A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 249
upstream of the riffle. This would occur in the event of organic
pollution, in the form of sewage effluent for example. Therefore
it would be expected that a change in the dissolved oxygen con-
centration of a riffle would in some way affect at least a part of
the biological community.
The inhabitants, however, of a slower flowing silted reach
would be more of a crawling, burrowing, or tube-building type of
organism, concerned mainly with the breakdown of organic matter
and naturally adapted to low oxygen concentrations and silted
conditions. These organisms would not be affected by even large
changes in dissolved oxygen concentrations and so in this respect
their study, with a view to the detection of organic pollution or
indeed chemical polluting agencies, would not be a wise path to
pursue.
Sensitivity to Organic Loading
Not only are riffles sensitive to change in the dissolved oxygen
concentration, but they are also very sensitive to changes in the
organic loading of the water. This is a term used to describe the
amount of suspended matter, in the form of organic solids, in the
river water. If the polluting discharge is heavy and consistent
for even a short period of time, silting results downstream of the
discharge over riffles and on aquatic plant life.
The reason for this sensitivity is that most inhabitants of a
riffle community rely on the river water carrying their food to
them, in the form of organisms, organic detritus and mineral
salts. All three provisions, however, are being constantly lost in
the water flow and so their presence is but transitory.
Any increase therefore, in the material, other than from
natural sources, will be quickly reflected in a change of the riffle
community. This may be a change in the relative numbers of a
genus or the establishment of other transitory genera. A silted
reach would not show such a marked response to an increase in
organic loading; it is a less dynamic system than a riffle.
Ecological Similarity
If rivers are to be compared biologically, then the environment
must be similar ecologically for the comparison between a natural
and polluted state to be valid. The riffle is such a system, which
is not only easily defined physically, but the organisms which exist
under these defined conditions vary little from one river to another
under natural conditions. These organisms are highly specialized
to high rates of river flow. The reason for this investigation is to
study the method by which these biological communities become
readapted to unnatural conditions resulting from pollution.
A PHYSICAL AND CHEMICAL ASSESSMENT OF THE RIFFLE
HABITAT
The most important physical factors are the rate and volume
of flow. In order to determine these, the approximate dimensions