Assessing the ecological vulnerability of natural flood defences and risks to agricultural land following seawater flooding
Anthropogenic climate change is intensifying extreme weather events, affecting the natural environment by altering ecosystems, and leading to habitat loss. Pressingly, the frequency and severity of storm surge flooding in low-lying coastal regions is projected to increase under future climate scenarios. There is a growing need to understand how these events will impact coastal ecosystems, and the measures that can be taken to mitigate against these effects and protect natural resources. Furthermore, with increased pressures placed on agriculture to meet food demand, protecting arable land resources in coastal regions from the rising threat of flooding is imperative. This thesis explores how simulated storm surge flooding will affect natural and cultivated coastal ecosystems. Chapter 2 investigates how coastal grassland plant species respond to pulse episodes of simulated seawater flooding in laboratory-based experiments. Taxonomic trends in species responses indicate Fabaceae as extremely sensitive, while Poaceae exhibited considerable tolerance to the imposed flooding scenarios. More widely, this showed that storm surge events are expected to cause shifts to natural coastal plant communities, with high likelihood that key plant families will be filtered out of coastal habitats following pulse episodes of seawater flooding. Coastal plant communities have a vital role in flood defence, protecting valuable terrestrial land resources such as agriculture. Any loss to their functional capacity could reduce ecosystem resilience and result in heightened flood risk to inland areas from future storm surge events. Chapter 3 investigates how archetypal pioneer marsh species, Spartina anglica, responds to pulse episodes of simulated seawater flooding in a greenhouse-based experiment. The results demonstrate that this species exhibits a strong tolerance to short-term immersion, with no mortalities and minimal effects on growth and biomass. These findings suggest that established marsh communities dominated by resilient pioneer species may maintain critical ecosystem functions, such as shoreline stabilisation and flood defence, even after disturbances from storm surge events. Additionally, the use of compositionally different seawater substitutes (complete marine aquarium salts vs NaCl only solutions) can influence observed physiological plant responses, providing important methodological considerations for future studies. Chapter 4 explores how four common commercial crop species cultivated in coastal regions expected to be at risk of storm surge flooding respond to acute simulated seawater flooding in greenhouse-based experiments. While crop performance was negatively affected, responses between species varied considerably. Indeed, barley and oilseed rape were tolerant, while wheat and rice were extremely sensitive. Furthermore, aside from species-specific differences, relative tolerance shifted with crop ontogeny. These results are critical to understanding which species can be reliably grown in ‘at risk’ coastal areas and can inform on sowing and harvest regimes based on when crops will be most vulnerable to seasonal flood events. In Chapter 5, ecophysiological processes related to the effects of pulse seawater flooding on soil decomposition and crop–herbivore interactions were investigated in greenhouse-based experiments. Results indicated that key soil processes can be impeded, leading to reductions in decomposition and nutrient cycling, with probable cascading effects for crop efficiency. Additionally, the likely negative physiological impact on crop plants following seawater flooding elicited reduced aphid survival and fecundity. The impact of plant selection by herbivores on crop plant yield and (by inference), plant species interactions in (semi-) natural coastal ecosystem plant community is consequently, likely to shift following seawater flooding. Overall, this thesis contributes to understanding how plant communities are set to experience the effects of storm surge flooding. Not only do I demonstrate how community composition is likely to be altered in key coastal ecosystems like dunes, salt marsh and grasslands, but that coastal agriculture may be greatly influenced. These results could contribute to more informed management in coastal regions that focus on reducing climate pressures on coastal vegetation through accommodation strategies (such as through managed realignment) to increase ecosystem resilience and allow enhancement of nature-based defences.
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