Forests of large brown algae, commonly known as kelp, play a vital role in mitigating global climate change and maintaining the stability of marine ecosystems. These fascinating ecosystems absorb large amounts of CO₂ due to their rapid growth. As kelp only grows on hard surfaces, such as rocks or stones, the CO₂ it absorbs cannot be stored directly in the soil at the site. However, dead algae that end up in the deep sea, along with the CO₂ they have absorbed, can become trapped in the sediment there. Furthermore, brown algae help counteract ocean acidification and protect coastal areas from erosion. However, these underwater forests, some of which can reach heights of up to 40 metres, are disappearing at an alarming rate worldwide. Therefore, kelp forests are set to be reintroduced in many places.
Researchers at Leibniz University Hannover have investigated how these underwater rainforests can be restored more efficiently. A team from the Ludwig Franzius Institute of Hydraulic, Estuarine and Coastal Engineering has been investigating the conditions required for kelp to establish itself and thrive on the seabed, using AI-assisted video analysis and other methods. The conventional method of planting algae individually into the seabed is extremely time-consuming and expensive. Therefore, scalable approaches such as the Green Gravel method are becoming increasingly important in order to counteract the decline of kelp. In this method, researchers grow kelp spores on small stones in laboratories before releasing them into the ocean. However, this method is only successful if the stones remain in place at the target location until the kelp has firmly attached itself to the rocky substrate. If waves and currents carry them prematurely into unsuitable areas, such as those that are too deep or lack light, the kelp will not be able to take hold.
The success of this method is determined within the first few weeks at sea. Before the kelp can anchor itself firmly to the seabed using its holdfasts, the stones must remain stable. In wave current flumes, the research team has systematically investigated the forces required to set kelp-rock systems in motion. The researchers used AI-supported video analysis to precisely document how the algae act like tiny sails, greatly increasing the surface area exposed to the water current.
The study provides insights that will greatly facilitate the planning of future reintroduction projects. Even kelp plants as small as 10 centimetres in length can significantly reduce the stability of stones. Therefore, the environment must be designed in such a way that as many pebbles as possible can remain in place. Analyses have shown that the roughness of the rocky seabed has a much greater impact on stability than the size or shape of the stones themselves. The stones become wedged on uneven surfaces such as coarse gravel, enabling them to withstand much higher loads than on smoother surfaces such as rock or concrete. As soon as the algae start to grow, stability decreases. However, beyond a certain point, this effect is offset by the friction of the algae fronds against the ground, which can in turn have a braking effect. Contrary to expectations, the team was surprised to find that moderate slopes of up to 15 percent played a rather minor role in the initial rolling of the pebbles, provided the surface was sufficiently rough.
A calculation model was developed using the data that accurately predicts the weight of the stones needed for the kelp. This allows researchers to determine in advance whether the stones at each location will be able to withstand the local waves and remain securely in place. These findings will enable reintroduction projects to be planned more precisely, significantly increasing the chances of success for these important marine carbon sinks.
The study is freely available at: 10.1371/journal.pone.0345763
The research project is part of the sea4soCiety project within the DAM CDRmare research mission, funded by the Federal Ministry of Research, Technology and Space, and federal states in Northern Germany.
Note to editors:
For further information, please contact PhD Maike Paul, Ludwig Franzius Institute of Hydraulic, Estuarine and Coastal Engineering (tel. +49 511 762 2584, email paul@lufi.uni-hannover.de).
