Past trajectory of a Northwest Europe socio-ecosystem at the land-sea interface: Case study of the northern watersheds of the Bay of Brest over the last 150 years

The Bay of Brest (BB) is a macro-tidal estuarine environment that has been exposed to strong anthropogenic pressures over the past few decades, especially after the Second World War. This makes the BB an ideal site for studying coastal ecosystem transformations since the Industrial Revolution. We analysed 5 sediment cores collected in 2 key BB areas with varying exposure to marine hydrodynamic processes: i) Elorn sector (3 cores) and ii) Bay of Daoulas (2 cores) in the inner BB, close to the mouth of the Daoulas river. Our study used palynological (pollen and dinoflagellate cysts) and benthic foraminiferal assemblages, sedimentological data, and elemental geochemistry, at a very fine temporal resolution (Daoulas core: 2-year resolution; Other cores: 2 to 16-year resolution). Working at a local spatial scale (i.e. BB) reveals robust correlations between the different driving forces and environmental changes at a high temporal resolution (i.e. sub-decadal) over the last 150 years. Our statistical analyses of these paleoecological time series reveal major breakpoints, specifically 4 key threshold periods: 1925–1938, 1960–1967, 1980–1987, and 2005, which are essential for understanding temporal changes in protist communities and BB landscapes through time. By combining these findings with a reanalysis of regional precipitation patterns (modelling approaches using meteorological reanalyses), nutrient concentration data, and historical accounts of land-use practices and industrial/mining/Second World War activities, we can identify the main drivers of coastal ecosystem transformations. Our findings emphasize the strong influence of agricultural practices on trophic changes and water quality decline in the BB, with coastal observatory series showing a notable increase in toxic algal blooms since the 1980s.