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 δ15N 2 to 4‰ higher. The low δ15N signatures of small cyclopoids and shrimps suggest they are feeding on nitrogen-fixing cyanobacteria (picoplankton or larger forms), while the higher δ15N signature of larger copepods suggest mixed feeding on large algae and small zooplankton and/or cyanobacteria. Medusae were slightly enriched in δ15N 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.

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