Light in Lakes

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 5 10 15 20 25 30 1 2 3 4 5 6 7 8 9 10 Temperature (°C) 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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? 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 whenentire 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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (3) Late Spring With the density difference established, the epilimnion “floats” on the colder hypolimnion

40. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (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. Temperature (°C) 5 10 15 20 25 30 1 2 3 4 5 Depth (m) 6 7 8 9 10 How do lakes stratify? (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