Ana Bratoš Cetinić

1 Introduction Ocean acidification (OA) is a well-documented threat to marine ecosystems caused by the extensive anthropogenic emissions of carbon dioxide (CO2). In the past 250 years, CO2 levels have increased by up to 50% as compared to pre-industrial times (Friedlingstein et al. 2022). The oceans absorb around 27% of the released carbon dioxide, leading to a significant change in the carbonate chemistry of seawater (Doney et al. 2009).

Abstract Ocean ecosystems have been subjected to anthropogenic influences for centuries, but the scale of past ecosystem changes is often unknown. For centuries, the European flat oyster (Ostrea edulis), an ecosystem engineer providing biogenic reef habitats, was a culturally and economically significant source of food and trade. These reef habitats are now functionally extinct, and almost no memory of where or at what scales this ecosystem once existed, or its past form, remains.

Abstract Anthropogenic activities have impacted marine ecosystems at extraordinary scales. Biogenic reef ecosystems built by the European flat oyster (Ostrea edulis) typically declined before scientific monitoring. The past form and extent of these habitats thus remains unknown, with such information potentially providing valuable perspectives for current management and policy.