Light in Lakes III: Heat in Lakes (Fall 2000)
I. Heat in Lakes
A. Median Frequency of light hitting a lake =?
Approx. 50% of the RE is >750 nm = Red, IF = HEAT
90% absorbed in the first meter
99% absorbed in the first 2 meter
absorbance is increased by dissolved organics and other
>>50% of ALL incoming radiation is absorbed in the first 1 – 2 meters
Heat is a SURFACE PHENOMENON!!!!
B. Potential impacts of this heat energy
1. Regulation of nutrient solubility and availability
2. Regulation of biological enzyme systems
3. Regulation of biotics
4. Regulation of water density & density difference per degree C
lowering,
C. Regulation of Stratification and Mixing in Temperature Zone Lakes
(PATTERNS)
1. winter  ice cover lost rapidly  Spring water T Spring
mixing period, which varies in duration from days to weeks, leading to a uniform heatin g of the lake.
Resistance to
mixing small 04°C range & heats
rapidly –1 – 3°
0°
4°
4°
2. Then the lake continues heating until it reaches around 14°C and at this point (Fig. 2-3 above) the upper 1 – 2 m
heats more rapidly leading to 2-3° difference in temp. during a calm period leading to less dense upper waters than cooler more dense
lower water (Per degree density diff is very high at 14
3. This leads to thermal stratification of the lake into three temperature “zones”
depth
Epilimnion
Metalimnion
hypolimnion
10-14°C Temperature
a. epilimnion – the upper stratum of uniformly warm, turbulent water
b. metalimnion – area of steep lthermal gradient (intermediate water density)
contains the thermocline the plane of maximum rate of decrease in T with depth (z)
c. hypolimnion
- the lower layer of uniformly cool calm waters, where no to little mixing occurs
d. The thermocline may form at various depths or not at all depending on Wind – T relations.
Strong wind in spring leads to uniform heating  Tmc forms deep in the lake and then rises
No wind  T induced TMC forms in the upper few cms and sinks down as wind and T balance out.
Hypolimnion increases in T as summer progresses and so Tmc moves deeper into the lake
II. Where the Tmc forms (the seasonal or parent) determines a lot about the lake and its overall productivity, especially in relation to
the photosynthetic aspects of light (Zcd)
Case 1: An oligotrophic clear, low “productivity” lake- phytosynthesis is largely restricted above the Tmc – why?
Thermocline
2
Low O2
Case 2: A lake with very high turbidity. What happens to the productivity as the producers are swept below the
TEMPERATURE
Compensation
depth as the epilimnion mixes.
0
Compensation Depth
DEPTH
Thermocline
Case 3: A highly productive lake: The Tmc and Cd may almost coincide as algal populations absorb both heat and light
radiation components
Z
Compensation Depth
Thermocline
All of these relationships are
Regulated by :
Light
Sediment, Particles, organisms as particles
Wind (mixing)
III. Loss of stratification occurs in the Fall
Air T declines  surface waters cool (epilimnion)  surface water becomes more dense than underlying warmer waters (that
important high specific heat of water) surface waters sink  leading to a “fall overturn” or fall mixing period in the lake
IV. Winter Inverse Stratification
As air T continues to decline in the north temperate regions ice eventually forms at the surface at –0°C and yet the water
under the ice continues to receive some incoming solar radiation and stays warmer, usually settling in at an average temperature of
4°C(remember this is the most dense water).
V. Overall summary of seasonal lake heat patterns in North Temperate Regions = “DIMICTIC LAKE”
Spring(4°)
Summer
Fall
Winter
This dimictic lake then redistributes nutrients during periods of circulation, uses nutrients during the summer and winter,
while settling much of the nutrients to the benthos.
3
Examine, understand, and be able to explain this isotherm diagram from Wetzel(p.75).
