Comparison of Manila clam (Ruditapes philippinarum) diet between 2006 and 2024 in a changing coastal lagoon

Environmental changes are extensively reported in the literature, sometimes leading to long-term trends. This study focuses on Arcachon Bay, a coastal lagoon in SW France, which has been the scene of several environmental modifications over the past decades, including a severe decline of seagrass meadows and changes in its physicochemical features. In this context, this study compared the diet of Ruditapes philippinarum , an abundant and commercially exploited bivalve in the lagoon, between 2006 and 2024 using carbon and nitrogen stable isotopes. Univariate and Bayesian statistics identified a significant shift in the clam δ 13 C values, higher than the expected change due to the Suess effect (−0.63 ‰, in addition to the Suess effect), and a shift in δ 15 N values (+0.68 ‰). The isotopic niches overlapped between the two years (41%, CI 19-74%), and mixing model outputs indicated phytoplankton as the main food source, with an increase in its contribution to the clam diet between the two years (66 to 88%). Concomitantly, the proportion of decayed seagrass, microphytobenthos and macroalgae decreased in the diet. While these results are based on two isolated time points, similar findings reported in the local, national and international literature from annual surveys support the hypothesis that these changes should be interpreted beyond interannual variability in Arcachon Bay suspension-feeding bivalves δ 13 C, and δ 15 N. Further work, particularly on the quality of food resources and trophic competition, would be required to better identify potential repercussions on ecosystem functioning, especially given the recent phytoplankton biomass decrease (2022-2025).