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close this bookBiological Monitoring: Signals from the Environment (GTZ, 1991)
close this folderBioindicators and biomonitors in aquatic ecosystems with special attention to potential applications in developing countries
View the document(introduction...)
View the document0. Abstract
View the document1. The context
Open this folder and view contents2. Means of detecting water pollution
Open this folder and view contents3. Biological assessment
View the document4. Water evaluation by remote sensing
View the document5. Summary: Use of bioindicators and biomonitors in developing countries
View the documentReferences

4. Water evaluation by remote sensing

Certain biological phenomena that are directly or indirectly due to water pollution, and changes in them, can be studied in their entirety without detailed analysis of species and biocoenoses. As is illustrated by documentation of the continuing expansion of the Sahara by means of aerial and satellite photography, this approach can also be used for observation of water contamination in larger systems. Surveillance of marine waters has been carried out in Europe and North America to detect contamination with used oil let off by ships (236). The spread and movement of oil slicks caused by tanker accidents are also observed from the air and documented. "Less harmful" contamination of waters with readily decomposable organic substances or plant nutrients can also be observed and photographed from the air. Analysis of the photographic evidence can then reveal much about the intensity of contamination and, if the observations are repeated, any changes undergone by it. Such analytical methods are suitable for both marine and large inland waters. The best spectral range for detecting differences in vegetation is in the near infrared (700-850 nary). Difficulties are encountered by this method, whether observations are made from the air space, when there is a high atmospheric moisture content, since the mentioned wavelengths are absorbed by water vapor, thus disguising differences between vegetation types and densities. This phenomenon increases in intensity with distance.

The multispectral scanning (MSS) method can be used for surveying both vegetation and water resources. From Landsat satellites, which were originally designated as Earth Resources Technology Satellites (ERTS), infrared photographs can be made in which each raster dot corresponds to an area of approx. 60 x 80 m. Photographs made in various different infrared bands are stored on tape, and can later be separately played back in 64 steps or bands between total reflection and total absorption. One channel is then used to, for example, clearly distinguish a lake from its surroundings and to depict its outline. The other channels can be used for classification of water quality, depending on color composition, ranging from oligotrophic (= low plant nutrient levels and phytoplankton population) to eutrophic (= high nutrient levels and dense phytoplankton population). An example is shown in Figure 17 in (237). Photographic techniques of this kind can also be used for identifying the sources of water pollution (238, 241).

For studying and depicting the morphology of water bodies that are not excessively deep, e.g. estuaries and coastal waters, radar can be used. This also allows water levels with varying degrees of turbidity to be distinguished. Due to its high contrast, this method is also well-suited for surveying catchment areas and mapping the morphology of river basin, borders between saltwater and fresh-water areas in coastal zones, former river tributaries, etc. (239, 241).