Action de la pollution organique sur la faune aquatique d'eau douce
Directeur de Recherches honoraire à l'Institut National de la Recherche Agronomique (I.N.R.A.)
River pollution in France is principally due to organic matter from built-up areas, cattle-rearing farms and agricultural factories. Other sources include industrial wastes such as acids, bases, hydrocarbons, detergents and, above all, pesticides. After briefly summarizing the sensitivity of invertebrates to certain substances such as acids and alkalis (salts being less toxic) and the very low toxicity threshold (considerably less than 1 ppm) of organic chloride pesticides (e.g. DDT, hexachlorocyclohexane), the author describes organic matter pollution and the resulting self-purification in which all but a few of the organisms present will die. During polluted stream. Self-purification by anaerobic and aerobic fermentation is essentially a biological phenomenon brought about by organic-matter-consuming organisms such as fungi, algae, protozoa, rotifera. gastrotrichaw, oligochaete worms and diptera. The wiry bacteria Sphaerotibus plays an important part in self-purification. Little or no oxygen, abnormally high carbon-dioxide or toxicsubstance contents and water turbidity will give rise to conditions in which all but a few of the organisms present will die. During self-purification, the properties of the environment-more particularly the oxygen content-will gradually improve. In the highly polluted zone, there is a decreease in the variabilily index of the population i.e. the number of species diminish (see fig. 1 and 2) and the number of individuals within a species increase. If there is too great an excess of phenolic of ammonium compounds, the fauma may disappear entirely. As self-purification proceeds and dissolved oxygen goes up, the number of species increase. However, the rate at which this occurs will vary with season, temperature and maximum and minimum stream discharge. Thus, the highly polluted zone, the zone below it where regeneration is taking place and the species which are present in both, will depend essentially on local overall conditions (see fig. 1). Different zoological groups behave differently as pollution indicators. For instance, certain species or, rather, biocoenoses can only thrive in clean water e.g. a population consisting of Heptageniida larvae and nymphs (ephemeroptera), Perlidae (plecoptera) and fixed-shealh trichoptera. However, care should be exercised before conclusions are reached since resistance varies considerably from species to species in the larvae of the three above groups. This is the case for trichoptera (Hydropsychidae) aquatic coleoptera (fig. 2) and the lower crustacea (Gammarus). Diptera larvae are mainly to be found in polluted waters though there are exceptions e.g. Diamesa novorienda, Cricolopus absurdus, Liponeura sp. Molluscs live in moderately polluted water, although Bylhinella auslriaca is found only in highly oxygen-rich water. Several loval ecological factors may change the environment and, consequently, the indicator value of a species or an association of species. Such factors are erosion, flow variation, soil type, position of tributaries, predatory action etc. After having criticized Kolkwitz and Marson's Saprobic system, which nonetheless retains some general value, the author presents a practical method used in France for diagnosing pollution. The method consists in estimating a conventional "biotic index", running from 1 to 10, from the forms encountered (called Systematic Units) and from the predominant zoological group (called the Reference Fauna Group) (see fig. 3 and 4).
© Société Hydrotechnique de France, 1972