Book Reviews
Chad was studied as the model of a tropical lake in a semi-arid region south of the Sahel Zone.
This monograph is the summary of 15 years of research. The field-work had to be interrupted in 1979. Lake Chad is one of the largest lakes of Africa, smaller only than the lakes in the Rift Valley. The aquatic ecosystem is characterized primarily by instability of the hydrological conditions and an abundance of macrophytes, Phragmites, Cyperus and Typha in particular. The studies from 1968 to 1971 revealed a relative steady state, although the lowering of the lake's level had already become noticeable. I n subsequent years, the environment and species populations underwent great modifications. J.-P. CARMOUZE describes the historical background since the beginning of the sixteenth century, when the lake was first discovered by Europeans. The following sections deal with the lacustrine environment and its evolution, the main types of communities and their changes during a drought period, including the physical and chemical characteristics ; the vegetation; the phyto-and zooplankton; the benthos; the fauna associated with the aquatic vegetation; and fish communities. An analysis was made to determine the balance of the lacustrine ecosystem during 'Normal Chad' and a period of drought. Special sections deal with the trophic relationships among plankton, fishes and birds. In the final section, the editors summarize the results of the multidisciplinary research. They point out tbat similar environments such as Lake Chilwa and the Okavango Delta are fairly abundant, not only in Sudanian Africa but also south ol the equator. Some of these are actually in danger, especially due to management projects, "and knowledge of their functioning is necessary to assist all attempts a t protection". After the periods of high water since 1972, the level of Lake Chad has lowered rapidly, and the north basin dried up in 1975. In 'Lesser Chad' only the south basin encompassing c. 40 Ol0 of the area remained under water. Several species disappeared, but marsh fish and algae typical of mesocarbonated waters increased. The ecosystem found new ways of functioning, enabling overall productivity, including the yields of fish, t o remain fairly high. The editors regret that in Lake Chad,it was not possible to etudyin detailthe disturbances to the populations caused by the drought period, but the authors "have gathered the most spectacular aspects". The monograph on Lake Chad is a valuable contribution to our knowledge of tropical limnology. It also demonstrates that the study of such a large ecosystem can only be performed under the auspices of a multidisciplinary research program. The work in Chad is a long-time meteorological based experiment.