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The distribution of heat in lakes

1. Why is the distribution of heat important to understand?

2. How and why do lakes stratify?

3. What are the major types of mixing patterns?

pg. 38- 45 in Dodson

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Why care about heat in lakes?

Controls rates of biological reactions

Controlling factor for distribution of organisms

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Recall the

exponential decay

of light in lakes

Kalff 2002

Does heat show

the same

pattern?

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NO!

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Temperature (C)

Depth(m)

Epilimnion

Hypolimnion

Metalimnion

Thermocline

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Epilimnion

Upper Layer

Warm

Well mixed

Hypolimnion

Lower layerCooler than epilimnion

Two separate water masses between which there is

little mixing

THERMOCLINE

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This condition of two, non-mixing layers is

known as THERMAL STRATIFICATION

Stabilitylikelihood that a stratified lakewill remain stratified.

This depends on the density differencesbetween the two layers.

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Examples:

Epilimnion Hypolimnion Result

8C 4C Not much density difference22C 7C Large density difference,

Strong stratification

30C 28C Large density difference,

Strong stratification

(tropical lakes)

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(1) Density relationshipsof water

Why do lakes stratify?

(2) Effect of wind

Less dense water

floats on deeper

water

Molecular diffusion of heat is slow

Wind must mix heat to deeper water

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How deeply the wind can mix the heat

depends on the surface area relative to the

depth

Fetch distance over which the wind

has blown uninterrupted by land.

Changes depending on which

way the wind blows

Influenced by a lakes surroundings

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How do lakes stratify?

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Temperature (C)

Depth(m)

Example:

10 m deep lake in Lake County, IL

(1) Early Spring

No density difference

No resistance to mixing

Heat absorbed in surfacewater is distributed throughout

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Spring Turnovertime of year whenentire water

column is mixed by the wind

Duration of spring turnover depends on the surface

area to maximum depth

In very deep lakes, the bottom water stays at 4C, in

more shallow lakes, can get up to > 10C.

Can last a few days or a few weeks.

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Temperature (C)

Depth(m)

(2) Mid Spring

Longer and warmer days

mean more heat is

transferred to the surface

water on a daily basis

Surface waters are

heated more quickly

than the heat can be

distributed by mixing

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This increase in surface waters relative to the restof the water column often occurs during a warm,

calm period

Now have resistance to mixing.

Hypolimnion water temperature will not change

much for the rest of the year.

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Temperature (C)

Depth(m)

(3) Late Spring

With the density

difference

established, the

epilimnion floatson the colder

hypolimnion

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Temperature (C)

Depth(m)

(4) Late Summer

The epilimnion has continued

to warm

Strong thermal stratification

In very clear lakes, can get

direct hypolimnetic heating

The decomposition of dead

plankton may result in loss of

oxygen from the hypolimnion

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78

9

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Temperature (C)

Depth(m)

(5) Early Autumn

Thermocline deepens

and epilimnion

temperature is reduced

Heat is lost from the

surface water at night

Cool water sinks and

causes convective

mixing

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78

9

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Temperature (C)

Depth(m)

(5) Mid-late Autumn

As epilimnion cools,

reduce density difference

between layers

Eventually, get Fall

Turnover

Turnover returns oxygen to

the deep water and nutrients

to the surface water

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78

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Temperature (C)

Depth(m)

(7) Winter

Surface water falls

below 4C and

floats on 4C

water

Get inverse

stratification

Ice blocks the wind from

mixing the cooler water

deeper

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isotherms

How to represent seasonal patterns in one graph?

Depth-time diagram, isopleth diagram

Wetzel 2001

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Basin morphometry

Geography

Water clarity

Weather

Mixing patterns are influenced by:

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1.Amicticnever mix because lake is frozen. Mostlyin Antarctica. Some in very high mountains.

2.Holomicticlakes mix completely (top to bottom)

3.MeromicticNever fully mix due to an accumulation

of salts in the deepest waters.

