Autotrophic Carbon Sources for Fish of the Central Amazon

Effective management of the Amazon's commercial fish populations requires an understanding of the factors controlling their production. A fundamental step in the investigation of fish production is to identify the plant groups that contribute energy to fish foodwebs. Stable isotope data for plants and 35 fish species were used to identify autotrophic carbon sources for the central Amazon fish community. Adult fish, aquatic macrophytes, tree parts, periphyton, and phytoplankton were collected in lakes and other flooded environments along the central Amazon floodplain and analyzed for carbon stable isotope composition (°¹ ³C) by mass spetroscopy. °¹ ³C values for plants ranged from —39.4 to —11.9% with averages of —33.3, —28.8, —27.6, —26.2, and —12.8% for phytoplankton, flooded forests trees, C₃ aquatic macrophytes, periphyton, and C₄ macrophytes, respectively. The average for all C₃ plants (phytoplankton, trees, C3 macrophytes, and Periphyton) was —29.1%, while the average for C₄ plants (mainly C₄ macrophytes) was —12.8%. Mean °¹ ³C values for adult fish ranged from —37.0 to —19.8% with an average of —28.8%. Fish and plant data were used in an isotope mixing model to estimate the relative contribution of different plant groups to fish carbon. C₄ macrophytes, which contributed over half of the primary production on the floodplain, accounted on average for only 2.5—17.6% (minimum to maximum) of the carbon in fish. The C₃ plants, as a group, were the primary carbon source for 34 fish species, and accounted for an average of 82.4—97.5% of the carbon in all species. Phytoplankton, a minor C₃ producer, accounted for a minimum of 36.6% of fish carbon on average, and was the principal carbon source for the commercially important characiform detritivores. Several alternative hypothese are proposed to explain the apparent selective transfer of C₃ carbon through Amazon fish foodchains.