250 THE ESSEX NATURALIST
of the riffle must be measured, its average depth and the rate of
flow. The latter is determined by timing the passage of a float
along the riffle between its upper and lower limits. The average
stone size of the riffle bed, pH and temperature are measured by
normal methods. The estimation of both chemical and bio-
chemical oxygen demand is of paramount importance in assessing
the state of pollution. Indeed the value of the Biochemical
Oxygen Demand (B.O.D.) is used to define whether or not a river
is polluted. A further discussion of the B.O.D. will be found in
Appendix I.
While the B.O.D. is an organic oxygen demand, two other tests
worth mention are used. One, the Dichromate value, estimates
the chemical oxygen demand (C.O.D.), and the other the Dissolved
Oxygen (D.O.) test, which determines the oxygen concentration
in the water at any specific time. The B.O.D. and C.O.D. are
expressed in parts per million (mg/litre) and the D.O. as percent-
age saturation. Other important tests are the determination of
suspended matter, which gives some indication of the organic
loading of the river and the amount of ammoniacal nitrogen and
oxidised nitrogen as nitrate. The latter indicates the efficacy of
protein degradation, especially if the polluting source is organic.
The determination of the calcium, magnesium, potassium, phos-
phate and nitrate concentrations, which affect the organisms
present do so indirectly, by influencing the growth of the plant
community which provides them with food and shelter. All of
these tests describe some part of the riffle complex and help to
define the environment. An example of a chemical analysis of a
riffle habitat follows (the results are expressed as parts per million
or milligrams per litre (p.p.m.) unless otherwise stated).
(S.S.) Suspended solids = 9
(C.O.D.) Chemical Oxygen demand=4.7
(B.O.D.) Biological or Biochemical Oxygen demand = 30
(D.O.) Dissolved Oxygen
Above riffle habitat = 98% Saturation.
Below riffle habitat=104% Saturation.
Temperature °C pH
Above riffle habitat=15.5 Above 81
Below riffle habitat=15.0 Below 81
Chloride ion concentration (Cl) = 48
+ + + +
Total hardness as calcium plus magnesium (Ca) + (Mg) = 382 p.p.m.
expressed as CaCO3
Potassium (K) = 68
Phosphate (PO,) = nil.
Nitrate as (nitrogen) = 1.6
Ammoniacal Nitrogen = 10
By chemical standards this river water could be designated as
"clean" due to the low B.O.D. The D.O. is good and shows but
little change over the riffle, while the temperature shows a drop