Chapter 11: Seasonal Temperature Cycles continued

1
Chapter 11 Seasonal Temperature
Cycles(continued)
2
Temperature Cycles Lake Stratification

• Most lakes mix during some seasons and become
  stratified during other seasons.
• These terms refer to the vertical circulation of
  water Mixing circulation, Stratification
  lack of mixing (development of layers)
• The mixing pattern has a large effect on lake
  chemistry and the biota
• Lakes have traditionally been classified
  according to their annual mixing pattern or
  mixing regime (amictic, monomictic, dimictic,
  etc.)

3
Temperate zone Dimictic Lake
Mixing
Stratified
Stratified
Mixing
4
Thermal zones in a stratifed lake
Metalimnion
5
Allens Lake, MISept 7, 2007
Epilimnion
Metalimnion
Hypolimnion
6
Seasonal Cycle in a Temperate Dimictic Lake

• After ice melts in spring, the lake is cold and
  isothermal (same temperature from top to bottom)

0
Z
Zmax
Temperature
4
7
Seasonal Cycle in a Temperate Dimictic Lake

• As air temperature and solar radiation increase,
  there may be a period of isothermal warming,
  where warmer surface waters are mixed downward by
  wind and wave energy

0
Z
Zmax
Temperature
4
8
Seasonal Cycle in a Temperate Dimictic Lake

• Eventually, the heating of the surface water will
  outpace the capacity of wind and waves to mix the
  heat downward

0
Z
Zmax
Temperature
4
9
Seasonal Cycle in a Temperate Dimictic Lake

• The warm surface layer (epilimnion) floats on the
  colder, denser layer (hypolimnion)

0
Z
Zmax
Temperature
4
10
Seasonal Cycle in a Temperate Dimictic Lake
11
Seasonal Cycle in a Temperate Dimictic Lake

• Over the summer the epilimnion may continue to
  warm, but the hypolimnion temperature will change
  very little

0
Z
Zmax
Temperature
4
12
Seasonal Cycle in a Temperate Dimictic Lake

• In the fall, the epilimnion begins to cool, and
  the process goes in reverse. The thermocline
  will deepen.

0
Z
Nutrients mixed upwards
Zmax
Temperature
4
13
Seasonal Cycle in a Temperate Dimictic Lake

• Fall Overturn followed by isothermal cooling

0
Z
Zmax
Temperature
4
14
Seasonal Cycle in a Temperate Dimictic Lake

• Inverse stratification and ice formation

0
Z
Zmax
Temperature
4
15
Seasonal Cycle in a Temperate Dimictic Lake
16
Seasonal Cycle in a Temperate Dimictic Lake

• Wind from strong storms can have a effect on the
  thermal profile, causing storm thermoclines.

0
Z
Zmax
Temperature
4
17
Mixing Regimes

• Dimictic Mixes in spring and fall
• Monomictic
• Cold high latitudes or elevation, Mixes all
  spring summer and fall. Stratified under winter
  ice.
• Warm never freeze in winter. Mixes all fall,
  winter, spring. Stratified in the summer. (Great
  Lakes as well)
• Amictic never mix. Antarctic lakes always ice
  covered and inversely stratified
• Polymictic Mix many times annually. Usually
  shallow lakes

18
Lake Thermal Isopleths
19
Winter Conditions
4
Temperature
0
10
2
Mean Fetch (km)
20
Effective Fetch
Prevailing winds
N
21
Thermal Bars

• In spring, near-shore areas of lakes heat faster
  than offshore areas. Also, inflowing water from
  tributaries is usually warmer than winter lake
  temperatures.
• These two factors lead to Thermal Bars, which are
  usually spring features of large lakes.

