A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 257
food and "living accommodation" Possibly also because it does
not live under stones like Gammarus and would be swept away in
a riffle. Both crustaceans may be regarded as scavengers, occupy-
ing similar niches in a riffle but only Asellus is capable of tolerating
a very wide fluctuation in temperature and dissolved oxygen.
Diptera were well represented by chironomid and simulid
larvae, the former being by far the most abundant. The chiron-
omids were similar in distribution to the gammarids. It is of
interest to note that the Stone Loach also shows this type of
distribution, as will be shewn in a later mention of the food-web
method of presentation. Chironomid and simulid larvae are not
characteristic of a riffle, as they are normally found in sluggish,
well silted reaches where they inhabit the muddy bottom, in both
polluted and unpolluted streams. Their occurrence in an un-
polluted riffle may be regarded as rare. The presence of a very
large number of chironomids in the riffle was almost certainly
due to "overflow" from the silted reach upstream and the fact
that some niches were created by the actual silting of the riffle
bed with gross organic loading.
As would be expected, Hemiptera were not well represented
in the riffle as these are characteristic of slow-moving water.
Coleoptera, like the Hemiptera, were rare and can safely be
regarded as vagrants from the Sparganium habitat upstream.
Of the gastropods (snails) Ancylus fluviatilus predominated,
but Limnea pereger and Planorbis planorbis which were also
present should be regarded as adventitious.
A more comprehensive list of organisms found is given in
Appendix II.
CONCLUSIONS
The distribution of the more abundant organisms in the riffle
was surprisingly even, which may serve to emphasize the
uniformity of the physical factors of the habitat studied.
The very high population density of the erpobdellid leeches is
almost certainly related to the abundance of their food, i.e.,
chironomid and simulid larvae. In the unpolluted riffle Erpobdella
octoculata and the simulids occurred frequently, while the
chironomids were almost entirely absent. It is evident that the
presence of Erpobdella octoculuta in very large numbers is indica-
tive of pollution, but it cannot of itself be regarded as a primary
indicator of pollution since its distribution is so wide and varied.
Mann (1954) says of it: —"This leech occurs in all kinds of habitat,
in hard and soft water, in running and standing water. In running
water it is almost always the most numerous leech present,
irrespective of the hardness and moderate organic pollution and is
associated with a particularly high population density".
The presence of a large number of Stone Loach (Nemacheilus
barbatula) may be explained in much the same way as Erpobdella
octoculata. It is there in large numbers because the food supply