Studies on the Ecology of Lake Qarûn (Faiyum-Egypt).

Naguib, Monir (1958) Studies on the Ecology of Lake Qarûn (Faiyum-Egypt). Open Access Kieler Meeresforschungen, 14 (2). pp. 187-222.

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Abstract

The productivity of lake Qarun is declining rather quickly and the present work was undertaken as a preliminary contribution for explaining the causes of the prevailing conditions in the lake. The important physico-chemical factors affecting thebiological productivity in lake Qarûn were studied. Water samples taken from 13 stations, chosen to represent in a general way the different regions in the lake, were analysed. Salinity, pH, alkalinity and oxygen content, as well as nutrients including nitrates, nitrites, phosphates, 'silicates' and iron were determined in these samples. The fact that the lake is a closed basin for drainage water is responsible for the observed increase in salinity with the lapse of time due to the accumulation of salts conveyed into it. The seasonal salinity variations are controlled by the fresh drainage flood invading the lake in autumn and early winter on the one hand, and by the excessive evaporation caused by high temperature in summer on the other hand. Minimum and maximum salinities of about 18 ‰ and 25.5 ‰ respectively were recorded in the year 1954. The pH of water did not show marked seasonal variations, oscillating all the time around 8. The pH values of the sediments were slightly lower than that. During autumn this decrease in pH was more eminant (pH 7.1) apparently due to the products of fermentation formed in that season. This process of bottom sediments fermentation causes an oxygen-deficient bottom during autumn, while during the other seasons the bottom is well aerated. The lake was found to be of the bicarbonate type, the concentration of carbonates present is rather low. The factors affecting carbonate-bicarbonate intertransformation is discussed. The lake was found to be very rich in nitrates and organic nitrogen. These substances are conveyed into the lake annually by the drainage water coming from lands fertilized with nitrogenous manure. The fact that the income of these nitrogenous material exceeds the arnount utilized, caused the accumulation of these substances in the lake. On the other hand nitrites were occasionally present as a link in the nitrogen cycle discussed. Phosphates in water was found in low concentration in winter, spring and autumn while completely absent in summer. A correlation between phosphate content and phytoplankton production was observed. Since a big decrease of phytoplankton coincided with the period of the absence of phosphates probably indicates that phosphate is a limiting factor in phytoplankton production in the lake. The silicate cycle in the lake is discussed, and it appears that silicate content does not represent a limiting factor in the life of the lake. The seasonal changes in the concentration of the nutrients in the surface water was found to follow the distribution of drainage water in the different parts of the lake. On the other hand, the increase in the concentration of the bottom water nutrients was found to be during the time in which the fermentation of the bottom sediments took place, and that happens in autumn, as indicated by the low oxygen content and high organic production in the bottom water, as well as high sulphide formation in the sediments during that season. Quantitative examination of the alimentary canal contents of the four species of fish predominating in the lake showed that these fish depend, to a certain extent, on a planktonic diet. Defficiency in the planktonic crop, whether phytoplankton or zooplankton, together with the other direct limiting, and sometimcs lethal faclors as: changes in salinity, high summer temperature, deficiency of oxygen, etc., as is discussed later, seem to be responsible for low fish production in the lake.

Document Type: Article
Additional Information: Printausgabe in Bibliothek vorhanden - print edition available
Refereed: No
Open Access Journal?: Yes
Publisher: Institut für Meereskunde
Date Deposited: 23 Feb 2022 10:22
Last Modified: 23 Feb 2022 10:22
URI: https://oceanrep.geomar.de/id/eprint/55345

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