Investigating factors shaping epifaunal rocky shore communities using citizen science

Bruno Serranito^1,2, Cam-Ly Rintz^1,2,3, Agathe Bouet⁴, Isabelle Le Viol⁵, Célia Mebarki¹, Athénaïs Beauvois⁶, Frederic Ysnel^7,1,2, Eric Goberville¹, Eric Feunteun^1,2,3, Boris Leroy¹

¹Unité 8067 Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Muséum national d’Histoire naturelle, Sorbonne Université, Université de Caen Normandie, CNRS, IRD, Université des Antilles, France - ²Station Marine de Dinard, Muséum National d’Histoire Naturelle, France - ³EPHE-PSL, France - ⁴Association Planète Mer, France - ⁵Centre d’Ecologie et des Sciences de la Conservation (CESCO), Muséum national d’Histoire naturelle, CNRS, Sorbonne Université, Station Marine, France - ⁶Centre Max Weber (UMR 5283), CNRS, École Normale Supérieure de Lyon, Université Jean Monnet Saint-Étienne, Université Lumière Lyon 2, France - ⁷Université de Rennes, France

Rocky shores are dominated by canopy-forming brown macroalgae that sustain high biodiversity and provide essential ecosystem services. However, these environments face increasing pressure from multiple stressors, with notable gaps in understanding their impacts on biodiversity. While the effects of eutrophication on macroalgae have been well-documented, its influence on epifaunal biodiversity, particularly gastropods inhabiting these biogenic habitats, remains understudied. Factors such as substrate type, shore height, wave exposure, and canopy density further complicate the detection of potential eutrophication effects on these communities. Nonetheless, rocky shore communities are accessible and easy to identify, making them suitable for monitoring through citizen science initiatives. The ESPOIRS project was developed to investigate the multiscale drivers of rocky shore epifaunal community changes, focusing on eutrophication and biogeographical gradients. This study builds on the BioLit citizen science program, launched in 2011, which has already documented changes in epifaunal communities caused by anthropogenic stressors. To disentangle the effects of various environmental factors, a spatial sampling design was co-developed with citizen scientists to target estuaries in Brittany (France). Sampling took place from spring to fall 2024 in seven estuaries with varying levels of inorganic nitrogen (NO₂ + NO₃). Using a multispecies hierarchical modelling approach, we investigated the occurrence, density, and size of key gastropod species within mid-eulittoral macroalgae belts (e.g., Fucus vesiculosus) to examine the responses of epifaunal communities to multiple drivers. This study highlights the potential of citizen science to monitor biodiversity under anthropogenic pressures, offering new avenues for assessing environmental quality and fostering public engagement in coastal ecosystem conservation.