River ecosystem resilience to extreme flood events

Abstract

Floods have a major influence in structuring river ecosystems. Considering projectedincreases in high-magnitude rainfall events with climate change, major floodingevents are expected to increase in many regions of the world. However, there is uncertainty about the effect of different flooding regimes and the importance of floodtiming in structuring riverine habitats and their associated biotic communities. In addition, our understanding of community response is hindered by a lack of long-termdatasets to evaluate river ecosystem resilience to flooding. Here we show that in ariver ecosystem studied for 30 years, a major winter flood reset the invertebratecommunity to a community similar to one that existed 15 years earlier. The community had not recovered to the preflood state when recurrent summer flooding 9 yearslater reset the ecosystem back to an even earlier community. Total macroinvertebrate density was reduced in the winter flood by an order of magnitude more thanthe summer flood. Meiofaunal invertebrates were more resilient to the flooding thanmacroinvertebrates, possibly due to their smaller body size facilitating greater accessto in-stream refugia. Pacific pink salmon escapement was markedly affected by thewinter flood when eggs were developing in redds, compared to summer flooding,which occurred before the majority of eggs were laid. Our findings inform a proposedconceptual model of three possible responses to flooding by the invertebrate community in terms of switching to different states and effects on resilience to futureflooding events. In a changing climate, understanding these responses is importantfor river managers to mitigate the biological impacts of extreme flooding effects.