Light in lakes l.jpg
This presentation is the property of its rightful owner.
Sponsored Links
1 / 61

Light in Lakes PowerPoint PPT Presentation


  • 104 Views
  • Uploaded on
  • Presentation posted in: General

Light in Lakes. Light is energy. Major energy source to aquatic habitats Productivity controlled by energy used in photosynthesis Thermal character of lake determined by solar energy. Light is energy. Solar radiation Capacity to do work Can be transformed into other energy forms.

Download Presentation

Light in Lakes

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Light in lakes l.jpg

Light in Lakes


Light is energy l.jpg

Light is energy

  • Major energy source to aquatic habitats

  • Productivity controlled by energy used in photosynthesis

  • Thermal character of lake determined by solar energy


Light is energy3 l.jpg

Light is energy

  • Solar radiation

  • Capacity to do work

  • Can be transformed into other energy forms


Light from the sun l.jpg

Light from the sun

  • Pulsating field of force, endless series of waves

  • Packets of energy - photons

  • Energy proportional to frequency (high-high), inversely to wavelength (high-short)


Light from the sun5 l.jpg

Light from the sun

  • Mixture of wavelengths, energies

  • Most (50%) striking lake surface is infrared, visible (especially red part of spectrum)


Light from the sun6 l.jpg

Light from the sun

  • Amount striking lake surface dependent on:

  • Latitude

  • Season

  • Time of day

  • Altitude

  • Meteorological conditions


Light and atmosphere l.jpg

Light and atmosphere

  • Light absorbed by particles in atmosphere

  • Less atmosphere to pass through, more light makes it to earth - angle of incidence

  • Shorter wavelengths selectively absorbed by O2, ozone, H2O vapor, CO2

  • Red sky at dawn, dusk


Indirect light l.jpg

Indirect Light

  • Some solar radiation reaches lake indirectly

  • Scattered light

  • Light scattered as it passes through atmosphere (20%)

  • Mostly UV and short wavelength visible (blue)


Indirect light9 l.jpg

Indirect Light

  • Importance of indirect light changes with angle of incidence

  • Contribution of indirect small when sun directly overhead

  • Contribution significant (~20-40%) when sun low in sky


Reflected light l.jpg

Reflected Light

  • Significant fraction of light striking lake surface may be reflected

  • Amount increases with decreased angle of incidence

  • Wave action increases reflection only at low angles of incidence


Other losses of light l.jpg

Other Losses of Light

  • Reflection comprises ~1/2 of light lost from water

  • Remaining half lost by scattering

  • Deflection by water molecules, dissolved substances, suspended particles

  • Varies with depth, season, particle loading


Lake color l.jpg

Lake Color

  • Scattering and absorption of light give lake part of its characteristic color

  • Clean water - blue color

  • More and bigger particles scatter longer wavelengths and absorb shorter wavelengths

  • Blue-green, green, yellow


Light attenuation l.jpg

Light Attenuation

  • Radiant energy diminished with depth

  • Results from both scattering and absorption

  • Absorption - loss of solar energy with depth by its transformation to heat


Light attenuation14 l.jpg

Light Attenuation

  • In distilled water lake, >1/2 of light energy transformed into heat with first 1 meter


Light attenuation15 l.jpg

Light Attenuation

  • Absorption not same for all wavelengths

  • Longer wavelengths more readily absorbed than shorter wavelengths


Light attenuation16 l.jpg

Light Attenuation


Light attenuation17 l.jpg

Light Attenuation

  • Few distilled water lakes

  • Dissolved, suspended stuff affects absorption

  • Less absorption, greater transmittance in clear, unproductive lakes than in productive, murky waters


Light attenuation18 l.jpg

Light Attenuation

  • Blues disappear, greens penetrate, reds change with productivity

  • Transmission drastically affected by cover of cloudy ice, snow

  • Shuts down photosynthesis, reduces O2 supply


Euphotic zone l.jpg

Euphotic Zone

  • Region from surface to depth at which 99% of the surface light has disappeared

  • Minimum intensity of subsurface light that permits photosynthesis is ~1% of incident surface light


Water transparency l.jpg

Water Transparency

  • Measuring light penetration before instrumentation - Secchi disk

  • Depth at which disk disappears/reappears from/to sight


Water transparency21 l.jpg

Water Transparency

  • Secchi disk transparency X 3 used as a “rule of thumb” estimate of depth of euphotic zone

  • Highly variable (e.g., Lake Erie 5X)


Heat density layering l.jpg

Heat & Density Layering


Light to heat l.jpg

Light to Heat

  • Loss of light = gain in heat

  • Should temperature profile parallel light profile?

  • No


Light to heat24 l.jpg

Light to Heat

  • Uniformly mixed layer of water near surface of same temperature

  • Often extends below euphotic zone

  • Mixing of upper layers of water by wind distributes heat downward


Direct thermal stratification l.jpg

Direct Thermal Stratification

  • Lighter, warmer layer overlying denser, cooler layer

  • Lake divided vertically into 3 regions

    • Epilimnion

    • Metalimnion

    • Hypolimnion


Direct thermal stratification26 l.jpg

Direct Thermal Stratification

  • Epilimnion - uniformly warm layer mixed by wind


Direct thermal stratification27 l.jpg

Direct Thermal Stratification

  • Hypolimnion - uniformly cool lower layer unaffected by wind


Direct thermal stratification28 l.jpg

Direct Thermal Stratification

  • Metalimnion - intermediate zone where temperature drops rapidly with increasing depth

  • Also referred to as thermocline - plane between two depths between which temperature change is greatest


Slide29 l.jpg

A Thermally Stratified Lake

5

10

15

20

25

30

1

2

3

4

5

6

7

8

9

10

Temperature (°C)

Epilimnion

Metalimnion

Thermocline

Depth (m)

Hypolimnion


Slide30 l.jpg

Two separate water masses between which there is little mixing

Epilimnion

Upper Layer

Warm

Well mixed

THERMOCLINE

Hypolimnion

Lower layer

Cooler than epilimnion


Slide31 l.jpg

STABILITY OF THERMAL STRATIFICATION

Stability—likelihood that a stratified lake will remain stratified.

