Seasonal variations in fish δ¹³C and δ¹⁵N in two West African reservoirs, Sélingué and Manantali (Mali): Modifications of trophic links in relation to water level§

§ Revised version of a paper presented at the 4^th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, April 19–23, 2004, Wellington, New Zealand.

Abstract

Most regions in the tropics undergo high seasonal precipitation that produces cyclic patterns of riverine discharge, resulting in periods characterized by low and high water levels. Many chemical and bio-logical factors are affected by this hydrologic seasonality, and it therefore appeared to be very likely that aquatic food webs would also differ during the low and high water periods. Available carbon sources for fish are thought to be less varied during low water periods, but flooding during high water periods could bring fish into contact with a greater abundance and diversity of food sources such as terrestrial plants or the biofilms that grow on submerged terrestrial plants. At low water levels, higher fish densities may lead to more piscivory and less omnivory when compared with the high water periods. Therefore, trophic links within the fish communities may then be modified by water level changes in tropical reservoirs. To address this prediction, we performed stable isotope analyses of the most common species in Sélingué and Manantali, two large reservoirs in Mali (West Africa). Allochthonous and littoral carbon sources were shown to support fish production to a significant extent, even during low water periods. However, the allochthonous or littoral carbon contributions that sustained the top-predators production were indeed greater during the high water periods as expected. The expected higher omnivory in the high water period might have shortened the food chain when compared with the low water period. Some carnivorous fish species were shown to feed at lower trophic levels during high water periods in both reservoirs, but this was not a general pattern. Flooding did not, therefore, necessarily result in a shorter food chain when water levels were high.

§ Revised version of a paper presented at the 4^th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, April 19–23, 2004, Wellington, New Zealand.

Keywords:

• Carbon sources
• Carbon-13
• Diet
• Fish
• Natural variations
• Nitrogen-15
• Rivers
• Seasons
• Trophic relationships

Acknowledgements

We are grateful to the UR 070, IRD, for the field work and logistics and especially to J. Raffray, L. Tito de Morais, J. Panfili and J.J. Albaret. We once more thank J.J. Albaret and L. Tito de Morais for their advisory review, as well as D. Paugy and Dr G. Lebrun for help in plant identification and B. Matthews for checking the English. We would also like to thank the three anonymous reviewers whose comments definitely improved our manuscript. This project was funded by the program ‘Algal bloom in tropical aquatic systems’ (UR 098, IRD).

Notes

§ Revised version of a paper presented at the 4^th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, April 19–23, 2004, Wellington, New Zealand.