Seasonal isotopic shifts in fish of the Pantanal wetland, Brazil

Abstract

Seasonal inundations shape the floodplain characteristics of the Pantanal, a large wetland in Central South America. In the first study combining stable carbon and nitrogen isotope analysis with classical stomach content analysis in this region, we investigated the influence of the annual inundation on diet and isotopic composition of floodplain fish. Apart from potential food items, 33 fish species from the Coqueiro Lake were analyzed, 10 of which were present during both the wet and dry season 1999. A δ13C and δ15N plot of the floodplain ecosystem allowed us to assess a foodchain of 3–4 trophic levels. However, the wide overlap of nitrogen values suggested that the organisms act on trophic continua rather than on distinct levels. The foodweb was based mainly on C3-plant carbon. However, fish species capable of feeding on terrestrial invertebrates (e.g., Brycon microlepis) had δ13C values above –25‰, indicating 13–30% intake of C4-plant based carbon during the flooding period. The novel use of vector coordinates and 2-dimensional ANOVA showed that the seasonal isotopic shifts of δ13C and δ15N were highly significant for some feeding guilds. δ15N values increased from the wet to dry season in most fish species, and these shifts were highly significant for omnivores (Astyanax bimaculatus, Triportheus nematurus, Tetragonopterus argenteus and Moenkhausia dichroura), and significant for invertivores (Gymnogeophagus balzanii and Poptella paraguayensis) and carnivores (Serrasalmus spilopleura). Average carbon isotope ratios decreased at the same time in the herbivores (Methynnis mola: 3.4‰) and detritivores (Psectrogaster curviventris: 5.3‰), but they did not change in the hypostomatic herbivore Sturisoma robustum. We explain these shifts by abundant and variable food sources during the inundation period and increasing carnivory and starvation during the dry season when the lake is confined to its central basin. Isotopic shifts between seasons were more prominent in less specialized species of omnivores, invertivores and some carnivores, whereas more specialized herbivores and detritivores appeared to be more influenced by changes in the carbon isotope ratio of the diet affected by biogeochemical processes such as respiration and methanogenesis. A general model for the interpretation of isotope data of floodplain fish considering different time-scales is given.