The Orinoco basin is one of the few locations where a large unregulated river adjoins a minimally-altered floodplain. The Orinoco therefore presents special opportunities for the study of natural floodplain phenomena. Past studies of mass flux along the Orinoco main stream and of the phasing in physical, chemical, and biological events on small segments of floodplain have produced several lines of evidence that indicate an unexpectedly high importance of algal organic matter (phytoplankton, periphyton) in sustaining the aquatic food webs of the Orinoco floodplain ecosystem. Other prominent sources of organic matter for the system include particulate material from upstream terrestrial environments, riparian litterfall, and aquatic vascular plants. However, current evidence indicates that the majority of biomass from these sources is degraded microbially without passing efficiently into consumer food chains. Stable isotopes appear to offer the best prospect for a direct test of this hypothesis. However, the usefulness of stable isotopes is to some degree site-specific because of variations in isotope signatures and in the number of compartments that must be resolved through the use of these signatures. For this reason, the present proposal outlines a plan for testing the feasibility of using stable isotopes in resolving the broad hypothesis concerning algal carbon in the Orinoco floodplain. Means and variances will be established for stable isotope signatures of C, N, and S in major organic-matter compartments at a representative site on the Orinoco floodplain, and methods for physical separation of functionally different compartments such as detritus and suspended algae will be tested. This information will be used in quantifying the resolving power of stable isotopes in the Orinoco floodplain.
