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Physical factors and some river weeds. 1971. Proceedings of the European Weed Research Council 3rd International Symposium on Aquatic Weeds, 29—38.



INTRODUCTION



The distribution of macrophytes in British watercourses is affected by many physical, chemical and biotic factors (including management), any of which may be of overriding importance. The presence of a waterweed shows that it can at least tolerate the prevailing levels of all factors. Its absence, in contrast, can be due to varied causes, such as: flow too fast, water too turbid or too polluted, substrate too coarse, bedrock not limestone, river recently dredged, toxic contents of a tanker recently added to the water. Hence the presence of a waterweed is more ecologically illuminating than is its absence.