Biodiversity analysis around our underwater breakwater in Benin

All over the world, ecosystems are being squeezed. The culprits? Climate change, human activities and extreme weather conditions. Tropical islands are particularly vulnerable. The Atlantic Ocean guzzles up more and more land in the West-African state of Benin. This threatens not only houses and harbours, it also causes floods and soil erosion. Therefore, from 2018 to 2021, we built an undersea breakwater inspired by nature in Benin. Its foremost purpose was preventing erosion, but also creating a safer swimming area and perhaps also a hotspot for aquatic life. Three years later, we came back to assess the results of this construction.

Early 2018, we started huge coastal protection works off the coast at Avlékété. The Benin government was the client for this project. Concretely, we built a 5-kilometre-long undersea breakwater parallel to the coastline and carried out beach nourishment works at the same time. Thanks to this breakwater, the waves could still reach the coast, but with reduced force.

Centred around nature-inspired design

To design the breakwater, we were inspired by nature, coral reefs in particular. With this breakwater, we imitated the shape of a barrier reef. We positioned rocks where the subsoil was sand. These rocks formed a stable basis onto which marine organisms could attach. This created a reef-like habitat where sponges and corals thrive, but which can also serve as breeding ground for fish.

By focusing on this nature-inspired design from the start, we extended the purpose of this breakwater from a simple coastal protection device to a base for the creation of a rich and blooming ecosystem.

More than just building

In Benin, we didn’t stop at the handover of the project. Three after having completed the breakwater, we came back to the project location. Why? To investigate the long-term impact of the breakwater on the socio-ecological dynamics and biodiversity of the environment. On the one hand, we organised interviews with the fishing communities at Avlékété to verify the impact the breakwater had on fisheries. On the other, we investigated the marine biodiversity to check whether it had increased since the breakwater was built. For this, we used three methods:

We mapped out fish species

We monitored and identified the species of fish in nets cast by the fishermen in areas along the breakwater. We compared these with the species we found in areas without breakwater. Here, the taxonomic expertise of our local partners played a crucial part. The first results of this research are definitely promising. The areas around the breakwater contain a larger diversity of marine life than areas without such structures.

We took sediment samples

To analyse the marine life growing on the rocks and present inside the surrounding sand, we took sediment samples and sieved out the smaller organisms. At the same time, a team of professional divers collected corals, sponges, algae and other marine life growing on the rocks. To identify all these organisms, we worked with the University of Abomey-Calavi in Benin and the Institute for Agriculture and Fisheries (ILVO) in Belgium.

We applied the eDNA method

Finally, we also implemented a new technique: environmental DNA (eDNA). With this method, we extract genetic material from environmental samples, such as seawater, and we analyse it to detect and identify the species present in a specific environment.

In-depth analyses in 2024

In 2024, together with our partners, we start analysing all the data collected in Benin. This should give us a complete picture of how the ecosystem develops long-term and what the socio-ecological impact is of the breakwater we built between 2018 and 2021. Moreover, there are still many unknowns about marine biodiversity in the Central-African region. There too, we hope gathering new knowledge with our research results.