This depends on the density differences between the two layers.


Slide32 l.jpg

Examples:

EpilimnionHypolimnionResult

8°C4°CNot much density difference

22°C7°CLarge density difference, Strong stratification

30°C28°CLarge density difference, Strong stratification

(tropical lakes)


Slide33 l.jpg

Even a Hurricane Can’t Break Stratification

Thermal resistance to mixing


Slide34 l.jpg

Why do lakes stratify?

(1) Density relationships of water

Less dense water “floats” on deeper water

(2) Effect of wind

Molecular diffusion of heat is slow Wind must mix heat to deeper water


Slide35 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

Example:

10 m deep lake in Lake County, IL

(1) Early Spring

No density difference

No resistance to mixing

Heat absorbed in surface water is distributed throughout


Slide36 l.jpg

Spring Turnover—time 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 4°C, in more shallow lakes, can get up to > 10°C.

Can last a few days or a few weeks.


Slide37 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(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


Slide38 l.jpg

This increase in surface waters relative to the rest of 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.


Slide39 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(3) Late Spring

With the density difference established, the epilimnion “floats” on the colder hypolimnion


Slide40 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(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


Slide41 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(5) Early Autumn

Heat is lost from the surface water at night

Cool water sinks and causes convective mixing

Thermocline deepens and epilimnion temperature is reduced


Slide42 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(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


Slide43 l.jpg

Temperature (°C)

5

10

15

20

25

30

1

2

3

4

5

Depth (m)

6

7

8

9

10

How do lakes stratify?

(7) Winter

Surface water falls below 4°C and “floats” on 4°C water

Ice blocks the wind from mixing the cooler water deeper

Get “inverse stratification”


Slide44 l.jpg

Seasonal Stratification in a Temperate Lake

Direct

Inverse


Dimictic lakes l.jpg

Dimictic Lakes

  • Complete circulations (turnovers) in spring and fall separated by summer thermal stratification and winter inverse stratification

  • Very common in temperate regions

  • Many other types based on circulation patterns


Slide46 l.jpg

Mixing Patterns

  • 1.Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.

  • Holomictic—lakes mix completely (top to bottom)

  • Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.


Slide47 l.jpg

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


Slide48 l.jpg

Holomictic:lakes mix completely

Cold monomictic lakes — one period of mixing

Frozen all winter (reverse stratification)

Mix briefly at cold temperatures in summer

Arctic and mountain lakes

Meretta Lake, CA

Kalff 2002


Slide49 l.jpg

Holomictic:lakes mix completely

Kalff 2002

Warm monomictic lakes — one period of mixing

Thermal stratification in summer

Does not freeze, so mixes all winter

Lake Kinneret


Slide50 l.jpg

Holomictic:lakes mix completely

Dimictic—two periods of mixing and two periods of stratification

Freeze in winter (inverse stratification)

Thermally stratify in summer

Wetzel 2001


Slide51 l.jpg

Holomictic:lakes mix completely

Cold polymictic lakes — mix many times a year

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


Slide52 l.jpg

Holomictic:lakes mix completely

Warm polymictic lakes — mix many times a year

Never ice covered

Tropical lakes

May stratify for days or weeks at a time, but mixes more than once a year


Slide53 l.jpg

Mixing Patterns

  • Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.

  • Holomictic—lakes mix completely (top to bottom)

  • Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.


Slide54 l.jpg

Meromictic: lakes are chemically stratified

Mixolimnion

Thermocline

Chemocline

Monimolimnion


Slide55 l.jpg

Meromictic: lakes are chemically stratified

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)


Slide56 l.jpg

Can get interesting thermal profiles

Warmer water below colder water above 4ºC

Recall salinity increases density


Slide57 l.jpg

Mixing Patterns

  • Amictic—never mix because lake is frozen. Mostly in Antarctica. Some in very high mountains.

  • Holomictic—lakes mix completely (top to bottom)

  • Monomictic lakes: Cold / Warm

  • Dimictic lakes:

  • Polymictic lakes: Cold / Warm

  • 3. Meromictic—Never fully mix due to an accumulation of salts in the deepest waters.


Slide58 l.jpg

Geographic Distribution


Slide59 l.jpg

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.

What is meant by “shallow” and “deep enough” is determined by the fetch and depth


Slide60 l.jpg

A lake with a maximum depth of 4m can stratify if it is in a protected basin

Bullhead Pond

Surface Area = 0.02 km2

Maximum fetch < 300 m


Slide61 l.jpg

A lake with a maximum depth of 12m can be unstratified if the fetch is long enough

Oneida Lake, NY

Surface Area = 207 km2

Maximum fetch = 33 km


  • Login