Zonation

Abstract

Classification of a watercourse as an entity may be useful in a general descriptive survey, but the categories are of necessity wide and subjective. On the other hand, a zonation scheme delimits areas within the system and permits a second order categorization of faunal groups and a degree of objective comparison between rivers in different geographical regions. The third order refinement, involving definition of the biogenic relationships within the habitat mosaic of a zone, requires an intimate knowledge of the ecological characteristics of the biota which, even for the well-studied European region, is not yet fully available. The desirability of classifying rivers or sections of watercourses has been challenged by a number of authors (e.g. Badcock 1954, Armitage 1961, Thorup 1966) on the grounds that all divisions are arbitrary and not representative of any true discontinuity in the biotope. Others (Hynes 1960, 1970, Macan 1961a) have indicated that in their opinion just not enough is known of the fauna to enable differentiation of zonal biocoenoses in the complex and intergrading mosaic of microhabitats that constitute most river channels. Both points are valid, but it is also relevant to note that when discernible changes in faunal associations occur in structurally similar biotopes (as far as can be determined), there may be a case for designation of a zonal boundary.