A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 259
others. The biological response of macro-aquatic organisms is
generally the same and may be conveniently summarised
diagrammatically in Fig. 7, in which the curves are adapted from
Bartsch (1948), although it is not known what sampling techniques
or places sampled were used in their production. This shews
three main biological zones which represent respectively areas of
gross, moderate and light organic pollution. The curves shew
that if the pollution is merely moderate then immediately below
the source of polluting discharge the tubificid zone would be
absent and the first zone would be that of the chironomids.
It is at once evident that the zone of recovery studied in this
paper is that represented by the chironomids with the degree of
overlap of distribution curves varying with the type of organic
pollution, its intensity and the station selected for sampling, etc.
It would seem that similar curves could be plotted for Erpobdella
and Nemacheilus.
Fig. 8. Possible food-web in the riffle habitat studied