Examining the role of habitat quality in determining river ecosystem resilience to extreme flood events

Extreme flood events have occurred more frequently in recent years throughout the UK due to changing weather patterns, and are predicted to further increase in regularity. Although floods naturally maintain ecosystem diversity and redistribute energy, nutrients, sediments & biota, extreme floods can significantly impact river ecosystem biodiversity, ecosystem functioning & instream habitat quality. Understanding these impacts on river ecosystems is a major priority for the Environment Agency (EA) who are under pressure to balance competing needs of environmental protection policy (e.g. EU WFD, EU Habitats Directive) with pressures from government, businesses and the public for adequate flood protection.
Limited evidence suggests that river ecosystem resilience to extreme floods is dependent on instream geomorphic complexity and to the magnitude and timing of previous high flow events. However, instream habitat modifications are extensive throughout England, with 52% of lowland & 42% of upland rivers affected by bank reinforcement and/or re-sectioning largely for flood protection. This habitat alteration has contributed to widespread failures of UK rivers under the EU WFD.
Recent advances in river habitat modelling have allowed the extrapolation of semi-quantitative survey data of habitat modification throughout England's river networks. Large-scale habitat quality & river modification data can now be paired with long-term flow and ecology datasets, to test how habitat alteration may influence the ecological quality of the river ecosystem & resilience to extreme events. Data held by the EA (co-located macroinvertebrate samples, river habitat survey (RHS) & river flow data) consists of >12 years of continuous data from >16000 sites thereby affording possibilities for advanced statistical modelling of the resilience of macroinvertebrate populations (e.g. persistence, extinction, invasion) and community assembly/disassembly responses to floods. A pilot data-matching exercise on the dataset identified 37 sites in the North West area alone where flood, invertebrate and RHS data were sufficiently temporally and spatially paried to allow the use of these sites in a statistical model, thus proving the applicability of this dataset & approach. The addition of long-term river flow data from EA gauges (available via CEH's NRFA database and via the EA) to identify extreme flow events, in combination with additional collection of macroinvertebrate and RHS data by the student to validate & extend database records will allow the unique opportunity to identify flow thresholds (flow magnitudes or durations from long term flow data above which invertebrate communities may not recover from), flood-sensitive species or habitats for monitoring flood recovery. This would provide a predictive capacity of how river restoration efforts could mitigate high flow impacts through the provision of flow refugia.
Using this approach, the student will be able to test the following hypotheses; (1) habitat alteration & loss of complexity reduces the resilience of macroinvertebrate communities to extreme flood events, (2) RHS habitat quality indices can be used to predict macroinvertebrate community recovery and resilience to flooding events; (3) relationships between ecological community resilience measures and habitat alteration indices can inform guidelines for appropriate management and restoration approaches to mitigate flood effects.