River ecosystem resilience to extreme flood events

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.

Keywords: climate change; extreme floods; macroinvertebrates; meiofauna; recovery; resilience; salmonids.

Figure 1

Discharge of Lemon Creek, Juneau, SE Alaska in 2014 with some events 8× median flow. Solid line = 2014 discharge; broken orange line = long‐term (30 yr) median

Figure 2

Cross sectional profiles of the WPC channel following both flooding events. (LB = Left Bank). Elevation is denoted in relation to a fixed bankside datum

Figure 3

(a) NMDS plots for macroinvertebrates from 1986 to 2015 using mean abundance data from ten replicates collected in August/early September and (b) meiofauna from 1994 to 2015 using mean abundance data from five replicates collected during the same time period. The immediate postflood years are outlined in red and black and the arrows highlight significant shifts in the macroinvertebrate community structure between the groups of years. Numbers represent years

Figure 4

Taxonomic richness and total abundance of macroinvertebrates and meiofauna over time. (a) macroinvertebrates from 1986 to 2015 using mean numbers from ten replicates collected in August/early September; and (b) meiofauna from 1994 to 2015 using mean numbers from five replicates typically collected in summer. Vertical dashed lines indicate the approximate timing of the two flood events and represent a flood occurring after that particular year's sampling

Figure 5

Comparison of the abundance of Diamesa davisii grp. and Pagastia partica (+/− 1SD). A significant negative relationship (p < 0.01) was evident from a generalized least squares (GLS) regression of log₁₀ abundance

Figure 6

Conceptual changes in river invertebrate communities following two extreme flooding events occurring at different times T₁ and T₂. Event T₁ may shift a community from state A to state B which is less complex and at an earlier successional stage, and therefore (a) less resilient, (b) more resilient, or (c) unchanged in resilience to a further extreme event at T₂ (see text)