The chemical composition of freshwater are varied under different conditions, but an average percentage composition (eg. Cole, 1983) can be shown.
Average percentage composition of fish water
| Cations | Anions | ||
| Ca++ | 60.9 | CO3= = | 72.4 |
| Mg++ | 19.0 | SO4= = | 16.1 |
| Na+ | 16.6 | CI- 11.5 | |
| K+ | 3.5 | ||
The chemical composition (percentage) of sea water is quite distinct from that of freshwater, as shown:
| Cations | Anions | ||
| Na+ | 83.5 | CI- | 87.2 |
| Mg++ | 10.3 | SO4= | 12.2 |
| Ca++ | 3.3 | CO3= | 0.3 |
| K+ | 3.0 | Br- | 0.3 |
The salinity of water can be defined as the concentration of all cations, significantly Na+, K+, Mg++ and Ca++ and of the anions CO3= and SO4= and halides, all HCO3- being converted to CO=3. The average composition of freshwater given above is that of average river water. In soft waters Ca++ and CO=3 might be much reduced. In acid waters sulfate may be dominant.
Chemical composition of water can be represented by ionic diagrams (Fig. 1). Thus the dominant ions can characterise the types of natural waters, eg.
Na+ and CI- in sea water
Ca++ and HCO3- in calcareous (calcium bicarbonate) water;
SO=4 and HCO3- in sulfato-carbonate water and
Na+ and Mg++ and HCO-3 in sodium - magnesium bicarbonate water (cf. soda lakes in Africa).
Further discussion on this and types of waters can be seen in Hutchinson (1957).
Fig. 1. Ionic Diagrams - Arrows indicate dominant ions (After Hutchinson, 1957)
Ionic composition of waters of verious freshwater bodies in Africa, along with information on conductivity and depth, for calculating an index of productivity (MEI - Morpho-adephic index, Ryder et al, 1974) are presented in Wellcome (1972) and Henderson and Wellcome (1974). This will be referred to again under “Biological Productivity” (Chap. 10).
