A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 245
biological contents of the river which will determine its degree of
pollution. Pollution may be said to have occurred when the dis-
charge of a substance into a stream alters appreciably the nature
of the biological communities of the stream. A measurement of
the polluting agencies may help to explain their effect on this
biological community.
Aquatic habitats are by nature very complex. Not only does
there exist a dynamic equilibrium between the biological com-
ponents but there are also many physical and chemical factors to
be considered before a true picture can be formed of the habitat
itself.
It is virtually impossible to define the biological and chemical
states at any moment since it is a continually changing system.
Within certain limits the physical and chemical conditions can be
better defined than the biological. The biological condition is
defined by the physico-chemical nature of the environment.
Each organism in the system will occupy a niche as near as
possible representing conditions at an optimum for its survival.
Indeed, so critical is the optimum that two species of a single
genus may be found living in relative abundance a short distance
apart. The environment of any one species of a genus may be
defined as a micro-habitat, while that occupied by a number of
genera as a macro-habitat. This paper is concerned with the
general biological distribution of aquatic organisms over a given
part of a stream or river, not with micro-habitats.
How may the biological and the physico-chemical systems be
defined ? Consider first the biological system as a separate entity,
independent of the physico-chemical factors which are influencing
it. An organism feeds to live; this is fundamental to all living
creatures. Therefore, if the biological state could be expressed as
a food web a more realistic picture could be obtained of this state
at the time of sampling. There is little information on food webs
in rivers, though a general example including the microfauna and
flora may be : —
Bacteria ------> ciliate protozoa -------> Rotifers
-------> chironomid larvae leeches -------> fish
A simplified presentation of a food web in a stream would be
as in Fig. 1.
The more adaptable the organism, the more likely it is to survive
large or small environmental changes. It is the environmental
change which is defined by the physico-chemical system.
The existence of an organism therefore in any particular
habitat is entirely dependent upon : —
(a) Its tipper and lower tolerance levels for variations in the
numerous physical and chemical factors.
(b) Its feeding habits whether omnivorous, carnivorous or
herbivorous, and also its method of feeding, whether by filter,
sucking, biting or some other type.