1. Light in Lakes

2. Light is energy Major energy source to aquatic habitats
Productivity controlled by energy used in photosynthesis
Thermal character of lake determined by solar energy

3. Light is energy Solar radiation Capacity to do work
Can be transformed into other energy forms

4. 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 (high-short)

5. Light from the sun Mixture of wavelengths, energies
Most (50%) striking lake surface is infrared, visible (especially red part of spectrum)

6. Light from the sun Amount striking lake surface dependent on: Latitude
Season
Time of day
Altitude
Meteorological conditions

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

13. 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

14. Light Attenuation
In distilled water lake, >1/2 of light energy transformed into heat with first 1 meter

15. Light Attenuation Absorption not same for all wavelengths
Longer wavelengths more readily absorbed than shorter wavelengths

16. Light Attenuation

17. Light Attenuation Few distilled water lakes
Dissolved, suspended stuff affects absorption
Less absorption, greater transmittance in clear, unproductive lakes than in productive, murky waters

18. 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

19. 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

20. Water Transparency
Measuring light penetration before instrumentation - Secchi disk
Depth at which disk disappears/reappears from/to sight

21. 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)

22. Heat & Density Layering

23. Light to Heat Loss of light = gain in heat
Should temperature profile parallel light profile? No

24. 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

25. Direct Thermal Stratification
Lighter, warmer layer overlying denser, cooler layer
Lake divided vertically into 3 regions
Epilimnion
Metalimnion
Hypolimnion

26. Direct Thermal Stratification
Epilimnion - uniformly warm layer mixed by wind

27. Direct Thermal Stratification
Hypolimnion - uniformly cool lower layer unaffected by wind

28. 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

29. A Thermally Stratified Lake
Temperature (°C) 5 10 15 20 25 30 1 2 3 4 6 7 8 9
Epilimnion
Metalimnion
Thermocline
Depth (m)
Hypolimnion

30. Two separate water masses between which there is little mixing
Epilimnion
Upper Layer
Warm
Well mixed
THERMOCLINE
Hypolimnion
Lower layer
Cooler than epilimnion

31. STABILITY OF THERMAL STRATIFICATION
Stability—likelihood that a stratified lake will remain stratified.
This depends on the density differences between the two layers.

32. Examples:
Epilimnion Hypolimnion Result
8°C 4°C Not much density difference
22°C 7°C Large density difference, Strong stratification
30°C 28°C Large density difference, Strong stratification
(tropical lakes)

33. Even a Hurricane Can’t Break Stratification
Thermal resistance to mixing

34. 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

35. How do lakes stratify? (1) Early Spring No density difference5 10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (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

36. 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.

37. How do lakes stratify? (2) Mid Spring5 10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (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

38. 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.

39. How do lakes stratify? (3) Late Spring5 10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (m)
(3) Late Spring
With the density difference established, the epilimnion “floats” on the colder hypolimnion

40. How do lakes stratify? (4) Late Summer5 10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (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

41. How do lakes stratify? (5) Early Autumn10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (m)
(5) Early Autumn
Heat is lost from the surface water at night
Cool water sinks and causes convective mixing
Thermocline deepens and epilimnion temperature is reduced

42. How do lakes stratify? (5) Mid-late Autumn10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (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

43. How do lakes stratify? (7) Winter5 10 15 20 25 30 1 2 3 4 6 7 8 9
Temperature (°C)
Depth (m)
(7) Winter
Surface water falls below 4°C and “floats” on 4°C water
Ice blocks the wind from mixing the cooler water deeper
Get “inverse stratification”

44. Seasonal Stratification in a Temperate Lake
Direct
Inverse

45. 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

46. Mixing Patterns
1. Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.
Holomictic—lakes mix completely (top to bottom)
Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.

47. 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

48. 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
Meretta Lake, CA
Kalff 2002

49. 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

50. 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

51. 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

52. 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

53. Mixing Patterns
Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.
Holomictic—lakes mix completely (top to bottom)
Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.

54. Meromictic: lakes are chemically stratified
Thermocline
Mixolimnion
Chemocline
Monimolimnion

55. 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)

56. Can get interesting thermal profiles
Warmer water below colder water above 4ºC
Recall salinity increases density

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

58. Geographic Distribution

59. They have similar temperatures top to bottom.
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

60. 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

61. A lake with a maximum depth of 12m can be unstratified if the fetch is long enough
Oneida Lake, NY
Surface Area = 207 km2
Maximum fetch = 33 km