Experiment: Determine the 1% depth
Problem: You want to determine at what depth there is still enough light for primary production.
Therefore you are diving with an irradiance meter, which measures the downwelling irradiance at wavelengths in the visible light (400-700 nm).
Because you have no oxygen tank, you can operate the instrument to a depth of only 2 m.
However, you want to determine the depth with 1% of the surface irradiance (≅ depth of euphotic zone).
Just below the surface you measure 100% irradiance (W m-2), at 1 m depth only 53% and at 2 m depth only 30% is left.
Question: Sketch the irradiance profile on the linear scale (left), then on the log scale (right).
Answer ↓ ↑
Irradiance profile on linear (left) and log scale (right)
relative downwelling irradiance (%) 100, 53, 30, 1 at depth (meters) 0, -1, -2, -7.5 (we measure as minus from surface)
Question: How deep is the euphotic zone?
Answer ↓ ↑
Answer: 7.50 m
Can you detect what is wrong in the right graph? At what depth in the water do you expect to measure only 0.1 % of the surface irradiance?
Lambert-Beer's Law
You just derived Lambert-Beer's law, an empirical relationship
that relates the absorption of light to the properties of the material through which the light is travelling
and presumes that attenuation of sunlight in water is an exponential function of water depth.
Lambert-Beer's Law ↓ ↑
The Lambert-Beer Law describes the exponential decrease in irradiance with depth,
as light is absorbed and scattered in the water column and is expressed in the following equation:
where E(z) is the irradiance at a given depth;
E(0) is irradiance at the surface;
KD is the attenuation coefficient for downward irradiance;
and z is the depth in (negative) metres.
European Directives ↓ ↑
Because they provide consistent agreed indicators across national boundaries,
remotely sensed products may be critical for monitoring performance under international agreements and conventions.
For example, the European Union has established environmental directives which require monitoring of coastal water quality indicators,
including transparency, colour and suspended solids. Ocean-colour sensors are recognized as offering a standard,
cost-effective way to monitor compliance.
| Directive |
Subject of concern |
Variables to be monitored |
| 76/160/EEC |
Quality of bathing water |
Coliforms, thermo-tolerant colibacteria,transparency, pH, faecal Streptococcus |
| 79/923/EEC |
Quality required of shellfish waters |
Salinity, oxygen, pH, temperature, colour, suspended material, hydrocarbons, coliforms,... |
| 91/271/EEC |
Urban waste treatment |
Biological Oxygen Demand, suspended solids, total phosphorus, total nitrogen |
Source: IOCCG
Light and life: You now learnt about the role of light in water.
The concepts behind transparency and attenuation have been presented together with satellite-based maps of these parameters.
From these maps a link was made to safety (for the Bathing Water Directive an underwater minimum visibility is required).
Another link was made to growth in particular the available light for algae and water plants to grow.
The major sources for attenuation (suspended sediment in the near coastal zone and algae in the clearer waters) are presented.
