Lilia Serrano-Grijalva, Raul Ochoa-Hueso, Salvador Sánchez-Carrillo

Approximately 95% of the total carbon (C) in wetlands is typically found as particulate organic matter (POM). In this study, we evaluated the main C sources of a semiarid floodplain wetland in central Spain under disturbance. For this, we used stable isotopes (13C and 15N) and the Bayesian mixing model SISUS. We show that the allochthonous C derived from wastewater discharges have distinctive δ13C values compared to the C of sites that did not receive wastewater discharges. Wastewater discharges affected the isotopic signatures of dissolved organic carbon as well as δ15N values of primary producers (epiphyton, filamentous algae and Scirpus maritimus). δ15N values showed a greater response to the effects of environmental degradation. The salt-tolerant marginal tree Tamarix canariensis was the main autochthonous contributor to POM (69 to 84%), while the main source of energy for the secondary production was the submerged macrophyte Ceratophyllum submersum. Tamarix canariensis was also the main component of the diet for macroinvertebrates, followed by POM. Other species such as Cladium mariscus, an emblematic aquatic plant in the region, showed a very low contribution to POM (less than 20%), although it represented a relatively important energy source for zooplankton (up to 47%). This relatively low contribution is probably due to the drastic reduction in the abundance of this species during the last 50 years. In contrast, Phragmites australis, which has multiplied its abundance by twenty-two since 1950, does not seem to contribute significantly to POM or to the diet of invertebrates (less than 22%), although it can represent up to 54% zooplankton’s diet. Isotopic results confirm that in this eutrophied and disturbed wetland, the contribution of macrophytes to the food web as autochthonous organic sources is lower than expected based on their biomass.