Evaluating eutrophication effects on rockys-shore epifauna with citizen science

Rocky shores, a key component of coastal ecosystems, are characterized by canopy-forming brown macroalgae that support high biodiversity and provide essential ecosystem services. These environments are increasingly impacted by multiple stressors, yet there remain significant knowledge gaps regarding their impacts on biodiversity. For instance, the effects of eutrophication on rocky shore biodiversity have been extensively studied in macroalgae, but much less is known about these effects on epifauna biodiversity inhabiting these biogenic habitats, predominantly characterized by gastropod species. 

Disentangling the effects of eutrophication from other natural or anthropogenic drivers is challenging as epifaunal communities exhibit complex ecological responses to local conditions such as the substrate type, shore height, wave exposure, or canopy density preference. However, rocky-shore communities are easy to sample and monitor due to their accessibility and ease of identification. These characteristics make them potentially valuable for monitoring changes in coastal environments through citizen science programs. 

Our main research question here is whether a citizen science program focusing on epifauna can help assess the effects of eutrophication on rocky shore gastropods, which in turn would make it a suitable indicator to monitor changes in coastal ecosystems. We explore this question in a transdisciplinary initiative driven by a co-construction process between scientists and citizens, the ESPOIRS project. This project is based on an existing citizen science project “Algues Brunes et Bigorneaux” (“Brown Algae and Winkles”) which was established in 2011 and previously documented changes in epifaunal rocky shore communities due to anthropogenic stressors at a regional scale. To correctly address these questions with a citizen-science protocol, a spatial sampling design is necessary to control for confounding factors. To this end, we collaboratively developed with citizen scientists a sampling framework focusing on estuaries where eutrophication is most pronounced, while minimizing wave exposure, a factor known to significantly shape intertidal communities. Together with volunteers, we investigated seven estuaries across Brittany (France), representing diverse environmental conditions and varying concentrations of inorganic nitrogen (NO₂ + NO₃) from spring to fall 2024. In each site, we estimated the density and size structure of key gastropod species found in mid-eulittoral brown macroalgae belts (e.g., Fucus vesiculosus and Ascophyllum nodosum) with a 0.1 m² quadrat. Additionally, the macroalgal communities within and around each quadrat were described to both control for other local effects and investigate whole community responses. This study provides new insights into understanding responses of rocky shore biodiversity to anthropogenic pressures and opens new prospects for assessing environmental quality through citizen science.