Suspended Particulate Matter (SPM)

This picture of Lake Tekapo on the South Island of New Zealand shows the brilliant blue of the water, indicating a lack of colored dissolved organic matter and chlorophyll a contamination. However, upon close inspection, the water is not transparent, as a result of high concentration of glacial silts. These glacial silts scatter the light but are not very absorbing leaving a bright blue, turbid lake.

SPM consist of soils and suspended bottom sediments, decaying plant material and plankton. They primarily absorb blue light, like CDOM, but additionally scatter light. Scattering is the more dominant optical result of SPM.

    There are three important attributes of suspended particulate matter that affect the extent of scatter and absorption. They are the:

  • Concentration
  • Composition
  • Grain size distribution

of the suspended particles. All of these factors affect the extent of light attenuation in different ways.

    A few typical absorption and scattering spectra for suspended particles are shown in the graph below. The graph shows three different types of suspended materials. They all scatter much more strongly than they absorb light. Some show linear scattering, while others can show a more exponential decrease across wavelengths. As stated earlier, they all seem to absorb blue light in the shorter wavelengths more strongly than in the longer wavelengths, however, overall absorption is minimal. Their main effect is on the clarity of the water, which is reduced by a high concentration of SPM.