VI. Major Lake Types (mixing)
A. Amictic lakes – polar with perrenial ice cover; sterile to oligotrophic with an algal based food chain.
B. Cold Monomictic – found in lakes formed by glaciers or in contact with permafrost
ice cover melts during the summer, but water remains at around 4°C all summer
cold adapted organisms with a large summer food web pulse
C. Dimictic – most temperate region lakes
ice only in the winter with 2 mixing periods and two food web pulses
D. Warm Monomictic – coastal (esp. Pacific NW) lakes in climates without long winter freezes
Stratify during summer, but circulate during the winter (>4°C) and never freeze
E. Oligomictic – mostly lakes found in “aseasonal” climates like the tropics where temperature s do not vary much. These
lakes remain stratified constantly and are “constant energy” systems with water T around 25°C. What might be the
stimulant factor that might initiate one of the “few mixes”.
F. Polymictic – Lakes at high altitudes are constantly changing temperatures and thus have frequent to continuous mixing.
G. Holomictic – the entire lake mixes
H. Meriomictic – low productivity lakes that never circulate and are constantly stratified. Why?
20,000ppm salt
Compensation Depth
mixolimnion
Chemocline
Monimolimnion
200,000ppm salt
4
I. The type of lake is largely determined by Altitude and Latitude (Fig. 6-7)
VII. Types and Characteristics of meriomictic lakes
A. Types:
1. Ectogenic meriomixis – low salt/low density upper, high salt, higher density lower
Typical of coastal lakes that are consistently inundated by oceanic storms, midwest lakes with road runoff, and
lakes in deforested (erosive salt origins) areas.
2. Crenogenic – Underwater saltwater spring origin in lake
3. Biogenic – mineralization by anaerobic decomposition in lake bottoms (when dimictic lakes fail to mix);
concentration of soluble compounds increases in lake bottoms
4. Cryogenic – as water freezes the concentration of salts increases in the remaining water.
B. Characteristics
1. Interesting T profiles: (You can tell lake type simply by its temperature profile)
Summer
Temperature
a. Radiation increases surface
temperature
Chemocline
DEPTH
Bacterial
decomposition
b. metabolic heat from anaerobic
decompositon processes also
contributes heat to the monimolimnion
Winter
Temperature
a. Surface iceinverse stratification
down to the chemocline
Chemocline
DEPTH
Low to no O2
b. Below Ccm heat income declines
and since no mixing  permanent
anoxia in the monimolimnion
5
Transition
Temperature
Summer
condition
Chemocline
DEPTH
A lag effect occurs as the mixolimnion
begans to warm in the spring.
Winter condition
2. Life in Meriomictic lakes
Photsynthetic algae, mixing, high O2
DEPTH
Green to purple photosynthetic bacteria, Life here is restricted by decreased O 2 and decreased
penetration of long  light for photsynthesis
Anaerobic sulfur reducing bacteria
3. Where are meriomictic
lakes
found???
Almost anywhere,
Lake Mary, WI
b. metabolic
heat
from anaerobic
decompositon
VIII. Thermal Storage Capacity
of also
Lakes
– Annual heat
HeattoBudgets
and Cycles
processes
contributes
the
Since heat drivesmonimolimnion
T and stratification, all lake metabolism, heat cycles within lakes are very important. For example: how
much heat is required to develop stratification etc.
A. Birge developed the idea of the annual heat budget or the maximum heat content.
1. Definition: The amount of heat necessary to raise the water of the lake from the minimum winter temp to the
maximum summer temperature
a. the summer heat budget is the amount of energy needed to increase from the isothermal 4°C state to the
maximum summer temperature
b. the winter heat budget is the amount of heat needed to raise from the minimum temperature (0°C?) to 4°C and
of course includes the heat of fusion and melting.
2. Can be calculated based upon
a. size of lake and depth (z)
b. latitude
c. lake type
d. Lake Baikal = largest, Lake Michigan
temperate lakes are intermediate
Artic and tropical lakes have low heat budgets
Meriomictic and amictic have very low heat budgets
Where would tropical lakes fit in????
3. According to Welch the heat content of a lake º
  Zcm (T s  T w)
m
m
where
Tm 
Ti  Ti  1 Vi

2
Vt
IX. Lake Valentine Light on 9/16/98 and 9/16/00
6
1998
2000