A warming climate pressures existing ecosystems, threatening its biological diversity as well as any ecosystem services provided. Coastal ecosystems are of particular concern in this regard as they are both under climactic stress and often support human societies. This project aims to better understand the ancient history of a Baltic coastal ecosystem. Environmental DNA (eDNA) is extracted from sedimental cores dating back 8,000 years from the island of Gotland. Ecosystem diversity is studied over the past centuries, during which Baltic sea fluctuated between states of freshwater lakes and salty oceans. We track key species e.g., cod (Gadus morhua) and bladder wrack (Fucus vesiculosus) asking if their presence/absence concurs with previous knowledge regarding ocean/lake status.