Ørsted and the World Wide Fund for Nature Denmark are testing how 3D-printed reefs can benefit biodiversity in the Kattegat, a strait between Denmark and Sweden, which is experiencing a historically low cod stock. This is the first time 3D-printed reefs are used in Danish waters, and they will complement existing boulder reefs that Ørsted established when constructing the Anholt Offshore Wind Farm in 2012-13. The reefs are made from 70 % sand and 30 % pozzolanic cement made from volcanic ash and portland cement. These are natural materials and will not be harmful to the surrounding environment, even if parts of the reefs should erode over time. The 3D-printed reefs are biocompatible due to the absence of synthetic or toxic substances of industrial origin and the moderately basic pH of the structures at around 8.5-9. Each reef weighs between 200 and 550 kg. They are 1 cubic meters in size with a height of 1 meter. The 3D-printed reefs are designed and developed based on collaboration between WWF Netherlands and Reef Design Lab. The reef structures are produced by the Italian company D-Shape.Ørsted and WWF have deployed 12 3D-printed reef structures on the seabed between the wind turbines at Anholt Offshore Wind Farm in the Kattegat, which is part of the Greater North Sea ecosystem. Among other things, overfishing, increasing oxygen depletion, and habitat loss have resulted in a decline of the cod stock in the Kattegat for the past 20 years. It is now alarmingly low. This creates a negative domino effect in the Kattegat ecosystem, impacting biodiversity and the marine ecosystem’s resilience against climate-driven changes.The two partners behind the project hope that it will have positive effects on the Kattegat cod stock and in turn contribute to a healthier, more resilient marine ecosystem with improved biodiversity.In recent decades, attention has been drawn to the fact that the extensive overconsumption of marine resources has had enormous consequences. Globally, these consequences include dwindling fish stocks, oxygen depletion near the seabed, and a loss of habitats for marine life, such as massive reduction of eelgrass meadows. Cod is an important top predator, meaning it preys on other marine species and thereby contributes greatly to maintaining the balance in the marine ecosystem. When cod gets greatly reduced in numbers, the abundance of their prey – such as the green crab – will grow. As a result, seagrasses decline because the crabs eat both the seeds of the eelgrass and many of the snails that themselves keep the eelgrass free from being overgrown with algae.This is a problem, as eelgrass is itself of great importance for both biodiversity and the climate. It provides important habitats for marine life, such as juvenile fish, it produces oxygen, and it stabilises the seabed. Crucially, eelgrass also very effectively stores carbon in its root network, preventing it from ending up in the atmosphere and contributing to the global temperature rise.