Mixing Patterns

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Holomictic:lakes are classified by the frequency of mixing

Monomictic lakes: one period of mixing

- Cold

- Warm

Dimictic lakes: two periods of mixing and two

periods of stratification

Polymictic lakes: mix many times a year

- Cold

- Warm

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Cold monomictic lakes one period of mixing

Frozen all winter (reverse stratification)

Mix briefly at cold temperatures in summer

rctic and mountain lakes

Kalff 2002

Meretta Lake, CA

Holomictic: lakes mix completely

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Warm monomictic lakes one period of mixing

Thermal stratification

in summer

Does not freeze, so

mixes all winter

Lake Kinneret

Kalff 2002


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Dimictictwo periods of mixing and two periods ofstratification

Freeze in winter (inverse stratification)

Thermally stratify in summer

Wetzel 2001


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Ice covered in winter, ice free in summer

May stratify for brief periods during the summer,but stratification is frequently interrupted

Shallow temperate lakes (< ~20 m) with large

surface area

mountain or arctic lakes

Cold polymictic lakes mix many times a year


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Never ice covered

Tropical lakes

Warm polymictic lakes mix many times a year

May stratify for days or

weeks at a time, but

mixes more than once a

year


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Estimated distribution of lake typesKalff 2002

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1. Amicticnever mix because lake is frozen. Mostlyin Antarctica. Some in very high mountains.

2.Holomicticlakes mix completely (top to bottom)

3.MeromicticNever fully mix due to an accumulation

of salts in the deepest waters.

Mixing Patterns

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Meromictic: lakes are chemically stratified

Monimolimnion

Thermocline

Chemocline

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Recall that salinity increases density

The water in the monimolimnion does

not mix with the upper water

The mixolimnion can have any mixing pattern

(e.g., dimitic, monomictic)


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Meromictic lakes are classified by how the deepwater became salty

Biogenic meromixis

Ectogenic meromixis

Crenogenic meromixis


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Lake Tanganyika

A. Biogenic Meromixis An input of salts due to

biological activity (decomposition)

Two main ways this can happen:1. Great depth:

Zm > 1400 mSurface Area = 32,000 km2

2 Sheltered lakes with relatively small surface area

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How much work can the wind do?

2. Sheltered lakes with relatively small surface area

relative to depth

Many years of

Incomplete mixing

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b. Ectogenic MeromixisAn external event brings

salt into a freshwater lake.

Salt water sinks and accumulates at bottom.

Often along marine coastal regions, strong storms

can wash in saltwater

Can be human induced

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Irondequoit Bay,

Lake Ontario

Chloride

concentration

increased 5 fold.

Road salt made this

Bay meromictic.

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c. Crenogenic MeromixisSubmerged saline spring

deliver dense water to deep portions of lake

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Recall salinity

increases density

Can get

interesting

thermal profiles

Warmer waterbelow colder

water above 4C

Mi i P tt

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1. Amicticnever mix because lake is frozen. Mostly in

Antarctica. Some in very high mountains.

2. Holomicticlakes mix completely (top to bottom)

Monomictic lakes: Cold / Warm

Dimictic lakes:

Polymictic lakes: Cold / Warm

3. MeromicticNever fully mix due to an accumulation ofsalts in the deepest waters.

Biogenic meromixis

Ectogenic meromixis

Crenogenic meromixis

Mixing Patterns

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What is meant by shallow and deep enough is

determined by the fetch and depth

All of these classification patterns are for lakes that are

deep enough to form a hypolimnion

Shallow lakes do not form a hypolimnion and are

therefore unstratified.They have similar temperatures top to bottom.

A lake with a maximum

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depth of 4m can stratify

if it is in a protected

basin

Bullhead Pond

Surface Area = 0.02 km2

Maximum fetch < 300 m

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depth of 12m can be

unstratified if the fetch islong enough

Oneida Lake, NY

Surface Area = 207 km2

Maximum fetch = 33 km

22 August 1993

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T e mp e r a t u r e ( C )

Depth(m)

T t K

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Epilimnion

MetalimnionThermoclineHypolimnionThermal stratificationStratification stabilityFetchSpring/Fall turnoverInverse stratificationIsopleth diagram

Isotherm

Terms to Know

Amictic

Holomicticmonomicticdimicticpolymictic

Meromicticbiogenicectogeniccrenogenic

ChemoclineMonomilimnionMixolimnion

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