Bimodality in stable isotope composition facilitates the tracing of carbon transfer from macrophytes to higher trophic levels.

Even though the suitability of macrophytes to act as a carbon source to food webs has been questioned by some studies, some others indicate that macrophyte-derived carbon may play an important role in the trophic transfer of organic matter in the food web of shallow lakes. To evaluate the importance of macrophytes to food webs, we collected primary producers-macrophytes and periphyton-and consumers from 19 South American shallow lakes and analyzed their carbon stable isotopes composition (δC). Despite the diversity of inorganic carbon sources available in our study lakes, the macrophytes' δC signatures showed a clear bimodal distribution: C-depleted and C-enriched, averaging at −27.2 and −13.5‰, respectively. We argue that the use of either CO or HCO by the macrophytes largely caused the bimodal pattern in δC signals. The contribution of carbon from macrophytes to the lake's food webs was not straightforward in most of the lakes because the macrophytes' isotopic composition was quite similar to the isotopic composition of periphyton, phytoplankton, and terrestrial carbon. However, in some lakes where the macrophytes had a distinct isotopic signature, our data suggest that macrophytes can represent an important carbon source to shallow lake food webs. [ABSTRACT FROM AUTHOR]