Historical changes in mollusc communities from a temperate chenier ridge system (Mont-Saint-Michel Bay, France)

Abstract

Abstract Coarse-grained chenier ridges contain abundant mollusc shells that help to reconstruct the evolution of marine ecosystems throughout the Holocene, but few temperate environments have been studied. Historical surveys of the mollusc death assemblages sampled in a chenier ridge system from the southern English Channel were compared. Present-day active ridges provide a regional-scale, time-averaged signal of the subtidal mollusc diversity. Back-barrier fossil ridges dated to the Early Medieval Period document pre-industrial baseline conditions. Shellfish farming and invasive species had little impact on mollusc death assemblages recovered from active ridges so far. However, detection of these recent changes is delayed due to taphonomic inertia as shell ages can span more than 1000 years, even in active ridges. Differences in taxonomic and functional composition between fossil and present-day ridges show a decline in grazing and carnivorous species related to macrophytobenthic habitats (seagrass beds and macroalgal belts) and a shift towards a suspensivore-dominated community. The study confirms water temperature as a major driver of ecological change in the English Channel, as warm-water species are more common in present-day ridges. Shelly chenier ridges are useful records of ecological changes in coastal environments that offer a temporal coverage far exceeding that of long-term ecosystem monitoring studies.