A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 261
experiments. River water is so complex chemically and biochemi-
cally that much has yet to be learned about the possible synergism
existing in such water when toxic polluting agents are present.
This means studying the effect of these agents on a biological com-
munity in situ, since the simulation of river conditions in the
laboratory is almost impossible. Once the upper and lower toler-
ance levels to polluting agents have been established for various
organisms in a given habitat, then a system of indicator organisms
can be introduced for various pollutants. The ultimate aim is
to diagnose a particular chemical pollution in terms of a specific
biological reaction.
Effect of Organic Pollutants
From the results it is seen that for gross organic pollution the
biological state of the river is easily defined. Indeed it would
be an easy matter to detect such pollution visually without resort
to either biological or chemical means. The detection of light
organic pollution is not so simple. Nevertheless the biological
response is more sensitive than chemical analytical means at
present available to us, since the chemical state of the river returns
more quickly to normal than does the biological state.
In conclusion I would suggest that the practical application of
biological survey work appears to be of threefold importance and
significance. First in detection and estimation of the severity of
past chemical or organic pollution. Secondly, in the determina-
tion by biological response to the presence of light pollution, which
by chemical analyses would not indicate the full effect of this
pollution on the stream life. Thirdly for the determination of the
rate of recovery from pollution by self purification, with severity
of pollution, time and distance from the discharge point, all con-
sidered.
APPENDIX I
A short consideration of the Oxygen demand of a river
The oxygen demand of a river is governed by two laws; one is the
rate of consumption of oxygen from the water and the other is the rate
of re-oxygenation.
The rate of consumption of dissolved oxygen by organic matter is
directly proportional to the weight of organic matter remaining at any
given moment; this may be represented as follows: —
d 1
— = KL
d t
This equation may be expressed in the form;— It = Le K't
It = Biochemical oxygen demand at time t
K = a constant
L = Original Oxygen Demand
K = A new constant