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Abstract:
After the closing of the dam in the Suriname River in 1964, the changes in the environment
and plant and animal life were followed continuously during three years. Several publications
about the vegetation, fish fauna and hydrobiology from different investigators
appeared.
In 1968 the author revisited the lake for some weeks and the hydrobiological observations in that period are given here, in addition to the data given at the IVL-Congress in
1965 (see: IVL-Handl. 16,1966).
In May 1968 the water gauge was 23l ft. In August 1967 the mark of 24 l fl. was reached
and the work was ready for use. An estimated 40 % of the lake was covered by waterhyacinth
(Eichhornia crassipes). The plants were controlled by sprayin g 2 - 4 D , from aeroplane
and boats. Information from 1970 reports that the water-hyacinth is practically absent
now.
As a result of the drowning of trees and the disappearance of water-hyacinth the effect
of wind on the water has ìncreased. This caused currents and mixing of the lake water.
Oxygen was found even at the bottom at a depth of 35 m. The vertical distribution of
oxygen was irregular and changed rapidly at all depths during the day. In sheltered stations
oxygen remained low and was absent below 5 meter. The observations in 1968 point to an
improvement of the oxygen content in the lake ; the information from 1970 however indicates
that no further progress is made, probably as a result of the large amounts of decaying
water-hyacinths, which must be present near the bottom by the radical control.
Fish developed well in the lake but there are few species, represented by Cichla
ocellaris and Serrasalmo rhombeus.
The pH at different stations ranged from 5.5 to 6.4 with the highest value at the surface.
Conductivity ranged from 20 to 40 µS at the surface and increased to the bottom. In
this environment Desmids develop in great numbers; diatoms and blue-algae were scarce;
plankton crustaceans and rotifers were present in great numbers. Compared to former
years, the rich development. of Volvox and Eudorína was decreasing.
