Ch. 4 - Light in Lakes

Light as an entity

  • The sun = ultimate source of energy

    • 99.9% of species are driven by the sun for use of photosynthesis

  • Until recently, we believed all ecosystems on Earth were based totally on the sun. We now know of 2 ecosystems not dependent on the Sun:

    • 1) deep ocean vents

      • in Mid-Atlantic Ridge

      • the are dependent on chemistry that drives up

      • they are chemosynthetic: they get their energy from breaking bonds

      • they do this process to substitute for sunlight

      • ex: Biota of the Mid-Ocean Ridges

    • 2) some cave ecosystems

  • Phytoplankton are mostly dependent on light which is why they live in open waters

  • Littoral zones: where rooted aquatic macrophytes can grow

    • aquatic macrophytes are higher plants that have gone back to water

    • Light depends on littoral zones (the further light goes, the further the littoral zone will reach) because it is based upon where the aquatic macrophytes can grow

  • Light is needed by animals so that they are able to see

Electromagnetic Spectrum in Limnology

  • Light quality: changing as you go deeper because of wavelengths

  • Light quantity: declines as you go deeper

  • Limnology only uses parts of the electromagnetic spectrum: UV, Visible, and Infrared zones

    • UV: for ozone depreciation

    • Visible: where photosynthesis occurs

    • Infrared: heating a lake (lake energy)

  • Different colours of light have different wavelengths (\lambda = Greek letter lambda)

    • E = hc/\lambda

    • E = energy

    • h = Plank’s constant

    • c = velocity of light

    • \lambda = wavelength

    • therefore, a high wavelength (\lambda) means a lower E, and therefore penetrates less into water

    • wavelength and energy are inversely proportional to eachother

      • if wavelength is long, energy will be lower, and will be absorbed first

      • if wavelength is short, energy will be higher, and will be absorbed last

    • So, in the electromagnetic spectrum, red will be absorbed first because it has a long wavelength, and blue will be absorbed last because it has a short wavelength

  • Transmission of light with depth is based on the amount and type of light

    • algae are different colours because of what light they have adapted to

      • algae at surface level of water will be most likely red

      • algae at a deeper sea level will be most likely green

  • These graphs are for totally clear/distilled water.

  • If lake water is coloured or not clear, these graphs above no longer hold

    • Lakes can be different colours due to different chemistry or environmental factors taking place

      • Ex. clay, salt

  • Clear water diagram
Eutrophic lakeHigh DOC (coloured lake)

Light Diagrams

  • The Direct Solar Radiation is light that directly makes it to lake surface

  • The Indirect Solar Radiation is light that is scattered or reflected before it reaches the lake surface.

  • Once the light in lakes is absorbed it can be processed into photosynthesis by aquatic macrophytes

  • If the light angle is not right than it will be reflected back and will not contribute to the energy available for photosynthesis

Snow and Ice

  • Ice can control light penetration, the main factors are the type, amount and snow

  • Black ice: freezes solid, removes some of the light being absorbed by the water

  • White ice: ice that melts a bit, then slushes, then freezes, it removes a lot of the light being absorbed by the water

  • Snow: does not let much light through into the water

Heating of Lakes

  • The greater the absorption and scattering, the greater the heating

    • clear lakes are not heated as much as those that are not clear when absorbing the sun’s energy to heat the lake

Physical Laws Affecting Light

  • Lambert’s Law: The absorption of light is exponential with the length of the light path

    • a.k.a. the deeper you go the less light you see

  • Beer’s Law: The absorption of light by any substance is proportional to the concentration of that substance

    • a.k.a. The more stuff you have in a lake the less light is being passed through (like a cloudy glass)

  • Ex. Lakes with high suspended solids (e.g. clays)

    • by Beer’s Law: the more lake you have the less light that will pass through

  • Ex. Lakes with high Dissolved Organic Carbon (DOC)

    • why is DOC critical? The more DOC you have the less light will pass through

  • Algal blooms can greatly affect light penetration and quantity

    • some algae have evolved to come to surface to get the sunlight

Techniques to Measure Light

  • Secchi disk: measuring how much light can pass through (lowering the disk until you can’t see the disk)

    • Secchi disk depth = corresponds to depth of approx. 10% of surface light

    • Photic zone = ~1.5 - 2X Secchi Depth

      • The photic zone is where enough light is needed for algae to photosynthesize

  • Underwater photometer: tells quality of light

  • Underwater spectrophotometer: tells quantity of light

  • Turbidity meter

Water Colour

  • Water colour dramatically affects the light quality and quantity

  • Apparent colour: includes suspended material (i.e. is not filtered)

  • True colour: filtered first, so only includes dissolved material

  • Water colour and transparency (not in distilled water) will be affected by:

    • biogenic factors (e.g. algae)

    • suspended solids (e.g. clays)

    • dissolved organic matter (e.g. DOC)

Summary

  • Reflection, scattering and absorbance all:

    • influence the quantity and quality of light

    • determine the temperature of water and stratification patterns