Lakes and Watercourses

Light conditions
Introduction
Light conditions are crucial for the survival of many organisms. Water
quality in this respect is assessed on the basis of absorbency readings
taken from filtered water at a wavelength of 420 nm in a photometer or
equivalent readings taken using a colour comparator, using brownishyellow
platinum chloride as a reference. A high water table, eg, in bogs
and marshes, results in run-off with a high humus content and hence a
higher colour figure. Various chemical, photochemical and biological
processes cause a certain amount of discoloration. This means that lakes
with a long retention time are less discoloured than those with rapid
turnover. From some points of view, a high concentration of humic
matter is advantageous, since it provides scope for complexing, which
reduces the toxicity of metals.
The turbidity of the water is assessed by analysing light dispersion
measured according to the FNU scale. These readings quantify the
particulate content of the water in the form of clayey matter as well as
organic matter such as humus floccules, plankton etc.
Turbidity is determined electronically using a turbidimeter in
accordance with the Swedish standard. Particles in the water scatter light
and, after calibration, the intensity of this light is used as a measure of
turbidity. One complication is that readings are affected by the concentration
as well as the nature of the particles. The turbidity of natural
running water is mainly caused by inorganic particles. The main source
of material like this, which causes turbidity, is probably erosion.
Inorganic matter has a high density and therefore sediments fairly
quickly. Lakes thus serve as clarification basins, where the predominant
cause of turbidity is usually organic matter.
Measuring the Secchi depth of lakes gives an indication of the optical
characteristics of the water. Secchi depth readings are taken using a
Secchi dish in situ, which indicates the overall effects of water colour and
turbidity on light penetration. The Secchi depth thus gives a direct,
simple measure of the optical characteristics of the water. It is generally
considered that the Secchi depth represents the depth reached by
approximately 10 per cent of natural light. A Secchi depth figure can be
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