¹University of Turku, Department of Biology, Turku, Finland
²Département des Eaux, Pêches et Pisciculture, Bujumbura, Burundi
³Tanzanian Fisheries Research Institute, Kigoma, Tanzania
⁴University of Jyväskylä, Department of Biological and Environmental Science, Jyväskylä, Finland
⁵Department of Fisheries, Mpulungu, Zambia
⁶University of Kuopio, Institute of Applied Biotechnology, Kuopio, Finland
⁷Centre de Recherche en Hydrobiologique, Uvira, R.D. Congo

^*Corresponding author: University of Turku, Department of Biology, FIN-20014 Turku, Finland , Fax +358 2 333 6550, E-mail jouko.sarvala@utu.fi
^§Deceased

Abstract

  Food web structure of the pelagic community in Lake Tanganyika was studied using the stable nitrogen and carbon isotopes 15N and 13C. Size-fractionated seston, zooplankton, shrimps, medusae and fish were sampled in the northern part of Lake Tanganyika. Picoplankton fractions as well as cyanobacteria-dominated nano/microplankton fractions had very low nitrogen isotope signatures typical for nitrogen-fixing organisms. Fractions containing mainly dead organic matter (and associated bacteria) or nano/microalgae (chlorophytes and diatoms) had ?15N 2 to 4‰ higher. The low ?15N signatures of small cyclopoids and shrimps suggest they are feeding on nitrogen-fixing cyanobacteria (picoplankton or larger forms), while the higher ?15N signature of larger copepods suggest mixed feeding on large algae and small zooplankton and/or cyanobacteria. Medusae were slightly enriched in ?15N relative to large copepods. Among fish, the signatures of Stolothrissa and small Lates stappersi suggested feeding on large copepods, while Limnothrissa and larger Lates were slightly more enriched, indicating partial piscivory. The enrichment of 13C between the putative trophic levels (2 to 3‰) was higher, while that of 15N (2 to 3‰) was lower, than usual in isotope studies. Our results indicate that picocyanobacteria and possibly also larger cyanobacteria are important producers in the pelagic food web of Tanganyika.