Flooding affects food web structure and basal sources supporting fish guilds in a subtropical wetland and shallow lake

Aquatic ecosystems exchange nutrients and organic matter with surrounding terrestrial ecosystems, and floods import allochthonous material from riparian areas into fluvial systems. We surveyed food web components of a wetland and shallow lake in a subtropical coastal region of Brazil to examine how community trophic structure and the entrance of allochthonous material into the food web were affected by floods. Stable isotope analysis was performed for samples of terrestrial and aquatic basal production sources and aquatic animals to trace the origin of organic matter assimilated by aquatic animals and estimate vertical trophic positions and food chain length. Lake and wetland trophic structures were compared for cool/wet and warm/dry seasons. Food web structure was hypothesized to differ based on hydrology, with the more stable lake having greater food web complexity, and seasonal flooding resulting in greater allochthonous inputs to the aquatic food web. We compared spatial and temporal variation in assemblage trophic structure using an adapted isotopic ellipse approach that plots assemblage elements according to δ13C on the x-axis and estimated TP on the y-axis. Lake trophic structure was more complex with longer food chains compared to that of the wetland. A greater contribution from terrestrial resources to animal biomass was observed in the wetland during the cool/wet period, and food chains in both habitats tended to be longer during the cool/wet period. Findings supported the hypothesis of greater assimilation of allochthonous sources during floods and greater trophic complexity in the more hydrologically stable system.