Advertisement

Hydrobiologia

, Volume 257, Issue 2, pp 107–120 | Cite as

Lake Muzahi, Rwanda: limnological features and phytoplankton production

  • R. Mukankomeje
  • P-D. Plisnier
  • J-P. Descy
  • L. Massaut
Article
  • 68 Downloads

Abstract

Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NH inf4 sup+ -N can be high.

Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)

In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.

Key words

Lake Muhazi tropical lakes phytoplankton production fish production Rwanda Africa 

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. A.P.H.A. (American Public Health Association), 1965. Standard methods for the examination of water and wastewater. New York, 769 pp.Google Scholar
  2. Beadle, L. C., 1981. The inland waters of tropical Africa. An introduction to tropical limnology. Longman Group Ltd., New York, 2nd edn., 475 pp.Google Scholar
  3. Damas, H., 1953. Les lacs du Rwanda et leurs problèmes. Soc. r. Zool. Belg. 84: 17–38.Google Scholar
  4. Damas, H., 1954a. Etude limnologique de quelques lacs rwandais. I. Le cadre géographique. Acad. r. Sci. Col. 14: 1–87.Google Scholar
  5. Damas,H., 1954b. Etude limnologique de quelques lacs rwandais. II. Etude thermique et chimique. Acad. r. Sci. Col. 14: 1–113.Google Scholar
  6. Descy, J.-P., P. Servais, J. Smitz, G. Billen & E. Everbecq, 1987. Phytoplankton biomass and production in the River Meuse (Belgium). Wat. Res. 21: 1557–1566.CrossRefGoogle Scholar
  7. Descy, J.-P. & J.-M. Théâte, 1990. Rapport de mission au lac Muhazi (Rwanda). UNECED (FUNDP), Namur, Belgium, 23 pp.Google Scholar
  8. Downing, J. A., C. Plante & S. Lalonde, 1990. Fish production correlated with primary productivity, not the morphoedaphic index. Can. J. Fish. aquat. Sci. 47: 1529–1536.CrossRefGoogle Scholar
  9. Ford, R., 1990. The dynamics of human-environment interactions in the tropical montane agrosystem of Rwanda: implications for economic development and environmental stability. Mountain Res. Dev. 10: 43–63.Google Scholar
  10. Gaudet, J. J., 1977. Uptake, accumulation and loss of nutrients by papyrus in tropical swamps. Ecology 58: 415–422.CrossRefGoogle Scholar
  11. Golterman, H. L. & R. S. Clymo, 1969. Methods for Chemical Analysis of Freshwaters. IBP Handbook 8, Blackwell Sc. Publ., Oxford and Edinburgh, 166 pp.Google Scholar
  12. Harper, D. M., 1991. Primary production in Lake Naivasha, Kenya. Verh. int. Ver. Limnol. 24: 1112–1116.Google Scholar
  13. Healey, F. P. & L. L. Hendzel, 1980. Physiological indicators of nutrient deficiency in lake phytoplankton. Can. J. Fish. aquat. Sci. 37: 442–453.Google Scholar
  14. Hecky, R. E. & P. Kilham, 1988. Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichments. Limnol. Oceanogr. 33: 796–822.Google Scholar
  15. Henderson, H. F. & R. L. Welcomme, 1974. The relationship of yield to morpho-edaphic index and numbers of fishermen in African inland fisheries. FAO, Rome, CIFA/OP1: 20 pp.Google Scholar
  16. H.M.S.O. (Her Majesty's Stationery Office), 1982a. Ammonia in waters. In Methods for the examination of waters and associated materials, Department of the Environment, London, 47 pp.Google Scholar
  17. H.M.S.O. (Her Majesty's Stationery Office), 1982b. Chloride in waters, sewage and effluents. In Methods for the examination of waters and associated materials, Department of the Environment, London, 46 pp.Google Scholar
  18. Kalff, J., 1983. Phosphorus limitation in some tropical African lakes. Hydrobiologia 100: 101–112.CrossRefGoogle Scholar
  19. Kifle, D. & A. Belay, 1990. Seasonal variation in phytoplankton primary production in relation to light and nutrients in lake Awasa, Ethiopia. Hydrobiologia 196: 217–227.CrossRefGoogle Scholar
  20. Kirk, T. O. J., 1983. Light and photosynthesis in aquatic ecosystems. Cambridge Univ. Press, Cambridge, 401 pp.Google Scholar
