecular diffusion of heat is slow
Wind must mix heat to deeper water
How do lakes stratify?
Temperature (°C)
Example:
10 m deep lake in Lake County, IL
5
1
(1) Early Spring
2
3
No density difference
Depth (m)
No resistance to mixing
4
Heat absorbed in surface
water is distributed throughout
5
6
7
8
9
10
10
15
20
25
30
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?
Temperature (°C)
(2) Mid Spring
5
1
Longer and warmer days
mean more heat is
transferred to the surface
water on a daily basis
3
4
Depth (m)
Surface waters are
heated more quickly
than the heat can be
distributed by mixing
2
5
6
7
8
9
10
10
15
20
25
30
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?
Temperature (°C)
(3) Late Spring
1
2
3
4
Depth (m)
With the density
difference
established, the
epilimnion “floats”
on the colder
hypolimnion
5
5
6
7
8
9
10
10
15
20
25
30
How do lakes stratify?
Temperature (°C)
5
(4) Late Summer
1
The epilimnion has continued
to warm
2
3
Strong thermal stratification
Depth (m)
In very clear lakes, can get
direct hypolimnetic heating
4
5
6
7
The decomposition of dead
plankton may result in loss of
oxygen from the hypolimnion
8
9
10
10
15
20
25
30
How do lakes stratify?
Temperature (°C)
(5) Early Autumn
5
Heat is lost from the
surface water at night
1
2
3
4
Depth (m)
Cool water sinks and
causes convective
mixing
5
6
7
Thermocline deepens
and epilimnion
temperature is reduced
8
9
10
10
15
20
25
30
How do lakes stratify?
Temperature (°C)
(5) Mid-late Autumn
5
As epilimnion cools,
reduce density difference
between layers
1
2
3
Eventually, get “Fall
Turnover”
Depth (m)
4
5
6
7
8
Turnover returns oxygen to
the deep water and nutrients
to the surface water
9
10
10
15
20
25
30
How do lakes stratify?
Temperature (°C)
(7) Winter
5
Surface water falls
below 4°C and
“floats” on 4°C
water
1
2
3
Ice blocks the wind from
mixing the cooler water
deeper
Depth (m)
4
5
6
7
8
Get “inverse
stratification”
9
10
10
15
20
25
30
5
14
2
6
7
8
18
17
11
10
ice
16
9
4
7
11
3
15
8
0
2
4
6
8
10
12
14
10
13
4
Apr
May
Jun
Jul
Aug
Sep
Oct
Temperature
0
0
5 10 15 20
2
Depth (m)
Depth (m)
Seasonal Stratification in a Temperate Lake
0
5 10 15 20
0
Nov
Dec
Feb
Mar
(0C)
5 10 15 20
Direct
4
Jan
0
5 10 15 20
Inverse
6
8
10
12
14
Spring mixing
Summer stratification
Fall mixing
Winter stratification
Dimictic Lakes



Complete circulations
(turnovers) in spring and
fall separated by summer
thermal stratification and
winter inverse
stratification
Very common in
temperate regions
Many other types based
on circulation patterns
Mixing 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.
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
Holomictic:
lakes mix completely
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
Kalff 2002
Warm monomictic lakes — one period of mixing
Thermal stratification
in summer
Does not freeze, so
mixes all winter
Lake Kinneret
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
Cold polymictic lakes — mix many times a year
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
Holomictic:
lakes mix completely
Warm polymictic lakes — mix many times a year
Never ice covered
Tropical lakes
May stratify for days or
weeks at a time, but
mixes more than once a
year
Mixing 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.
Meromictic: lakes are chemically stratified
Thermocline
Chemocline
Monimolimnion
Meromictic: lakes are chemically stratified
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)
Can get
interesting
thermal profiles
Warmer water
below colder
water above 4ºC
Recall salinity
increases density
Mixing 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)
Monomictic lakes: Cold / Warm
Dimictic lakes:
Polymictic lakes: Cold / Warm
3. Meromictic—Never fully mix due to an accumulation of
salts in the deepest waters.
Geographic Distribution
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.
What is meant by “shallow” and “deep enough” is
determined by the fetch and depth
A lake with a maximum
depth of 4m can stratify
if it is in a protected
basin
Bullhead Pond
Surface Area = 0.02 km2
Maximum fetch < 300 m
22 August 1993
A lake with a maximum
depth of 12m can be
unstratified if the fetch is
long enough
Te mpe r a t ur e ( C)
0
0
2
Oneida Lake, NY
Surface Area = 207 km2
Maximum fetch = 33 km
Depth (m)
4
6
8
10
12
5
10
15
20
25
30