The zooplankton of the Blue Nile overcomes the adverse conditions of the flood by entering into a state of diapause. Resting stages were found in the river bottom, flowing water and rain pools. They were blown to different localities by wind and can withstand long periods of desiccation. Some species of zooplankton go into diapause more than once a year, notably during their seasonal peaks of abundance. Release from diapause occurred in the temperature range 20-30°C and was facilitated by longer photoperiods and decreased concentrations of suspended silt. Various species of zooplankton were induced in facultative diapause when stored in complete darkness.

The Nile is the major source of water for irrigation in northern Sudan and Egypt. The need for water is most acute during the dry season, which coincides with the period of low-flow of the Nile. Consequently, intensive studies have been made, since 1904, on the hydrology of the Nile and its control, while dams have been built on the Blue Nile, the White Nile, and the River Nile (i.e. after the confluence of the Blue Nile and White Nile at Khartoum). The concept of storage in the equatorial lakes was also entertained, before the decision was taken on the current excavation of the Jonglei Canal to by-pass the ‘Sudd’ swamps.A vast literature is available on the hydrology and hydrobiology of the Nile. Nevertheless the present manuscript is seemingly the first attempt to present unpublished notes with a compilation of the published literature. I have also attempted to outline the existing Jonglei Canal Project, which is often confused with earlier-suggested projects.I advocate that the need for water and the present state of knowledge justify undertaking ‘Phase One’ of the Project, but not ‘Phase Two’. A future paper (now in preparation) will deal in detail with claims that the excavation of the Canal will have drastic ecological repercussions, and will affect the way of life of the local Nilotic tribes.

The Jonglei Canal Project should be weighed against its socio-economic impacts as well as environmental considerations. If taken in a short- or long-term cost-benefit perspective, the Project we think is a justified undertaking.One might argue: ‘let everything well alone; change is another word for disaster’. We are not contending that the Jonglei Canal Project, if duly completed, would not bring about a new mode of life for both Man and domestic and other animals. We are contending that the new mode will be a more satisfactory on than that which exists at present. Sudan aims at the transformation of traditional communities to better social and economic horizons, and not the intentional destruction of the traditional way of life. Many overlook the fact that the Nilotes are already changing, and that it is beyond the powers of the political and social systems to confine them to the past.Nevertheless there are indications that ecologists and environmentalists are far from agreed among themselves as to whether the Jonglei canal is, on balance, a good or a bad idea. But in any case we would like to stress in conclusion that the Jonglei Canal Project is being executed at a relatively fortunate time in Man's history of awareness of the way in which he should be using his natural resources.

With 6 figures in the text)Two lakes lie in a caldera at the top of Jebel Marra. One is shallow and highly saline, with a dense bloom of planktonic blue-green algae and a zooplankton consistingentirely of rotifers.The other is over 100 metres deep, is less saline, with a sparse phytoplankton of diatoms, and the dominant zooplankter is Eucyclops gibsoni. The marginal fauna of the deep lake includes a number of widespread insects, but is unusual in that it includes an aquatic glowworm (Family Lampyridae).In the shallow lake there is a superabundance of oxygen in the upper two metres, but no oxygen below six metres, except when strong winds mix the lake so that there is then about 40% saturation throughout the water column. In the deep lake it was found that after strong winds there was only about 12% saturation in the top 30 metres of water. The restricted fauna of these lakes appears to be a result of the combined effects of isolation, salinity and a variable oxygen regime.