STABLE CARBON AND NITROGEN ISOTOPES IN ALGAE AND DETRITUS FROM THE ORINOCO RIVER FLOODPLAIN, VENEZUELA

Stable isotope ratios of carbon and nitrogen in autotrophs and detrital organic matter were investigated during inundation of the Orinoco River floodplain in Venezuela. Sampling emphasized microalgae and fine particulate detritus and included organic matter on submersed surfaces Of C3 and C4 vascular plants as well as organic matter suspended in the water column and in surficial sediments. Algae and detritus were separated and collected for isotopic analysis by density gradient centrifugation in colloidal silica. Fine particulate organic matter in floodplain waters contains variable proportions of algal material, and the isotopic composition of the algal fraction was often distinct from that of the detrital fraction. Because most floodplain vascular plants assimilate atmospheric CO2, their deltaC-13 values proved to be typical of those reported for terrestrial plants. Algae were quite variable in deltaC-13; phytoplankton were most depleted (deltaC-13, -34.0 to -37.2 parts per thousand), while epiphytic algae spanned a different and wider range in deltaC-13(-23.4 to -33.0 parts per thousand). Depletion in C-13 of organic matter mixtures relative to the range for C3 vascular-plant sources is caused by the presence of algal carbon, whereas enrichment is caused by algal carbon or by carbon from C4 vascular plants that grow on the floodplain. Phytoplankton and aquatic macrophytes varied relatively little in deltaN-15(1.5 to 4.1 parts per thousand), but epiphytic algae were more variable (0.1 to 6.5 parts per thousand). Algal samples in which N-fixing cyanobacteria were abundant were most depleted in N-15.