What Is the Chemical
Composition of Lake Malawi Water? How Does it Compare with
Lakes Victoria and Tanganyika?
he classic paper by Talling & Talling (1965), source of the data tabulated 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.
Class I lakes (in the terminology of the Tallings) 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 complete publication of Talling & Talling (1965) can now be downloaded from my Bibliography page. The extract shown here 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 on the table: The original values from the Tallings' paper, which have been posted on this page since 1997, are shown in lightface. Joe Gargas, former Director of Research and Development for the Wardley Corporation and an expert in water chemistry and treatment, corresponded with me in April 2009 and has generously calculated the updated values in boldface in the table. These represent conversions of the Tallings' data to, in some cases, more modern units of measurement now in widespread use and more familiar to aquarists. Many thanks, Joe!
Water Chemistry of the African Great Lakes
Water Chemistry of the African Great Lakes
| Chemical Constituent | L. Victoria | L. Malawi | L. Tanganyika | Electrical conductivity (µmho = microsiemens) | 91-145 | 210-220 | 606-620 | Total solids (mg/L) | 76-118 | — | 460 | Total solids (mg/L) average | 58.24-118.0 | 134.4-140.0 | 387.84-460.0 | Sum of cations (meq/L) | 1.04-1.21 | 2.45 | 7.30-7.46 | Sum of anions (meq/L) | 1.08-1.77 | 2.5-2.59 | 7.25-7.71 | Total hardness as calcium carbonate CaCO3 (mg/L) | 19.12-70.83 | 60.58-85.60 | 186.33-224.0 | Total calcium as calcium carbonate CaCO3 (mg/L) | 12.50-37.50 | 41.0-49.0 | 23.0-44.0 | Total magnesium as calcium carbonate CaCO3 (mg/L) | 6.625-33.33 | 19.58-36.60 | 163.33-180.0 | pH | 8.0-8.8 | 8.5-8.6 | 8.66-9.06* | Na (sodium) (mg/L) | 10.4-13.5 | 21.0 | 57-63.6 | K (potassium) (mg/L) | 3.7-4.2 | 6.4 | 18.0-35.5 | Ca (calcium) (mg/L) | 5-15 | 16.4-20.7 | 9.2-17.6 | Mg (magnesium) (mg/L) | 1.59-8 | 4.7-8.8 | 39.2-43.3 | Alkalinity as calcium carbonate CaCO3 (mg/L) | 12.5-76.5 | 118.0-129.0 | 301.0-340.5 | HCO3 + CO3 (bicarbonate, carbonate) (meq/L) | 0.25-1.53 | 2.36-2.58 | 6.02-6.81 | Alkalinity as HCO3 + CO3 (bicarbonate, carbonate ions) (mg/L) | 7.62-46.66 | 71.98-78.69 | 183.61-207.7 | Cl (chloride) (mg/L) | 3.9-7.0 | 3.57-4.3 | 20.9-36.6 | SO4 (sulfate) (mg/L) (using ion-exchange resins) | 2.3 | 5.5 | 7.2-15.3 | SiO2 (dissolved silica) (mg/L) | 3-20 | 1.1-4 | 0.3-6.6 | NO3 · N (nitrate-nitrogen) (µg/L) | 11-29 | — | below detection limit |
PO4 · P (phosphate) (µg/L) | 3-13 | <7-30 | 7 | Total P (phosphorus) (µg/L) | 47-67 | — | [21 (doubtful)] |
*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:
Lake Tanganyika: pH Measurements, 8/93-7/94
Depth and  Sampling Station | Mean ± SD | Median | No. of Samples | Surface | Bujumbura (north end) | 9.01 ± 0.23 | 9.03 | 29 | Kigoma (east coast) | 9.06 ± 0.13 | 9.06 | 30 | Mpulungu (south end) | 9.02 ± 0.15 | 9.02 | 32 | 100 meters | Bujumbura (north end) | 8.66 ± 0.21 | 8.68 | 30 | Kigoma (east coast) | 8.79 ± 0.13 | 8.77 | 29 | Mpulungu (south end) | 8.86 ± 0.12 | 8.90 | 31 |