22
Lake Ontario Thermal Bar
http//www.on.ec.gc.ca/solec/nearshore-water/paper
/images/fig3.gif
23
Lake Michigan Thermal Bar
1982
1994
Beletsky and Schwab, 1991
24
Southern Lake Michigan Thermal Bar and Sediment
Plume

• Thermal bars have a large effect on the ecology
  of L. Michigan
• DOC and nutrient-rich tributary water held near
  shore and Bottom sediments resuspended by storms
  cause
• Resuspension of contaminants (PCBs, PAHs)
• Blocks light (reducing phytoplankton productions)
• Increased bacteria growth
• Increased zooplankton growth (feeding on
  bacteria)
• Ecology for entire year depends on extent of
  spring storms and thermal bar

25
Stability of Stratification

• Thermal Stability refers to the physical energy
  (wind mixing) required to completely destratify a
  lake.
• Stability is related to the difference in density
  between the eplilimnion and hypolimnion.
• Stability is also related to the depth of the
  thermocline

0
Ze
Z
Ze
Zmax
Temperature
26
Thermocline Depth

• Thermocline Depth of lakes is largely influenced
  by regional wind strength.
• Lakes located in windy regions (New Zealand,
  Scotland, Argentina, etc) will have a greater
  average depth of themocline than otherwise
  similar lakes in less windy regions.
• Within a region (similar wind strength)
  thermocline depth is influenced by lake
  morphometry, fetch, and water transparency

Log Thermocline depth (m)
Secchi Depth (m)
27
Effects of Zebra Mussels on Thermocline depth

• Small resevoir in Ohio was invaded by zebra
  mussels in 1993
• ZM are filter-feeders, consumed phytoplankton,
  reducing algal biomass by 1995

28
Effects of Zebra Mussels on Thermocline depth

• Reduced algal biomass resulted in greater
  transparency (secchi depth)

29

• Increased transparency permitted deeper
  penetration of light energy, resulting in a
  deeper thermocline
• Zebra mussels as ecosystem engineers

2003 light
2005
30
Heat Budgets

• Annual Heat Budget Amount of energy needed to
  heat lake from its coldest winter temperature to
  its warmest summer temperature.
• Temperate zone lakes have much larger heat
  budgets compared to tropical lakes (of similar
  size).
• Solar radiation accounts for gt 90 of heat gain
  in most lakes as opposed to advective heating
  (warm rain and river inflows)

31
Climate Change and Lakes

• For many lakes, a warming climate would
• Reduce annual period of ice cover
• Increase evaporative losses from lake and
  watershed
• Increase water residence time
• Increase salinity
• Cause earlier onset of stratification, longer
  summer stratified period
• Increase hypolimnetic hypoxia
• Favor warm water fish species (perch, bass,
  walleye)
• Cool water species (salmonids, whitefish) may
  have to migrate northward
• Less DOM inputs to water, therefore greater UV
  radiation

32
Lakes and Climate Change

• Northern Hemisphere Lakes
• Over last 150 years, freezing later in the year
• Thawing earlier in the year

33
Phytoplankton growth cycles (Typical
temperate-zone lake)

• Phytoplankton Growth Cycle are the product of
• Seasonal Temperature Cycles
• Light
• Nutrients

34
Nutrient Cycles

• Nutrient concentrations increase after spring
  thaw due to tributary input and isothermal mixing
  (whole lake in contact with sediments)
• Nutrient concentrations in epilimnion decline
  over summer because
• Tributary inputs decline (less external loading)
• Surface waters are not in contact with sediments
  (less internal loading)
• Nutrient concentrations in epilimnion increase
  after fall turnover

35
Ice-out
Turnover
Nutrients
Jan June Dec
36
Light

• Light available to organisms in the lake changes
  over the seasons
• Low light under snow/ice cover
• Increased light as snow melts and ice thins.
• Very low light during spring isothermal period
  (high turbidity, deep mixing)
• Light in epilimnion increases after
  stratification (longer daylength, increased
  clarity)
• Light levels decline at fall turnover

Turnover
Ice-out
Light
Jan June Dec
37
Temperature

• Temperature of the epilimnion follows a regular
  seasonal pattern

Turnover
Ice-out
Temperature
Jan June Dec
38
Combine all three factors
Turnover
Ice-out
Temperature

• For phytoplankton growth in the epilimnion
• During spring mixing, conditions are poor very
  low light
• Following spring stratificaton conditions are
  excellent. (high light, high nutrients, cool
  temperature)
• Nutrient depletion in epilimnion, High
  temperature causes high sinking rates. OK
  conditions
• Higher nutrients as hypolimnion begins to mix
  with epilimnion. Good conditions
• Poor conditions due to low light

Light
Nutrients
1
2
3
4
5
Jan June Dec
39
Annual Phytoplankton Growth Cycle