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The fate of organic matter in a papyrus ( Cyperus papyrus L.) dominated tropical wetland ecosystem in Nyanza Gulf (Lake Victoria, Kenya) inferred from δ 13 C and δ 15 N analysis

2005; Taylor & Francis; Volume: 41; Issue: 4 Linguagem: Inglês

10.1080/10256010500384739

1477-2639

John Gichuki, Ludwig Triest, Frank Dehairs,

Aquatic Ecosystems and Phytoplankton Dynamics

Abstract Papyrus swamps usually form at the interface between river inlet and open lake. From one such wetland ecosystem (the Kibos system located in the Nyanza Gulf, Lake Victoria, Kenya), three sediment cores were recovered using piston corer in order to determine the fate of organic matter derived from papyrus and possible nutrient pathways in this system. The coring represented a transect from the river through the floating papyrus mat to the lake. Two short cores were retrieved from the lake and river. One long core (2 m) was recovered on a floating papyrus mat. The C:N ratio showed similar trends down core from the three locations. This may possibly be due to diagenic processes such as autolysis, dissolution and microbial mineralisation occurring in the sediments. Statistical analysis through one-way ANOVA revealed no significant differences in the C:N ratios between stations. Results of the stable carbon isotope ratios revealed that the δ13C of the river and lake samples were persistently more negative than−20 ‰ over the whole profile indicating possible contribution from terrestrial derived carbon. Regarding the floating mat core, the δ13C values ranged from−18.99 ‰ on the top of the floating mat but gradually increased to−16.82 ‰ towards the bottom of the core indicating possible contribution of carbon from Cyperus papyrus that has a δ13C value of−13.45±0.62 ‰. Statistical analysis through one-way ANOVA revealed significant differences in the δ13C values between stations. The stable nitrogen isotope values were highly positive both in the river and in the lake station (δ15N>10 ‰), indicating possible contamination from sewage wastes. Values in the swamp were less positive suggesting first, the formation of ammonium depleted in 15N from intense organic matter mineralisation, secondly indicating the δ15N signal of papyrus and, finally that nitrogen fixation processes were possibly occurring in the swamp. Statistical analysis through one-way ANOVA revealed significant differences in the δ15N values between stations. The stable isotope findings suggested that carbon derived from papyrus is retained in the swamp. Impoverished oxygen concentration in the swamp suggests high mineralisation of organic matter in the swamp indicating that the retained papyrus-derived carbon is largely respired. We conclude that further studies should be undertaken to determine the respiration rates in the wetland. Keywords: Carbon fluxusCarbon-13Lake VictoriaKenyaNatural isotope variationsNitrogen-15Organic matterPapyrusSinkStable isotopesSwamp habitatsWetlands Acknowledgements The funding for this research was kindly provided by VUBAROS—the Council for Development Co-operation of the Free University of Brussels in Belgium. Gratefully, we acknowledge the support of Professor Dr. Dirk Verschuren from the State University of Ghent, who assisted recovering undisturbed cores for this study. We would like to thank the editors of Isotopes in Environmental and Health Studies for their patience during the preparation time of this article. Finally, we want to express our gratitude to the reviewers of the article, who gave important comments for the improvement of our text.