Trophic structure of Lake Tanganyika: carbon flows in the pelagic food web

The sources of carbon for the pelagic fish production in Lake Tanganyika, E ast Africa, were evaluated in a comprehensive multi-year study. Phytoplankt on production was assessed from seasonal in situ C-14 and simulated in situ results, using on-board incubator measurements and knowledge of the vertic al distributions of chlorophyll and irradiance. Bacterioplankton production was measured on two cruises with the leucine incorporation method. Zooplan kton production was calculated from seasonal population samples, the carbon contents of different developmental stages and growth rates derived from p ublished sources. Fish production estimates were based on hydroacoustic ass essment of pelagic fish biomass and data on growth rates obtained from leng th frequency analyses and checked against daily increment rings of fish oto liths. Estimates for primary production (426-662 g C m(-2) a(-1)) were 47-1 28% higher than previously published values. Bacterioplankton production am ounted to about 20% of the primary production. Zooplankton biomass (1 g C m (-2)) and production (23 g C m(-2) a(-1)) were 50% lower than earlier repor ted, suggesting that the carbon transfer efficiency from phytoplankton to z ooplankton was low, in contrast to earlier speculations. Planktivorous fish biomass (0.4 g C m(-2)) and production (1.4-1.7 g C m(-2) a(-1)) likewise indicated a low carbon transfer efficiency from zooplankton into planktivor ous fish production. Relatively low transfer efficiencies are not unexpecte d in a deep tropical lake, because of the generally high metabolic losses d ue to the high temperatures and presumably high costs of predator avoidance . The total fisheries yield in Lake Tanganyika in the mid-1990s was 0.08-0. 14% of pelagic primary production, i.e. within the range of typical values in lakes. Thus, no special mechanisms need be invoked to explain the produc tivity of fisheries in Lake Tanganyika.