Abstract
Floods have a major influence in structuring river ecosystems. Considering projected increases in high‐magnitude rainfall events with climate change, major flooding events 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 flood timing in structuring riverine habitats and their associated biotic communities. In addition, our understanding of community response is hindered by a lack of long‐term datasets to evaluate river ecosystem resilience to flooding. Here we show that in a river ecosystem studied for 30 years, a major winter flood reset the invertebrate community 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 years later reset the ecosystem back to an even earlier community. Total macroinvertebrate density was reduced in the winter flood by an order of magnitude more than the summer flood. Meiofaunal invertebrates were more resilient to the flooding than macroinvertebrates, possibly due to their smaller body size facilitating greater access to in‐stream refugia. Pacific pink salmon escapement was markedly affected by the winter flood when eggs were developing in redds, compared to summer flooding, which occurred before the majority of eggs were laid. Our findings inform a proposed conceptual model of three possible responses to flooding by the invertebrate community in terms of switching to different states and effects on resilience to future flooding events. In a changing climate, understanding these responses is important for river managers to mitigate the biological impacts of extreme flooding effects.
Highlights
Floods are a defining and natural feature of the flow regime of many rivers (Lake, 2000; Poff et al, 1997) but flood magnitude and frequency are increasing worldwide with climate change (Coumou & Rahmstorf, 2012; Milner, Robertson, McDermott, Klaar, & Brown, 2013) which will markedly alter their role in structuring riverine habitat and their associated biotic communities (Jones, 2013)
In 1986, a continuous study was initiated of the ecosystem of Wolf Point Creek (WPC), a newly formed river sourced from a basin with ~70% glacial ice cover (58°59′49.84′′N, 136°9′57.05′′W) in Muir Inlet, Glacier Bay, Alaska
Overall the turnover of the meiofaunal community was smaller following the summer 2014 floods than after the 2005 winter flood, suggesting the impact of recurrent summer flooding on the meiofaunal community resilience was less than for the macroinvertebrates, perhaps because reproduction rates were high in response to relatively high water temperature (Dole-Olivier et al 2000)
Summary
Floods are a defining and natural feature of the flow regime of many rivers (Lake, 2000; Poff et al, 1997) but flood magnitude and frequency are increasing worldwide with climate change (Coumou & Rahmstorf, 2012; Milner, Robertson, McDermott, Klaar, & Brown, 2013) which will markedly alter their role in structuring riverine habitat and their associated biotic communities (Jones, 2013). | 8355 flows and associated habitats in which biological communities exist (Ledger & Milner, 2015), our overall understanding remains in its infancy (Coumou & Rahmstorf, 2012) Another key aspect of floods in addition to peak flow magnitude is their timing throughout the year, causing potentially different impacts, with respect to biotic communities (George, Baldigo, Smith, & Robinson, 2015). Heavy precipitation continued into August with 222 mm of precipitation falling (fifth wettest summer month on record) (Menne et al, 2012) These events created an extreme high-frequency series of recurrent discharge peaks (Figure 1 Lemon Creek proximal to the study area). Specific objectives were to (a) examine whether the timing of the extreme events resulted in different biological effects, (b) assess how far each event reset the respective invertebrate communities, (c) determine the effect of the floods on the resilience of the different components of the biological community, and (d) develop a conceptual model of community response to extreme flooding events
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