Foodweb analysis of the Orinoco floodplain based on production estimates and stable isotope data

Stable isotope data and indirect methods for estimating consumer production were used in a foodweb analysis for the Orinoco River floodplain, Venezuel a. The ratio of annual production to mean annual biomass (P-a/(B) over bar) was estimated from mass at maturity for all major categories of consumers. Field data on (B) over bar then were used to estimate P-a, for each major category. Carbon sources for all categories of consumers were identified th rough delta C-13 analysis, and trophic shifts in delta N-15 were used in as signing trophic levels to consumers. The ultimate Carbon source for both in vertebrates and fish was algae (phytoplankton and periphyton), even though macrophytes and Litterfall from the floodplain forest composed 98% of poten tially available C. Production of invertebrate consumers (benthos, rhizofau na, and zooplankton), which occurred almost entirely through l(st)-level co nsumption, was estimated as 14 g C m(-)2 y(-1). Fish, which produced 11 g C m-2 y-(1), showed a mean delta N-15 trophic shift of 1.8 units above the a lgal C source, suggesting that similar to 20% of fish production was direct ly accountable to consumption of algal C (l(st)-level consumption), and tha t most of the remainder was attributable to l(st)-level carnivory. Data on production and trophic shifts implied trophic efficiency of 5% for inverteb rates and 20% for fish. Although the food web is focused on a very small fr action of potentially available primary C (algae), this C source can accoun t quantitatively for the observed production of both invertebrates and fish . The food web showed marked trophic compression (large consumers supported by trophic levels 1 and 2), which is the only means by which high producti on of large consumers can be sustained on a small fraction of the potential ly available C.