1. LAKES

2. Lakes vs. ponds
In lakes light doesn’t reach the whole way to the bottom
No photosynthesis = no O2 produced
Many organisms on or near the bottom require oxygen. How does it get there?

3. Overturn
Overturn is the process by which currents circulate the water in a lake or pond resulting in a movement of oxygen and nutrients

4. Spring Overturn Mixing of water in the springtime
Caused by melting and the cold water sinking to the bottom
The water is then mixed easily by wind
Temperature is the same throughout
Same temperature = same density
Same density allows for easier mixing
Brings nutrients up from bottom to the top

5. Spring Overturn

6. Spring Overturn

7. Summer Stagnation
In summer the sun heats the top layer faster than the wind can circulate it.
3 layers result
Different densities = 3 different levels
These layers don’t mix well

8. Summer Stagnation

9. Summer Stagnation

10. Thermocline or metalimnion
Epilimnion
Upper layer warmer
Water circulates freely
Hypolimnion
Cool layer
Does not circulate
No oxygen from the top
Thermocline or metalimnion
Middle layer
Transition from warm to cold

11. Fall Overturn Epilimnion (top layer) cools
The fall winds cause another mixing
The entire lake becomes a uniform temperature again

12. Fall Overturn

13. Fall Overturn

14. Fall Overturn

15. Winter Stagnation The upper layer (epilimnion) cools further and sinks
Increased cooling = increased density
Now the thermocline is reversed. The water on the bottom is near 4ºC and the water at the top is 0ºC (cold to warm)
Top layer freezes
Result = 3 layers
Epilimnion is ice
Hypolimnion is above or at 4°C

16. Winter Stagnation

17. Winter Stagnation

18. ZONES OF A LAKE

19. Trophogenic Zone Upper layer where most oxygen is produced
Organisms here produce food through photosynthesis

20. Tropholytic Zone
Lower part of the hypolimnion with high CO₂ and a low O₂
Organisms feed on decomposing matter

21. Compensation depth Zone between trophogenic and tropholytic zones
Oxygen production and use are equal

22. Limnetic zone Subzone of trophogenic zone
Part of open water where light penetrates the water
Phytoplankton are the main producers

23. Littoral zone Subzone of trophogenic zone
Region where light reaches the bottom (near shore)

24. AGING OF A LAKE

25. Eutrophication The process of aging
The increasing productivity of a lake
Means “adequate food”
Nutrient rich
Accumulations of nutrients from runoff, sewage waste

26. Comparing new and old lakes

27. Eutrophic Adequate food or nutrient rich
Oxygen levels decrease greatly at the bottom

28. Eutrophic

29. Oligotrophic “Lacking food” Nutrient poor
Oxygen decreases only slightly with depth
Young biological lake
Bedrock of limestone or dolomite (basic)
The pH normally drops as a lake ages
Decay releases acids into the water (carbonic acid)
Carbonic acid forms when the carbon dioxide from respiration reacts with the water

30. Oligotrophic

31. Mesotrophic Middle stage of lake life More acidic
The pH usually rises and becomes more basic

32. Young biological lake
Middle aged
biological lake
Old biological lake
basic
more
acidic
Oligotrophic
Mesotrophic
Eutrophic

33. GEOPHYSICAL PROPERTIES OF LAKES
Temperature
Nature of the bottom
Color
Turbidity
Transparency

34. Temperature
Each species of organism has its own optimum or preferred temperature
Some can stand a variation in temperature
But if it goes too far in a range = Death or move elsewhere
Increase of 5 ̊C can be lethal
Fish kills
Oxygen demand exceeds the available oxygen
Death occurs
TLM – Tolerance Limit Median - based on time to get out of a bad situation
High temperatures will increase the toxic effects of chemical pollutants in the water

35. Nature of the Bottom
Detritus
Submerged Plants

36. Color Suspended material Water is green = phytoplankton
Water is yellow or brown = dead algae, runoff

37. Turbidity Suspension of solids Phytoplankton and zooplankton
Dead wastes sewage
Inorganic clay and silt
Secchi Disk

38. Transparency How well light passes through water
Suspended solids in water
Secchi Disk
Low reading (1m) = high level suspended matter
High reading (8m) = quite clear
Based on compensation depth