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Functional responses of fish communities to environmental gradients in the North Sea, Eastern English Channel, and Bay of Somme

Abstract : The ensemble of biological, geochemical, and physical processes that occur within ecosystems is driven by the interplay between biological communities and the abiotic environment. Explaining the spatial and temporal dynamics of biological communities in relation to environmental conditions is therefore essential for understanding ecosystem functioning, and ultimately for achieving sustainable development. In marine ecosystems, fish communities are key to ecosystem functioning, and fisheries provide livelihoods for over 10% of the world’s population. However, understanding the processes structuring fish communities remains difficult because community structure varies with both natural environmental fluctuations and, increasingly, human pressures. Effectively managing fisheries and marine ecosystems under global change therefore requires better characterizing fish community dynamics over time and space and disentangling the underlying drivers and mechanisms. While fish ecologists have traditionally relied on species-based approaches (i.e., taxonomic approaches) to study community structure, trait-based approaches (i.e., functional approaches) are increasingly used because they can provide better insight into community assembly and the mechanisms driving community responses. To meet this need for a better understanding of biodiversity dynamics, the present thesis took advantage of long-term scientific monitoring data to characterize the functional responses of fish communities to environmental gradients in the North Sea, Eastern English Channel, and Bay of Somme. All three ecosystems experienced temperature rises and oceanographic changes associated with a warming phase of the Atlantic Multidecadal Oscillation (AMO), which rapidly impacted fish community structure. Consistent biological responses were observed across the three ecosystems despite their different spatial scales, demonstrating that fish communities were affected by environmental change through bio-ecological traits associated with habitat preference and life history. In the North Sea and Eastern Channel, pelagic species were the most responsive and contributed largely to community dynamics, which is likely explained by their greater mobility, higher dispersal rates, and fewer habitat requirements. However, beyond habitat preference, species with r-selected life histories (e.g., low size and age at maturity, low parental investment, small offspring) had the fastest environmental responses whether or not they were pelagic, likely due to their rapid population turnover and generation time. Importantly, the way these species’ responses shaped community structure depended on environmental context. R-selected, pelagic species rapidly declined in the Bay of Somme and Eastern Channel, but rapidly increased in the North Sea. This likely reflects environmental suitability, indicating that after the phase change of the AMO, the Eastern Channel became a less favorable environment for these species, while the North Sea became more favorable. Thus, species with high mobility and fast life history cycles appear capable of rapidly tracking environmental conditions, shifting in abundance in response to environmental suitability. Additionally, as these ecosystems have warmed over the last 30 years, community responses were characterized by increases in mean thermal preference. Importantly, the amplitude of community changes was partially determined by communities’ initial structure and redundancy of bio-ecological traits, showing that community responses depended not only on environmental changes but also on biodiversity itself. Lastly, while fish community responses were consistently associated with climatic changes, historical fishing pressure on large-bodied, demersal species appeared to render fish communities more sensitive to environmental changes by increasing the relative of abundance of pelagic and r-selected species.
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Matthew Mclean. Functional responses of fish communities to environmental gradients in the North Sea, Eastern English Channel, and Bay of Somme. Ecology, environment. Université du Littoral Côte d'Opale, 2019. English. ⟨NNT : 2019DUNK0532⟩. ⟨tel-02384271⟩

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