Lake Chad: Ecology and Productivity of a Shallow Tropical Ecosystem

583 Pages·1983·15.325 MB·English

by  Jean-Pierre Carmouze (auth.), J.-P. Carmouze, J.-R. Durand, C. Lévêque (eds.)

LAKE CHAD
MONOGRAPHIAE
BIOLOGICAE
VOLUME 53
Series editor
H. J. DUMONT
1983 Dr W. JUNK PUBLISHERS
a member of the KLUWER ACADEMIC PUBLISHERS GROUP
THE HAGUE / BOSTON / LANCASTER
LAKECHAD
Ecology and Productivity of a Shallow
Tropical Ecosystem
Edited by
J .-P. CARMOUZE, J .-R. DURAND and C. LEVEQUE
1983 Dr W. JUNK PUBLISHERS
a member of the KLUWER ACADEMIC PUBLISHERS GROUP
THE HAGUE / BOSTON / LANCASTER
Distributors
jor the United States and Canada: Kluwer Boston, Ine., 190 Old Derby Street,
Hingham, MA 02043, USA
jorall othercountries: Kluwer Aeademic Publishers Group, Distribution Center,
P .O.Box 322, 3300 AH Dordreeht, The Netherlands
Library of Congress Cataloging in Publication Data
Hain entry under title:
Lake Chad.
(Monographiae biologicae ; v. 53)
Includes bibliographies and index.
1. Lake ecology--Chad. Lake. 2. Biological
productivity--Chad. Lake. 3. Chad. Lake. I. Carmouze,
Jean-Pierre. H. Durand, Jean Rene. IH. LW@que, C.
IV. Series.
QP1.P37 vol.53 5748 [574.5'26322'096743] 83-4288
[QH195.C46]
ISBN-13: 978-94-009-7268-1 e-ISBN-13: 978-94-009-7266-7
DOI: 10.1007/978-94-009-7266-7
Cover design: Max Velthuijs
Copyright
© 1983 by Dr W. Junk Publishers, The Hague.
Softcover reprint ofthe hardcover 1st edition 1983
All rights reserved. No part of this publieation may be reprodueed, stored in a
retrieval system, or transmitted in any form or by any means, meehanical,
photoeopying, reeording, or otherwise, without the prior written permission of
the publishers,
Dr W. Junk Publishers, P.O. Box 13713, 2501 ES The Hague, The Netherlands.
Contents
Preface
(J. Daget) VII
Foreword
(The Editors) XI
Historical background
(J.-P. Carmouze)
I. The lacustrine environment and its evolution
1. Paleolimnology of an upper quaternary endorheic lake in Chad
Basin
(M. Servant and S. Servant) 11
2. The lacustrine environment
(J.-P. Carmouze and J. Lemoalle) 27
3. Physical and chemical characteristics of the waters
(J.-P. Carmouze, J.M. Chantraine and J. Lemoalle) 65
4. Hydrochemical regulation of the lake
(J.-P. Carmouze) 95
11. The main types of communities and their evolution during a drought
period
5. The aquatic vegetation of Lake Chad
(A. Iltis and J. Lemoalle) 125
6. The phytoplankton
(P. Compere and A. Iltis) 145
7. The zooplankton
(L. Saint-Jean) 199
8. The benthic fauna: ecology, biomass and communities
(C. Leveque, C. Dejoux and 1. Lauzanne) 233
9. The fauna associated with the aquatic vegetation
(C. Dejoux) 273
10. Fish communities of Lake Chad and associated rivers and flood
plains
(V. Benech, J.-R. Durand and J. Quensiere) 293
111. The balance of a lacustrine ecosystem during 'Normal Chad' and
aperiod of drought
11. Phytoplankton production
(J. Lemoalle) 357
12. Secondary production (zooplankton and benthos)
(C. Leveque and 1. Saint-Jean) 385
13. The exploitation of fish stocks in the Lake Chad region
(J.-R. Durand) 425
IV. Trophic relations
14. Trophic celations between the phytoplankton and the zooplankton
(A. Iltis and 1. Saint-Jean) 483
15. Trophic relations of fishes in Lake Chad
(1. Lauzanne) 489
16. The impact of birds on the lacustrine ecosystem
(C. Dejoux) 519
V
17. The lacustrine ecosystem during the 'Normal Chad' period and
the drying phase
(J.-P. Carmouze, J.-R. Durand and C. Leveque) 527
Systematic index 561
General index 571
VI
Preface
In 1963 the director general of the 'Office de la Recherche Scientifique et
Technique Outre-Mer' (ORSTOM) asked me to form a well structured team of
