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THE ESSEX NATURALIST
Fig. 1. Simplified food web in a stream or pond
Unlike a lake, a river is more subject to weather variation and
any one mass of water may travel many miles from source to
estuary, becoming modified all the time, depending largely upon
the type of country through which it flows. Indeed because of the
continuous loss of nutrients due to the river current, the organisms
depend mainly upon that supplied by drainage from the surround-
ing land. Since there is such a large variation in the physico-
chemical system in any one part of a river, it follows that there
will also be a very large variation in the type of organism found
in the river at any one place. It is apparent that the study of
only one point on the river is not representative of the whole river
and that no one system can be defined absolutely. Any change in
a particular habitat must be found by comparison.
All biological systems are basically cyclic in nature, whether
they be sexual, biochemical or nutritional. We speak of the carbon
cycle or the nitrogen and oxygen cycles and these must in some
way be directly related to the biological cycles of living organisms.
They must also be inter-related one cycle with another. The most
probable linkage is through the protein of the organism since this
is a source of all three elements above mentioned. Two other
cycles of importance those of sulphur and phosphorus, are often
found related to the protein molecule in one form or another.
The element Hydrogen is, like Carbon, Nitrogen, Oxygen and
Phosphorous a primary factor in biochemical relationships. Of
course, many other elements take an active part in the biological-