Ecological change in the early Anthropocene: Insight from death assemblages of the locally extinct European flat oyster (Ostrea edulis) in the Sylt-Rømø Bight

The European flat oyster (Ostrea edulis) provided relevant ecosystem functions and services as a keystone filter feeder and habitat forming species across the North Sea. At the turn of the last century, the species suffered a severe decline following a brief period of massive economic exploitation. Today, a few local populations still exist in e.g. the UK, the Netherlands, and Denmark. In the German North Sea, O. edulis is classified as functionally extinct.

The Sylt-Rømø Bight is one of the largest tidal back-barrier environments of the European Wadden Sea and is located on the border between Germany and Denmark. The environment underwent significant changes over the last millennia driven by natural landscape dynamics and, more recently, by anthropogenic claims to land and coastal resources. Despite its demise as a living organism, O. edulis shells are still widely found in the area and remain a part of the sedimentary environment that potentially bears evidence of past ecological and geomorphological change. Written and graphic accounts on the past state, composition and distribution of Ostrea reefs are rare and unreliable.

In this case study, we explore shell assemblage counts, geospatial data, and radiocarbon dating as tools to better understand ecological change over the past few centuries. More than a hundred dredge profiles were collected in the German part of the Sylt-Rømø Bight during two campaigns in summer 2021 and 2022. The results show marked spatial differences in the shell material regarding total dredged volume, overall species composition, size distribution, and taphonomic state. These differences can partly be explained by landscape dynamics and the economic history of the area (blue mussel cultivation). Radiocarbon dating of Ostrea shells (n=42) from three locations suggests that Ostrea banks at more exposed sites were short-lived and dynamic features, while ages from a less exposed site suggest the continuous presence of Ostrea over (at least) three thousand years.

This example showcases that methods from geoarchaeological and geoscientific contexts can help to shed light on the more recent paleoecology of coastal environments over historical timescales, where sufficient depth of information is lacking from other sources. Beyond pure curiosity, the information has a clear value as benchmark to define aims for marine conservation and restoration efforts.