We used the stable isotopes of carbon and nitrogen to examine the food webs of three small floodplain lakes (billabongs) in south-eastern Australia. With few exceptions, stable carbon isotope analysis could not be used to discriminate among the conspicuous potential sources of fringing, emergent or floating vegetation or benthic detritus. These primary sources showed little spatial or temporal variation in delta(13)C values, with means ranging from -28.5 to -26.8% in spring and -29.1 to -25.4% in late summer. Submerged vegetation had similar delta(13)C values to the above sources in spring but showed greater spatial variation and were less C-13- depleted, considerably so in some species, in late summer. Epiphytes and algae were C-13-depleted in spring compared with the other primary sources but became more C-13-enriched in late summer. Mean delta(13)C values for primary and secondary consumers were not only far more variable (-37.4 to -22.7%) but in general were more negative than the potential food sources, particularly in spring. Using the combined information from stable carbon and nitrogen isotope analysis, we could narrow down the list of potential primary sources driving food webs in these billabongs. The freshwater crayfish (Cherax) was one of the few taxa that appeared to obtain its biomass carbon from detrital material. Gastropods and leptocerid caddis larvae on emergent or submerged vegetation obtained a mixture of carbon from epiphytes and macrophytes; in both taxa, epiphytes contributed more to biomass carbon than did the macrophytes. However, other common grazers and collector/ gatherers sampled from macrophytes, e.g. baetid mayflies, chironomid larvae and atyid shrimps, were often too C-13-depleted even to have derived their biomass carbon solely from epiphytes. Many other primary consumers, including zooplankton, and mussels (Velesunio), and most of the secondary consumers, including water mites (Hydracarina), phantom midge larvae (Chaoborus) and fish, were also C-13-depleted. The enormous biomass of littoral and fringing vegetation could contribute to metazoan food webs in these billabongs only if an additional highly C-13-depleted source was consumed simultaneously. Methane released from billabong sediments could provide such a C-13-depleted carbon source that is reintroduced into metazoan food webs via the consumption of methanotrophic bacteria. Alternatively, food webs in these water bodies are largely driven by an unknown and inconspicuous C-13-depleted primary producer, such as planktonic Chlorophyta.