hydrobiologists based at Fort-Lamy (now N'Djamena) where a laboratory
could be built as a branch of the already existing ORSTOM Center. The
International Biological Program, IBP, had at this time recommended an
integrated study of ecosystems selected in proportion to their representation of
a particular zone or ecological conditions. I took this opportunity to propose
Lake Chad as the model of a tropicallake in a semi-arid c1imate as part of the
activity of the section Productivity of freshwater communities (PF). With the
efficient help ofDr B. Dussart a research program was established involving the
establishment of a permanent team of ten researchers in Chad.
Apart from myself, ORSTOM only had three hydrobiologists: Mr R. Gras,
Mr G. Loubens and Mr A. Iltis. These valuable researchers who had been
working for the 'Centre Technique et Forestier Tropical' and had acquired
considerable experience in African freshwaters, unhesitatingly accepted to join
me at Fort-Lamy. It was necessary to enlarge this team with so me young and
enthusiastic collaborators, namely, Mr C. Leveque, Mr C. Dejoux, Miss S.
Duwat, Mr L. Lauzanne and Mr. J. R. Durand, later joined by Mr J. P.
Carmouze and Mr J. Lemoalle.
At first we stayed briefly at the laboratory of the 'Institut de Medecine
Veterinaire et Tropicale' (IMVT) at Farcha until the construction of the
hydrobiology laboratory at Fort-Lamy was completed. It is with pleasure that I
pay respect not only to the scientific value of this first team which established
itself by producing seven Ph.D's but especially to the good atmosphere and
friendship established between the researchers and technicians at Fort-Lamy as
well as in the field. Due to this exceptional atmosphere, there were only
scientific problems to solve and the two years that I spent at Chad have left me
with unforgettable memories.
With the installation of the team in its laboratory, opened in 1965 and fairly
weIl equipped, the research program was well on its way and I returned to Paris.
Here, with the help of Dr B. Dussart and until1975, I supervised the scientific
Photo 1 The ORSTOM Hydrobiological team in 1966.
coordination of research and the recruitment of new scientists and technicians
to ensure the maintenance or reinforcements of the first team.
This experience proved to be extremely beneficial for the training of men who
have become highly qualified specialists as weil as for the importance of their
scientific results. The director general of ORSTOM took a certain risk by
regrouping all his hydrobiologists at the same time on the same program. What
appeared at first to be achallenge soon became a total success as the reader will
find out for himse\f when reading this book.
It should, however, be recognized that the interest and the scientific scope of
the study presented here partly depends on a happy conjuncture: the presence of
a team of scientists having had the time to study the state of the lake called
'Normal Chad' before and during the time when years of drought led to the state
of 'Lesser Chad'. This indeed appeared to be a cyclic phenomenon that had
already occurred at the beginning of the century as shown by Tilho's
observations, and which would recur again after a new period of high waters.
However, a researcher would only have the chance to observe this once during his
career. This complete team of hydrobiologists then, was able to follow in detail
the lowering of the lake level. If this event had occurred ten years earlier, there
VIII
would have been no one to study it; if it had occurred ten years later, it would
have passed unobserved as research in Chad would have been impossible.
