Feeding ecology of the Uruguay Tetra Cheirodon interruptus (Pisces: Characidae) in subtropical wetlands during dry and wet seasons based on stomach content and stable isotopes
The Cheirodon interruptus is a species that occurs in the South America that has an omnivorous, feeding behavior with a tendency to herbivory. Our goal was to investigate diet composition of C. interruptus and the relative importance of basal food sources sustaining this species in subtropical wetlands. We sampled fish and their main food sources in two freshwater ecosystems located within an Ecological Federal Reserve in Brazil called Taim. Food web components were obtained in two distinct lakes (locally know as Cacapava and Nicola). Both lakes are subject to annual hydrologic pulse associated with the rainy season. Therefore, we sampled during the dry and wet seasons with the expectation that flooding during winter would promote higher aquatic and terrestrial connectivity and this would reflect changes in diet. The stomach of each fish was analyzed based on frequency of occurrence, numerical abundance and volume occupied of each food item. In order to investigate the relative importance of basal food sources sustaining the Uruguay Tetra we determined the carbon and nitrogen isotope ratios of representative primary producers (macroalgae, periphyton, aquatic and terrestrial plants - C3, C4) and other organic basal sources as seston (phytoplankton and detritus) and organic matter in the sediment.
A two-way ANOVA showed no statistically significant differences in carbon isotope ratios (δ13C) between seasons (p>0.05), but revealed differences in δ13C between lakes during winter (Cacapava: -21.88‰ and Nicola: -24.37‰). The absence of differences in δ13C between seasons did not corroborate our initial hypothesis and suggest that the trophic link between this fish and basal food sources remain similar regardless the higher terrestrial-aquatic connectivity during winter. Nevertheless, it was possible to observe minor changes between seasons in carbon isotope ratios that could be linked to changes in fish diet. For instance, average δ13C decreased from to -23.25‰ to -24.37‰ at Nicola and this coincided with a higher consumption of 13C depleted C3 plants in their stomach content during the winter. Average δ15N values presented significant differences (p<0.05) between seasons at Cacapava. Nitrogen isotope ratios showed lower average values during the winter (6.52‰) than summer (7.91‰). An alternative explanation for this pattern could be that the flood pulse altered the overall nitrogen baseline of the lagoon between seasons (higher in summer than winter). Future studies encompassing additional predators and food web components are needed to evaluate the effects of the hydrologic pulse in the structure and dynamics of this subtropical wetland.