Using otolith stable isotopes to reconstruct trophic baselines in the European sea bass (Dicentrarchus labrax)Biogeochemical Cycling

Stina Kolodzey¹, Yvan Pailler², Pierre Stéphan², Gauthier Schaal³, Peter Grønkjær¹

¹Aarhus University, Denmark - ²LETG, IUEM, UBO, France - ³LEMAR, IUEM, UBO, France

Marine food webs have been fundamentally altered due to human exploitation, compounded by accelerating anthropogenic stressors, such as climate change. Yet, the long-term impact on trophodynamics within marine food webs remains poorly understood. All bony fish possess calcium carbonate otoliths in their inner ears, which incorporate elements from the surrounding water as they grow. The preservation of otoliths in archeological deposits offers the possibility of studying the lives and environments of past fish. In this study, we analyzed the stable isotopic composition (δ¹³C and δ¹⁵N) of organic matter extracted from otoliths of European sea bass (D. labrax) collected from Late Neolithic archaeological sites in the Molène Archipelago ( 4000 years BP) and compared them to modern otoliths. Fossil otoliths had significantly lower δ13C and δ15N values than modern sea bass otoliths. While the potential for diagenetic alteration of total carbon and nitrogen content cannot be fully excluded, the observed isotopic shifts align with environmental and ecological differences between the two periods. The lower δ¹³C and δ¹⁵N values in fossil otoliths likely reflect changes in diet, cooler sea surface temperatures (SSTs) and the absence of human agricultural runoff during the Late Neolithic. Cooler SSTs increase CO₂ solubility in seawater, resulting in a δ¹³C-depleted dissolved inorganic carbon (DIC) pool that is reflected in the isotopic composition of primary producers and subsequently consumers. Elevated δ15N values in modern D. labrax could be nutrient enrichment through anthropogenic pollution and its effect on the value of δ15N in basal organic matter source pools. These findings offer valuable insights into the baseline isotopic composition of an important marine predator prior to significant human-induced environmental changes. By highlighting the long-term shifts in isotopic values, this study contributes to our understanding of anthropogenic impacts on marine ecosystems.