Basal carbon sources and planktonic food web in a tropical lake: an isotopic approach

Trophic connections among habitats may be central to food-web dynamics in lakes. Lacustrine zooplankton can rely on basal carbon (C) sources from different origins and plays an important link between these and organisms in higher trophic levels. We investigated the basal C sources supporting the planktonic food web and the trophic relationships among zooplankton size fractions in a tropical lake (Carioca) in Brazil. To do so, we measured the C and nitrogen (N) stable-isotope ratios in basal C sources originated in terrestrial, littoral, and pelagic habitats and in zooplankton size fractions, and data were analysed through Bayesian mixing models. Mesozooplankton showed seasonal variation in resource use, specifically a smaller dependence on algae in the wet than in the dry season. In the wet season, mesozooplankton relied more on the detritivore food chain eating mostly microzooplankton (mode: 95.1%), which in turn consumed mostly terrestrial C in this season (mode: 74.7%). Zooplankton size fractions also occupied different relative trophic positions between seasons. These variations seem to follow the seasonal dynamics of in-lake primary production and of terrestrial C inputs. Also, all size fractions of zooplankton, and particularly Chaoboridae larvae, showed low C staple-isotope values, suggesting the consumption of a missing C source.