L7 - Lake optics and classification Flashcards
Optically active materials
Light in water is absorbed and scattered, wavelength specific.
Optically active materials include
- Water itself - absorbs red and infra-red light
- CDOM - coloured dissolved organic material (absorbs blue)
- Algae - absorbs blue and red, scatters a little green
- suspends sediments - scatters a lot and absorbs if coated with organics
Attenuation is a consequence of the combination of scattering and absoption
Light on algal cell is scattered away at a similar colour but the light absorbed is green because phytoplankton is green.
Colour of light that comes out the algal cell is the light which has not been absorbed, it’s green because the blue and the red has been absorbed.
The colour we see is the colour that is leaving the water collumn.
“Backscatter or reflected light”
The reason the lakes all look different colours was because the processes occuring are all different.
Light enters lake and hits a scattering particle, it will then hit a algal cell and another particle and it changes colour through the whole process.
Different proportions of optically active materials.
PAR - Photosynthetically available radiation
The waveband plants use for photosynthesis, we use this band as it tells us how much is there for photosynthesis. Primarily we measure light in the water because it tells us how much energy is there for primary productivity.
Both more related to scattering as apposed to attenuation.
We look at the edge of the disk and as it gets fuzzier it gets harder to see because of light scatter.
Kd and Secchi depth measure different things.
Inherintly sensible thing do to.
Big resevoir of data on this as its easy and people have been doing it for so long. Results show visability has declined since 1965.
Classification - an aid to understanding
Classification is a way to group together clusters of things that have broadly similar propeties so can be expected to behave in similar ways.
Lakes have traditionally been classified according to:
- Method of formation
- Mixing regime
- Nutritional status
- Southern Alps only
- approx 2 hectres to be classified as a lake
- Steep glacial catchments
- All over the continent
- often small and shallow
- traditionally eligotrophic
- vulnerable to lake shore development
- areas with often high biodiversity values
- affected by forestry
(lakes dependant on ground water which is being sucked up by pine etc)
- Often shallow
- Complex shoreline
- Gently sloping shoreline
- Landslide more ‘dam’ like
ICOLLS
(intermitantly closed & open lagoon or lake)
- Shallow
- Fertile (eutrophic)
- Young
Volcanic Lakes
- Often steep
- Round
- Small catchment
- Very old
Artificial Lakes.
Mainly reservoirs for portable water and hydroelectricity
Hydro lakes
Short residence time
Helps us to understand what to expect from a lake system.
