Flood events from climate extremes drastically shift prey energy densities

The variability and intensity of climate events is increasing with global warming. Estuaries are subject to more frequent and severe droughts and floods, which impact biota directly and through environmental changes. Impacts on prey propagate through food webs by affecting energy availability for higher trophic-level predators. This study investigated changes in energy density of macroinvertebrates and small-bodied fish in the estuary of Australia's largest river system, before and after the largest flood in 70 years, to test impacts on prey for birds and fish in this Ramsar listed wetland. The flood led to a pronounced shift in prey energy density and changes in several environmental conditions, including in the hypersaline southern parts of the ecosystem, where salinities substantially declined, allowing prey to colonise this region. The spatiotemporal trends in macroinvertebrate energy density were more distinct than for fish and largely followed salinity gradients. The area of greatest prey energy density shifted from near the river mouth before the flood to approximately 50 km into the coastal lagoon after the flood. Around 10 months after the flood peak, after an initial decline, macroinvertebrate energy density had increased again in the northern estuary region and remained elevated in the south, thereby expanding the area with moderate energy resources. The effects of this flood demonstrate that higher flows could help enhance energy provisioning for predators by reducing hypersaline conditions. Overall, this study improves our understanding of the effects of extreme climate events on prey dynamics and trophic functioning in estuarine food webs.