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THE ESSEX NATURALIST
the kind of biological community which exists in any one part of a
river are very complex.
THE SELECTION OF A SAMPLING STATION
Much work has been done in trying to classify rivers into well
defined zones. On the Continent, river zonation is often defined
or characterized by certain fishes (Huet, 1954); this has also been
attempted in this country (Carpenter, 1928). A classification of
rivers based on characteristic plant zones has also been used
(Butcher, 1933).
The text book definition of a river states that it rises in the
mountains, flows rapidly to the foothills, then on into the plains
and finally to the sea. Such a classification, associated with its
own characteristic type of river bed is applicable to most rivers.
Sub-division of this outline classification into smaller defined
areas, brings one to the conclusion that however commendable
these methods of zonation are theoretically, their application is
far from simple and for the most part impossible to apply with
any degree of certainty to more than just a few rivers. Probably
each river must be treated as an individual, showing its own
characteristic variations from one place to another along its entire
length. These variations are rarely duplicated in other rivers of
equal size and distance from their source. Hynes (1960) con-
sidered that because of the extreme complexity of streams and
rivers the definition of each zone was unnecessary.
From what has been noted above, it is clearly unwise to
attempt a biological survey of a river, with a view to comparing it
with the same zone of another. I have therefore rejected a river-
zone system for defining my sampling station in favour of a much
smaller area of a river—a riffle. Riffles may be considered to be
non-silted areas, having a fairly rapid flow, above 1 ft./sec. The
surface of the water is usually broken and it normally has a depth
of not more than 1 ft. In this case stable riffle beds were chosen,
that is those in which movement of the gravel-bed is negligible, a
moving bed proving a most inhospitable environment to biological
communities.
A riffle is a more easily studied habitat than say a sluggish
silted reach, in that it is normally more accessible and more can
be seen of the habitat. Consequently the sampling methods used
can be better standardised and the physical factors more easily
measured.
Sensitivity to dissolved Oxygen Fluctuations
The inhabitants of riffles normally have a high oxygen demand
which is satisfied by the high degree of physical aeration which
occurs over a riffle.
The oxygen balance existing in riffles between organisms'
requirements and degree of physical aeration is known to be very
sensitive to fluctuations in the dissolved oxygen concentration