Climate change

Can oyster reefs withstand rising sea levels?

A collaborative project involving Leibniz University has looked at whether oyster reefs and mussel beds on the German coast can adapt to rising sea levels caused by climate change.

A research alliance involving Leibniz University Hannover (LUH), Technische Universität Braunschweig and the Senckenberg am Meer Research Institute has investigated oyster reefs and mussel beds in Germany’s Wadden Sea mudflats. The aim was to establish whether the oyster reefs can, in terms of distribution and growth, adapt to tidal conditions and, as a result, also adapt to present sea-level rise. Oyster reefs play an important role in coastal protection within the Wadden Sea flats. The findings of this joint study by the above-mentioned three players have now been published in an article in the Frontiers in Marine Science journal.

Documenting growth dynamics of an oyster reef

Over the past two decades, and due to North Sea warming, the Pacific oyster (Magallana gigas) has expanded its range along the German coastline, displacing native species and turning mussel beds into oyster reefs. These new aggregations of oysters are – unlike the former mussel beds off Lower Saxony – extremely resistant to mechanical stress, and form reef-like structures that are continuously growing in both areal extent and height. The interdisciplinary research team has, over a two-year period, collected and classified drone data in order to determine both spatial distribution and increase in size of one of these reefs. This project has, for the first time, documented the growth dynamics of an oyster reef and of the driving factors at play, thanks to a combination of new remote-sensing techniques and photogrammetric analysis. Previously, it was chiefly other methods that had been employed to study reefs.

Oyster reefs adapt to local tide conditions

The major finding of this study (whose lead researchers were those of LUH’s Ludwig Franzius Institute of Hydraulic, Estuarine and Coastal Engineering) is that the oyster reefs in the tidal flats adjust to local tide conditions, enabling them to follow the observed rise in sea level caused by climate change. “The Pacific oyster forms a quasi-natural, growth-adaptable reef structure and thus assumes supportive coast-protection functions involving retention of sediment in the tidal flats and acting as a breakwater against wave energy,” explains the article’s lead author, Tom K. Hoffmann of the Ludwig Franzius Institute. Even an unusually long ‘dry fall period’ during the course of the study – with the reef exposed to frost over a long duration, and damaged both structurally and in terms of size – did not significantly affect the reef. It recovered rapidly and continued to grow strongly in the subsequent months.

Coastal and ecosystem protection

While these findings suggest that climate-induced transformation from mussel beds to oyster reefs along German coastlines is irreversible and profound, it also appears that that, given their robust nature, these reefs perform a valuable additional function in terms of coastal and ecosystem protection. This includes serving as breakwaters (to reduce wave energy), carbon sequestration, water filtration, nutrient supply and provision of new habitat.

Approach adopted in study transferable

The approach used by this study can be applied to future monitoring of other ecosystems. “It is essential for effective coastal management that we understand the dynamics of oyster reefs,” says Professor Torsten Schlurmann, director of the Ludwig Franzius Institute. Findings on the growth dynamics of reef structures and the advancement of model systems will enable planners and public authorities to assess the impact both of mussel beds and oyster reefs on the natural environment, and to devise appropriate policy measures for habitat and coastal protection.

BIVA-WATT joint project

This project, funded by Germany’s Federal Ministry of Education and Research (BMBF), forms part of a joint project called BIVA-WATT under which, between 2019 and 2022, several research programmes with different foci were conducted.