  21. Kiss, R., 1976. Etude hydrobiologique des lacs de l'Akagera moyenne. Inst. nat. Rech. Sci., Butare, 16, 167 pp.Google Scholar
  22. Lemoalle, J., A. Adeniji, P. Compère, G. G. Ganf, J. Melack & J. F. Tailing, 1981. Phytoplankton. In J.-J. Symoens, M. Burgis & Gaudet (eds), The ecology and utilization of African inland waters, UNEP Reports and Proceeding ser., Nairobi: 37–50.Google Scholar
  23. Liang, Y., J. Melack & J. Wang, 1981. Primary production and fish yields in Chinese ponds and lakes. Trans. am. Fish. Soc. 110: 346–350.CrossRefGoogle Scholar
  24. Lorenzen, C. J., 1967. Determination of cholorophyll and phaeopigments: spectrophotometric equations. Limnol. Oceanogr. 12: 343–346.CrossRefGoogle Scholar
  25. Marker, A. F. H., E. A. Nusch, H. Rai & B. Riemann, 1980. The measurement of photosynthetic pigments in freshwaters and standardization of methods: conclusions and recommendations. Arch. Hydrobiol., Beih. Ergebn. Limnol. 14: 91–106.Google Scholar
  26. McConnell, M. J., S. Lewis & J. E. Olson, 1977. Gross photosynthesis as an estimator of potential fish production. Trans. am. Fish. Sec. 106: 417–423.CrossRefGoogle Scholar
  27. Melack, J. M., 1976. Primary production and fish yields in tropical takes. Trans. am. Fish. Soc. 105: 575–580.CrossRefGoogle Scholar
  28. Oglesby, T. R., 1977. Relationships of fish yield to lake phytoplankton standing crop, production and morphoedaphic factors. J. Fish. Res. Bd Can. 34: 2271–2279.Google Scholar
  29. Pechar, L., 1987. Use of acetone: methanol mixture for extraction and spectrophotometric determination of chlorophyll a in phytoplankton. Arch. Hydrobiol. Suppl. 78, Algol. Stud. 46: 99–117.Google Scholar
  30. Plisnier, P.-D., 1989. Etude hydrobiologique et développement de la pêche an lac Muhazi (bassin de l'Akagera, Rwanda). ACDST (ULg)-UNECED (FUNDP)-MINAGRI-AGCD, final report (1986–1988), 178 pp.Google Scholar
  31. Plisnier, P.-D., 1990. Ecologie comparée et exploitation rationnelle de deux populations d'Haplochromis spp. (Teleostei, Cichlidae) des lacs Ihema et Muhazi (Rwanda). Ph.D. Thesis, Louvain-la-Neuve, Belgium, 328 pp.Google Scholar
  32. Plisnier, P.-D., J.-C. Micha & V. Frank, 1988. Biologic et exploitation des poissons du lac Ihema (Bassin Akagera, Rwanda). ORTPN, AGCD, CECODEL (ULg), UNECED (FUNDP), Presses Universitaires de Namur, Belgium, 212 pp.Google Scholar
  33. Sparre, P, E. Ursin & S. Venema, 1989. Introduction to tropical fish stock assessment. Part 1. FAO Fish. Tech. Pap. 306/1, Rome: 337 pp.Google Scholar
  34. Symoens, J.-J., 1968. Exploration hydrobiologique du bassin du lac Bangweolo et du Luapula. II, 1: La minéralisation des eaux naturelles. Cercle Hydrobiologique de Bruxelles, Bruxelles, 199 pp.Google Scholar
  35. Tailing, J.-F., 1965. The photosynthetic activity of phytoplankton in East African lakes. Int. Revue ges. Hydrobiol. 50: 1–32.Google Scholar
  36. Tailing, J.-F. & I. B. Tailing, 1965. The chemical composition of African lake waters. Int. Revue ges. Hydrobiol. 50: 421–463.Google Scholar
  37. Vollenweider, R. A., 1965. Calculation models of photosynthesis-depth curves and some implications regarding day rate estimates in primary production measurements. Mem. Ist. ital. Idrobiol. (suppl.) 18: 425–457.Google Scholar
  38. Vollenweider, R. A. (ed.), 1974. A manual on methods for measuring primary production in aquatic environments. IBP Handbook 12, Blackwell Scientific Publications, Oxford, 225 pp.Google Scholar
  39. Wetzel, R. G. & G. E. Likens, 1979. Limnological analyses. W.B. Saunders Company, Philadelphia, 357 pp.Google Scholar

Copyright information

© Kluwer Academic Publishers 1993

Authors and Affiliations

  • R. Mukankomeje
    • 1
  • P-D. Plisnier
    • 1
  • J-P. Descy
    • 1
  • L. Massaut
    • 1
  1. 1.Unit of Freshwater Ecology, Department of BiologyFUNDPNamurBelgium

Personalised recommendations