262 THE ESSEX NATURALIST
An exactly similar equation governs the rate of re-oxygenation of the
depleted water.
When these two equations, rate of oxygen consumed, to rate of oxygen
redissolving, are plotted as a single graph, a simple curve is obtained
called the "Oxygen Sag-Curve".
Fig. 9. Diagrammatic representation of an Oxygen Sag-Curve.
From A-B, the rate of de oxygenation is greater than re-
oxygenation between B-C; these rates are more or less equal, and
between C-D re-oxygenation exceeds deoxygenation.
D-E, on the intolerable oxygen depletion curve would corres-
pond to the re-establishment of plant and animal life.
It is obvious that the lower the Sag-Curve the less dissolved
oxygen there will be in the water and hence the recovery rate
from the pollution will take longer.
The natural recovery rate of a river from pollution by the
process of reoxygenation is termed "Self-Purification".
I will not consider further the processes by which deoxygena-
tion or reoxygenation occur, but would point out that riffles and
aquatic plants are sources of natural reoxygenation.
As a matter of interest the areas marked X,Y and Z correspond
roughly to the tubificid, chironomid and asellid zones of river
recovery shown in Fig. 7.
APPENDIX II
List of Organisms normally found in the Riffle Habitat
Phylum Annelida
Order Hirudinea
Piscicola geometra (r)