Light + Temperature

Define ‘Photosynthetically Active Radiation’ (PAR):

Light between 400-700nm in wavelength, most important for photosynthesis.

How does the proportion of PAR light, in comparison to the whole EMR spectrum, change as it travels through the atmosphere?

At the edge of the atmosphere, it makes up 30% of incident light, but this increases to 50% by the time it reaches Earth’s surface as other wavelengths are filtered out to a greater degree.

How does depth affect light intensity within a water body?

The intensity of light decreases exponentially with depth.

How does chlorophyll content affect light attenuation in water?

Some chlorophylls absorb in the blue + red region, making lakes appear greener.

How does silt content affect light attenuation in water?

Waters high in silt and /or decaying matter absorb shorter wavelengths; less red/orange light 

Define photosynthetic photon flux density:

The number of photons per unit area per unit time, measured in μmol m^-2s^-1

Define euphotic depth:

When photon flux density is at 10μmol m^-2s^-1, net photosynthesis within an algal cell is zero - the depth at which this occurs is the euphotic depth.

For phytoplankton, where does the euphotic depth tend to occur?

Where % irradiance is 1% of surface irradiance.

At what temperature is water densest?

4 degrees Celsius

Define an isothermal lake:

Where the water within is at a similar temperature across all depths.

Describe how dimictic lakes change over a year:

• In the winter, less dense cold water overlies warmer water

• In the summer, less dense warm water overlies colder water

• In between, wind mixing + low density differences prevent stratification

Where do dimictic lakes commonly occur?

Northern temperate regions, or areas where it is icy in the winter.

Where do monomictic lakes commonly occur?

Areas with mild winters/summers e.g. the UK 

Define amicitic lakes:

At very high latitudes/altitudes, some lakes are permanently covered in ice and never mix

What sort of lakes are commonly polymictic?

Shallow lakes

Define meromictic lakes:

These are always stratified and can form an anoxic bottom layer.