Climate change has made the Wadden Sea 1.5°C warmer in the last 30 years. When Pacific oysters were imported, people still thought the water along the North Sea coast is adequate only for growth, but too cold for reproduction. This soon proved to be erroneous. The larvae of these oysters drifted from oyster farms in the Netherlands and one near Sylt across the Wadden Sea, attached themselves in particular to common mussels and transformed mussel banks into oyster reefs. This has an impact on birds, like the eider duck, that eat mussels, but not oysters because of their thick and bulky shell. On the other hand, animal colonies and algae, which in turn provide small mobile animals and fish with an entirely new habitat, grow on oysters, which are constantly covered by water below the tidal zone. Japanese Sargassum seaweed was introduced with oysters in this way. It is tough, branched, grows to over three metres in length and forms a thick algae forest under water. In stormy seas waves grab the algae and carry them away along with the oysters hanging onto them. This leads to further spread. Due to the presence of Pacific oysters, there are more filtering organisms in the ecosystem as well as reef-like structures and more algae, but less to eat for the consumers in the upper part of the food chain. The combination of climate change and world trade with oysters has accelerated biological globalisation under water, thus increasing biodiversity at the regional level, but making biological differences between the coasts less distinct in a worldwide comparison. It just like with fast food restaurants, only that oysters and algae still do not appear on their menu.

The winter was icy this year and up to three quarters of the oysters did not survive it. The reefs with densely packed oyster shells have remained, however, and Sargassum seaweed grows as rapidly as before. The purifying effect of the cold spell will not be only of short duration. This change in the Wadden Sea is presumably irreversible.