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SOME OF THE IMPACTS OF THE ROREIRES RESERVIOR ON THE ECOLOGY OF THE BLUE NILE. BY F.A.SINADA, O.B .ALI and A. I. el MOGHRABY. PRELUDE.
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F.A.SINADA, O.B .ALI and A. I. el MOGHRABY
New reservoirs have important effects on the ecology of rivers. De-oxygenation of water from the rotting of drowned vegetation. The increased organic content of water appears to be in favour of the development of floating water plants in the early years of inundation.
The construction of Roseires dam in 1966 has exerted profound influence on the biological conditions of the Blue Nile at Roseires as well as downstream at Khartoum.
The numbers of zooplankton increased 7-316 times more than the river above the reservoir;
Before the construction of Roseires dam there existed an extremely sparse plankton population with large detritus suspension and few adventitious organisms in 400 km section of the river upstream Sennar dam.
Within this stretch the diatom Aulcoseira granulata (formerly named Melosira granulata) and the cyanobacterium Anabaena flos-aquae – two principal components of the Blue Nile plankton north of the Sennar reservoir - were present in very small numbers.
It seems that poor light penetration coupled with relatively high current velocity might have been the sole factor retarding the development of phytoplankton within this stretch of the Blue Nile before the construction of Roseires dam.
Shortly after filling Roseires reservoir for the first time, a moderately rich phytoplankton developed in the reservoir with Microcystis flos-aquae, previously a minor component, rapidly dominating the plankton forming a dense bloom imparting a bright green colour to the surface of the lake. Aulacoseira granulata and Anabaena flos-aquae always important components of the plankton of the Blue Nile at Khartoum were present throughout the sampling season.
During the following second season of filling of the reservoir (November 1967- May 1968), the phytoplankton of Roseires showed considerably heavier growth than during the first season of 1966-1967, with the dominant algae being the same, namely, the diatom Aulacoseira granulata, thecyanobacteria Microcystis flos-aquae and Anabaena flos-aquae.
During the third season of filling the reservoir (1969-1970) the Roseires reservoir showed further differences from the "usual" seasonal cycle though Aulacoseira granulata produced a heavy growth in November.
Unexpectedly Volvox aureus dominated the plankton in December 1969 but disappeared shortly after that to be replaced by a heavy growth of Pediastrum simplex var. duodenarium which continued to dominate for a short while.
It is apparent that the reservoir during the first three years of its filling (1966-1970) had not stabilized biologically.
The seasonal cycles and succession of planktonic algae differed from year to year.
No doubt, the formation of the Roseires reservoir has brought about changes in the species composition and seasonal cycles of the plankton in the Blue Nile along the whole stretch of the river within Sudan plain.
The recurrent seasonal cycle of the phytoplankton observed near Khartoum in the Blue Nile in the 1980s 1990s and by the turn of the millennium is very different from that reported earlier in the 1950s and 1960s before the construction of the dam.
The fisheries of the upper reaches of the Blue Nile were surveyed by Boulenger in 1907, Sandon in 1950 and Mishrigi in 1970. The latter was carried out after the construction of the Roseiers reservoir with the objectives of giving a list of fishes present and to determine the catch composition.
Only 10 families out of the 14 present in the Blue Nile are represented in the reservoir. Mishrigi reported 29 species while 46 were reported in earlier studies.
Four species have not been reported form the Blue Nile before. These are: Hydrocyon brevis, Disticodus rosratus, D. previpinis and Mormyrops anguilloidis.
The catch increases in March/ April and decreases in January and February. This was correlated to physical, chemical, and biological factors.
Mass mortality of fish occurred for three consecutive years after first filling. This coincided with the annual upstream breeding migration, in May.
The dam has no fish passage way.
Nine years after first filling the reservoir lost one third of its storage capacity due to siltation.
To date the reservoir is not operating according to its original design. The irrigation canals were never constructed and the deep sluice gates are never opened except during the peak of the flood. Instead the spillway gates (the flood gates) are used instead. The reason being a ‘dent’ developed in the ‘apron’ in front of the deep sluices as far back as 1967