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, its 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 cant 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
  lt2 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 Earths 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 waters
  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

• Henrys 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?