When I first became interested in Lake Chad, I was struck by two factors in
particular. In spite of the approximately 20000 km2 of area then known,
Hutchinson had not considered it worthy to be included in the list of African
lakes considered in his classical treatise of Limnology. Furthermore as far as
the fish fauna was known, there were no true lacustrine species present. Alestes
dageti, described as endemic to the lake by Blache and Miton, only appeared to
me as an ecophenotypic form of Alestes nurse characterized by its dwarfism, a
poorly known deterministic phenomenon, although it is relatively frequent in
some fish populations. In 1967, I wrote that 'Ecologically Lake Chad is closer
to the flood zones of large tropical rivers than true lakes .,. it thus appears
probable that a slight worsening of climate could involve a complete drying of
the lake'. The exceptional drought of the years 1972-73 which was dramatic for
the Sahelian populations showed that it did not behave as a simple system, as
drought first affected the north, i.e. the deepest part. It will be interesting to
read how the major types of population changed over the course of the drying
phase.
Finally, it is necessary to recall that the fisheries in Lake Chad were
developing around the sixties in relation to the introduction of nylon gill nets
and especially the development of transport facilities to carry the dried and
smoked fish towards Nigeria and the southern territories. Now, paradoxically,
the driest years turned the production rocketing to 220000 tons in 1974,
causing a huge concentration of fishermen in the lake. In addition the
productivity of these tropical waters and the resilience of their species was seen
to be much higher than provisionally estimated by comparison with other
environments.
Following several notes and papers published in specialized journals, the
summary of 15 years of research presented here in an easily accessible form
appeared welcome to me. It is not premature as the field-work had to be
interrupted in 1979 nor is it too late because the problems of management and
the improvement of the Chad basin are not yet solved.
I agreed and accepted the invitation to preface this scientific work with real
pleasure for it is a factual document. I would like to address my personal thanks
to the authors for their collaboration as weIl as to the editors who agreed to
publish it and distribute it to the international scientific community. I am sure
that it will obtain from the latter all the success that it deserves.
Jacques Daget
Professor, National Museum
of Natural History, Paris
IX
Foreword
Lake Chad is a large, endorheic, shallow basin south of the Sahel zone.
Although it is subject to important annual variations of area and volume, it is
one of the largest lakes of the African continent, following those in the Rift
valley (Victoria, Tanganiyka and Malawi).
The biotopes of Lake Chad are similar to those found in lakes in the central
Niger delta, southern Sudan, Lake Chilwa, Okavango delta and a number of
other less important aquatic ecosystems. They are characterized primarily by
the instability of hydrological conditions and the abundance of macrophytes
(Cyperus, Typha and Phragmites, in particular). Thus the interest of studies
conducted on Lake Chad extends beyond regional limits because many
problems are common to all these regions even if each has its own specificity.
Since the beginning, the research pro gram on Lake Chad was part of the
French contribution to the International Biological Program, entitled 'Study of
a Tropical Lake in a Semi-Arid Climate'. Its principal objective was to describe
the lake hydroclimate and evaluate the biomass and production of the main
groups of organisms in order to establish a balance of matter and energy and of
their cycles on the different trophic levels. In addition to the fundamental value
of this research for biological phenomena in a climatic zone that was fairly
unknown until now, this study could also have economic consequences for the
development projects for Chad basin, and particularly for the fisheries. Indeed
Lake Chad is a very productive environment due to its morphology. Its average
fisheries catch, estimated at more than 100 000 tons per year, provides an
important economic activity for the nearby communities.
Between 1964 and 1978, a team of several limnologists worked at the
ORSTOM Center at N'Djamena, where a modern laboratory of 220 m2 was
built in 1965. The first studies were limited to the east zone of the lake where it
was possible to make preliminary tests for sampling methods and analysis with
the Nausicaa, a 10 m long boat specially adapted to these shallow environ
ments.
It was soon apparent that a general view of the lake was essential for the
proper interpretation of several observed phenomena. Thus, a study of large