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Hydrobiologia

, Volume 36, Issue 1, pp 81–103 | Cite as

Seasonal changes in the waters of Lake Chilwa (Malawi) in a drying phase, 1966–68

  • A. Morgan
  • Margaret Kalk
Article

Summary

Lake Chilwa, Malawi (lat. 15°S; long. 36°E) has a potential area of open water of 700 km2; it is shallow and has no outlet. The lake level fluctuates in height with alternate wet and dry seasons, and the mean annual level rises and falls over a 6-year period. This study was conducted from the end of the wet season of 1966 to the end of the dry season of 1967, which was at the end of the 6-year cycle, and it preceded the complete drying of the lake in 1968 and its recovery in 1969, an event which last happened in 1922–23.

Forthnightly samples of water were examined for electrical conductivity, sodium, potassium, calcium, magnesium, iron, chloride, alkalinity, sulphate, phosphate, silicate, nitrite, nitrate, albuminoid nitrogen, dissolved oxygen, pH and temperature.

The lake water is characterized by chloride being half the concentration (in me/l) of sodium and equal to alkalinity. The conductivity initially 4210 µmho/cm, increased by a factor of 3 in 1966 and by 4 in 1967. Calcium and magnesium carbonate were precipitated in the hot dry months. The effect of a cyclone was to dilute the surface waters 20 times less than the previous month. The water was supersaturated with oxygen, except in the coolest months. The temperature varied between 21°C and 37°C and the pH between 8 and 11 units.

The pattern of chemical factors depends on dilution and concentration due to climatic influences, on interactions of some ions with the biota, with each other and in some cases with the exposed mud. The significance of this pattern for the biology of the lake is discussed. The ionic composition is compared with that of other African lakes.

This study, during an atypical period, is intended to assist plans to conserve the fishing industry, which in years of high lake level produces almost half the total catch of the country.

Keywords

Nitrite Cyclone Alkalinity Lake Level Total Catch 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Résumé

Le Lac Chilwa, Malawi (lat. 15°S; long. 36°E) a une aire potentielle d'eau de 700 km2; le lac est peu profond et n'a pas de sortie. Le niveau du lac varie en hauteur selon les saisons humides et sèches qui se succédant, et le niveau moyen se lève et tombe pendant une période de six ans. L'étude du lac a eu lieu depuis la fin de la saison humide en 1966 jusqu'à la fin de la saison sèche en 1967. Cette étude se faisait à la fin d'une période de six ans et précédait le désèchement complet du lac en 1968 et son rétablissement en 1969, une situation qui a eu lieu pour la dernière fois en 1922–23.

Tous les quinze jours on a examiné des échantillons d'eau quant à conductivité électrique, sodium, potassium, calcium, magnésium, fer, chloride, alcanité, sulfate, phosphate, silicium, nitrite, nitrate, substance albuminoide, oxygène dissolu, pH et température.

L'eau du lac est caracterisée par chloride étant un demi-concentration (en me/l) de sodium et égale aux substances alcalines. La conductivité, au commencement 4210 umho/cm, a été augmentée par un facteur de 3 en 1966 et par 4 en 1967. Calcium et magnésium carbonate étaient précipites pendant les mois chauds et secs. L'effect d'un cyclône devait les eaux superficielles vingt fois moins que le mois précédent. L'eau était sursaturée d'oxygène, exceptépendant les mois plus froids. La température variait entre 21°C et 37°C et le pH entre 8 et 11 unités.

La variation de facteurs chimiques dépend de la dilution et de la concentration dues aux influences climatologiques, des interactions de quelques ions avec les biota, mutuellement et en certain cas, avec la vase exposée. La signification de cette variation pour la biologie du lac est discutée. La composition ionique se compare avec celle d'autres lacs africains.

Cette étude, pendant une période non-caractéristique, a pour but de soutenir des projets pour conserver l'industrie de poisson qui pendant les années ou le niveau du lac est haut produit à peu près la moitié de la prise totale du pays.

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Copyright information

© Dr. W. Junk n.v. Publishers 1970

Authors and Affiliations

  • A. Morgan
    • 1
  • Margaret Kalk
    • 1
  1. 1.University of MalawiMalawi

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