EVPP 550 Waterscape Ecology and Management

1. EVPP 550Waterscape Ecology and Management Professor R. Christian Jones Fall 2007

2. Energy, Stratification, and Mixing • What happens to the energy as the light is absorbed by the lake? • 2nd law of thermodynamics state that all forms of energy eventually degrade to heat • As heat is added to water, temperature of water increases • Temperature of water determines its density and helps drive stratification

3. Heat vs. Temperature • Heat is the thermal energy content of a substance • Heat is created from other energy forms such as light and chemical bonds • The heat content of a substance is reflected in its temperature • Heat flows from substances of higher temperature to those of lower temperature

4. Stratification • Stable stratification results when waters of differing densities are positioned vertically in order of their density • In other words, more density (heavier) water lies below less dense (lighter) water • Work is required to break down this density gradient • Stability of stratification is the work required to uniformly mix a stratified lake

5. Mixis • Mixis is the state when the water body is mixed • If the entire water body mixes on a regular basis, it’s called holomixic • If only a portion normally mixes, its called meromictic • If the water body never mixes, its called amictic

6. Holomictic Lakes • Dimictic (temperate) Lakes • Stratification occurs in summer and winter • Mixing periods occur in spring and fall • Summer pattern

7. Dimictic Lakes – Annual Cycle Seasonal heating and cooling Wind creating turbulence

8. Dimictic Lakes – Annual Cycle Seasonal heating and cooling Wind creating turbulence

9. Dimictic Lakes – Annual Cycle • Note lakes regions defined by temperature profile • Metalimnion: zone where temp changes at least 1oC per m • Epilimnion: mixed layer above that • Hypolimnion: fairly stagnant layer below that

10. Dimictic Lakes – Annual Cycle • In addition to the main thermocline forming the metalimnion, temporary thermoclines can form within the epilimnion due to diel heating and cooling

11. Dimictic Lake – Annual Cycle • Annual cycle depicted by isopleths (Mountain Lake, VA)

12. Monomictic Lakes • Lakes that circulate once per year are called monomictic lakes • If the circulation is only in the warm season, they are called “cold” monomictic because these lakes are normally found in colder areas (near polar) • If the circulation is only in the cold season, they are called “warm” monomictic because these lakes are normally found in warmer areas (subtropical areas like southern US)

13. Warm Monomictic Lakes • Seasonal cycle similar to dimictic, but ice cover and winter stratification • Generally found in subtropical areas, but may be found as far north as New York if lake is deep and can’t cool below 4oC (ex. Cayuga L and L Ontario) Lake Windermere, England

14. Cold Monomictic Lakes • Seasonal cycle similar to dimictic, but no summer stratification • Restricted to polar areas; dependent on wind mixing to break down incipient stratification

15. Polymictic Lakes • These lakes stratify and mix many times per year • Often have a daily stratification and mixing cycle • Most common in the tropics

16. Polymictic Lakes • Shallow temperate zone lakes can also be polymictic including the GMU Pond • Note the daily stratification and mixing pattern

17. Lake Mixis Summary • Type of lake mixis can generally be predicted based on latitude and depth

18. Oligomictic Lakes • Mix very infrequently • Tropical areas with little temperature fluctuation • Or large lakes where cooling and the wind are not sufficient to mix the entire water mass • The infrequent mixings can result in release of large quantities of CO2 with lethal effects Lake Nyos

19. Meromictic Lakes • Entire water body never turns over • Permanent stratification • Bottom water generally has a high concentration of dissolved material which increases its density well beyond what it would be at 4oC

20. Meromictic Lakes • Causes of Meromixis • Biogenic: dissolved substances derived from bacterial decay of organic matter and diffusion from the sediments • Ectogenic: dissolved substances originate from mineral salts introduced from the surrounding watershed (or freshwater flows on top of an existing salt lake) • Crenogenic: dissolved substances originate from subsurface flows containing mineral salts

21. Merimictic Lakes • An additional requirement for merimixis, esp biogenic, is a deep, steeply sloped basin, protected from the wind (zr = 5-22% compared to <2% for most lakes)

22. Other Water Movements • Surface currents/Ekman drift • Wind-generated • In large deep lakes the wind generated current is deflected 45o due to Earth’s rotation • As lake size decreases, angle decreases approaching 0 in small lakes • In general water velocity is about 2% of surface velocity • Reverse current is generated in below surface waters

23. Water Movements • Langmuir circulation • Motions induced by wind turbulence can be organized into vertical helical currents in the upper layers of lakes • Spiral orient with the wind and result in accumulation of floating material and even organisms at downwelling (convergence) sites

24. Water Movements • Hypolimnetic currents • Even during stratification, some movements occur within the hypolimnion • Under ice, currents have also been demonstrated

25. Water Movements • Seiches are free oscillation of the entire lake following water displacement, normally by high sustained winds • External seiche is the oscillation of the water’s surface • Internal seiche is the oscillation at the thermocline

26. Water Movements • Seiche amplitude is a function of the energy applied (wind stress) (+) and the density difference between the two layers (-) • External seiches are generally quite small due to large density difference between water and air • Some typical values: Lake Mendota 1-2 mm, Lake Geneva 1.9 m, and Lake Michigan (1954) 3 m

27. Water Movements • Internal seiches are generally much greater in amplitude due to much smaller difference between epilimnion and hypolimnion • A surface seiche of 10 mm would correspond to an internal seiche of 6.7 m

28. Seiches and Profiles • Seiches can be determined by observing changing temperature profiles at a given point

29. General Summary of Water Movement in Lakes • This figure summarizes the range of types of lake movements that can occur in lakes

30. Chemistry of Lakes - Oxygen • Oxygen is the second most abundant element in the atmosphere (20%) • But is only weakly soluble in water (10 ppm) • Most aquatic organisms require 4-5 mg/L for survival • So… oxygen can be a limiting factor in aquatic systems

31. Chemistry of Lakes - Oxygen • Henry’s Law governs the solubility of gasses in water • Saturation conc = partial pressure x solubility factor • Solubility factor is a function of temp, in water it decreases with increasing temp • Altitude decreases partial pressure and decreases saturation conc • Dissolved solids decrease solubility factor thereby decreasing saturation conc

32. Lake Chemistry - Oxygen • Sources • Atmosphere • Photosynthesis • 6CO2 + 6H20 + light  C6H12O6 + O2 • Sinks • Atmosphere • Respiration • Chemical oxidation • Gas bubbles

33. Lake Chemistry - Oxygen • Diurnal variation • Oxygen can increase rapidly near the surface during the day due to photosynthesis • In L. Victoria polymictic conditions mean that the lake turns over virtually every evening

34. Lake Chemistry - Oxygen • Vertical Distribution • Varies with lake type • Very productive lakes lose oxygen during stratification

35. Lake Chemistry - Oxygen • The absence of oxygen can allow other chemicals like H2S to build up

36. Lake Chemistry - Oxygen • Very pure lakes can exhibit a different curve called a clinograde curve in which O2 increases with depth • Why? • What about curve c?

37. Lake Chemistry - Oxygen • Other unusual DO profiles – Redberry Lake, Saskatchewan • Any ideas about what is happening here?

38. Lake Chemistry - Oxygen • Does this help?