1Department of Biology, University of Turku, FIN-20014 Turku, Finland
2Département des Eaux, Pêches et Pisciculture, P.B. 1250, Bujumbura, Burundi
3Tanzania Fisheries Research Institute, P.O.Box 90, Kigoma, Tanzania
4Department of Biological and Environmental Sciences, University of Jyväskylä, P.O.Box 35, FIN-40351 Jyväskylä, Finland
5Fisheries Department, P.O.Box 55, Mpulungu, Zambia
6Institute of Applied Biotechnology, University of Kuopio, P.O.Box 1627, FIN-70211 Kuopio, Finland
7Centre de Recherche en Hydrobiologie (Uvira, République Démocratique du Congo), B.P. 254, Bujumbura, Burundi

Stable natural isotope ratios (15N, 13C) elucidate the food web structure in the pelagial zone of Lake Tanganyika

Food web structure of the pelagic community in Lake Tanganyika was studied using the stable natural nitrogen and carbon isotopes (15N and 13C). Size-fractionated seston, zooplankton, shrimps, medusae and fish were sampled in the northern part of Lake Tanganyika. Here we focus on the nitrogen isotope results. Picoplankton fractions had very low nitrogen isotope signatures typical for nitrogen-fixing organisms. Fractions containing mainly bacteria or larger algae had 2-4l higher signatures. Small cyclopoids and shrimps had low signatures suggesting feeding on picoplankton, while larger copepods had higher signatures suggesting feeding on larger algae. Medusae were slightly enriched relative to large copepods. Stolothrissa and small Lates stappersi had signatures suggesting feeding on large copepods, while Limnothrissa was slightly more enriched, indicating partial piscivory. Our results show that isotope ratios are useful for working out the trophic relations in the pelagic food web of Tanganyika.