he classic paper by Talling & Talling
(1965), source of the table below,
reviews the chemical
composition of African lake waters, compiling both previously published
and many original observations. In addition to providing a detailed table
with reference values for some 67 lakes, the Tallings classify
the lakes into three "conveniently, though arbitrarily, distinguished"
classes according to their total ionic concentrations and electrical
conductivity.
Conductivity, measured in
microsiemens (µS), or millionths of a
siemens [formerly called micromhos (µmho)],
is a sensitive index of total ionic
concentration. Crudely and somewhat imprecisely stated,
the higher the conductivity, the more total
ions —
the "harder"
the water.
Class I
lakes have low total concentrations of ions
[alkalinity
usually less than 6
milliequivalents per liter (meq/L); this term has
fallen out of use], and conductivity of
less than 600 µS (µmho). These lakes get their water
from direct surface runoff or rivers with little salt. Class I includes Lakes
Malawi, Malombe, Victoria, Tana, and George, among others.
Class II
lakes have higher total ion concentrations (alkalinity of about
6 to 60 meq/L), and conductivity of between 600 and 6000
µS. The higher ion concentrations arise from
"accumulation and evaporation in closed basins or by inflows rich in
solutes
and particularly sodium carbonate and bicarbonate" (p. 435),
especially drainage from alkaline lavas. Lakes Tanganyika, Kivu,
Edward, Albert, Turkana (Rudolf), Rukwa, and others are in this class.
Class III
includes the saline lakes, with alkalinity usually greater than
60 meq/L and conductivity of 6000-160,000
µS. These lakes, which often contain solid
mineral deposits such as
trona,
include Nakuru, Magadi, Natron, Manyara,
Eyasi, Katwe, and others.
The table below is extracted from Talling & Talling (1965). It gives
all the ranges of values tabulated for surface waters of Lake Malawi, with
those of Lakes Victoria and Tanganyika for comparison.
(Not repeated here are readings from 300 and 600 meters depth in
L. Malawi, far below the level where there is enough dissolved
oxygen for fishes to occur. Also omitted here are deep readings from
Victoria and Tanganyika. One value that the Tallings cited, but
considered doubtful, is included in brackets.) Notice that the values of
most constituents in L. Malawi water are intermediate between those of
the other lakes, and generally closer to those of L. Victoria.
(Note: Aquarists who want to know how to add salts to their water
to make it more like that of Lake Malawi may be interested in
George Reclos's article at Malawi Cichlid Homepage,
Make your Own Salt Mixtures.
A nice discussion of the chemical information below is contained in an
article by Carli Flenniken,
also at Malawi Cichlid Homepage.)
Water Chemistry of the African Great Lakes
*No pH values for L. Tanganyika were available to Talling & Talling (1965).
The values tabulated here were very kindly provided by Eric Coenen of
Lake Tanganyika Fisheries
Research, extracted from Technical Document TD/46 written by
Plisnier et al. (1996) of LTR
(Research for the Management of the Fisheries of Lake Tanganyika).
These pH values for L. Tanganyika were measured
on pelagic waters (near-surface and at various depths) every 1-2 weeks
for 1 year using the same pH meter and standardized sampling methods.
In more detail, the measurements at the surface and 100 m
are as follows:
Sampling Station
