A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 251
of only 0.5° C. The water may be regarded as "hard". For
comparison "moderate" hardness is 110-150 p.p.m. CaC03 while
"soft" waters are up to 90 p.p.m. CaCO3.
The potassium content is about average for Essex rivers. What
is somewhat surprising is that the river water at the sampling
station contained no phosphate, despite the sewage disposal works
discharges upstream. One can only conclude that this was either
removed by aquatic plants or the phosphate contribution from the
sewage works was negligible on the day of sampling. It is known
that the phosphate content of sewage effluent can vary consider-
ably from day to day.
The chloride ion concentration is below average for Essex
rivers, but large fluctuations are possible due to sewage disposal
works discharges. This ion normally remains unchanged on entry
to river water, its concentration decreasing by dilution rather
than by plant and animal utilization or physical absorption or
adsorption on organic or inorganic surfaces. From the above
chemical results it is concluded that there is little chemical
evidence of organic pollution.
The B.O.D. is average for clean river water. The oxygen
demand is low and this shows the river to have recovered satis-
factorily from previous organic pollution. The D.O. is high and
thus the available oxygen for aquatic animals is adequate.
While the temperature is rather high, 15°C (60°F) it would
have to rise to 25 C (78°F) or above to affect the aquatic animals
of the habitat to any observable extent. It is uncommon for
Essex rivers under dry weather flow conditions to reach or even
exceed 20°C in Summer. Some cooling of the river water over the
riffle is evident, but is not significant. Biologically the calcium
concentration of the river water at this sampling station is very
high. According to the snail criterion of hardness, water becomes
soft at or below 20 p.p.m. which shows that the calcium concentra-
tion of the habitat contains sufficient of this ion to meet the needs,
not only of the snail population, but also of all other organisms
present.
The pH of 8.1 is the value normally to be expected of Essex
rivers except in cases of gross organic and trade effluent pollution,
when it can drop to pH 5 or 6, but rarely below this value, since
river water is a very well buffered system. A pH of 5 is not
unfavourable to the aquatic life of a riffle.
In conclusion, the chemico-biological interpretation of the
analyses, suggests that the river is clean and unpolluted.
BIOLOGICAL ASSESSMENT OF THE RIFFLE HABITAT
The methods adopted for the estimation of the larger aquatic
vertebrates and invertebrates are already well known to biologists.
I have tried to standardise these methods, for while the principle
of collection may be the same, many variations are possible. This