Fishes in a seasonally pulsed wetland show spatiotemporal shifts in diet and trophic niche but not shifts in trophic position

We examine temporal (seasonal) and spatial (habitat) effects on consumers' diet, trophic position, trophic niche, and food-web topology in a subtropical oligotrophic wetland to illustrate how consumers and food webs respond to hydrologic pulsing in a spatially complex ecosystem. We ask if the annual flood pulse causes fishes to undergo a trophic shift or if fishes maintain a constant diet, trophic position, and trophic niche all year and across habitats, as is often assumed. Furthermore, we ask if the flood pulse alters food-web topology in different habitats and if invasive fishes overlap in trophic niche with native fishes in this ecosystem. We found that trophic dispersion (shift in trophic-niche size) was common (66% and 71% of spatial and temporal comparisons respectively), trophic displacement (shift in trophic-niche location in niche space) was ubiquitous (spatial – 92%; temporal – 82%), and shifts in trophic position were relatively rare (spatial – 11%; temporal – 4%). Trophic dynamics were primarily driven by differing amounts of piscivory, detrital consumption, and diet plasticity across habitats and seasons. In the dry season, food-web topology indicated decreased complexity in all habitats (decreased number of links, link density, and connectance) and instability in ponds that may facilitate invasions. Both stomach contents and stable isotopes revealed trophic-niche overlap among native and invasive fishes, notably centrarchids and cichlids. Diverse, flexible trophic responses to seasonality across habitats may be pivotal to nutrient and energy cycling and in maintaining ecosystem stability and resilience, especially in regularly perturbed environments. Seasonal fluctuation typical of wetlands may require inter-habitat relocation, leading to the types of food-web changes we document. We conclude that spatiotemporal trophic plasticity is probably common and deserves additional study given its ability to influence food-web structure and function.