Posidonia, often mistaken for algae, is actually a marine flowering plant that forms extensive meadows in shallow waters. These seagrass meadows play a crucial role in marine and coastal ecosystems, providing valuable ecosystem services such as raw material production, food sources, coastline protection, water purification, and carbon storage. However, they face various threats, including global warming and human activities such as anchoring and fishing practices. Protecting Posidonia is essential to maintain the balance and health of our environment.
Facts on Posidonia you’ll want to know
First and foremost Posidonia is not an algae !
Although it is often mistaken for an alga (especially when it’s dead floating around), Posidonia is actually part of the marine magnoliophyta, a group of flowering plants that has sixty different species spread across the globe. In fact, they can be found on every continent except in Antarctica. They form vast meadows lying between 0 and 40 meters deep.
Seagrass meadows are one of the most important ecosystems in the world given the ecosystem services they provide us, but they only occupy one percent of the oceans.
Today, their monetary value per hectare largely exceeds the one of coral reefs or of the Amazonian Forest, which clearly show how important they are.
Posidonia plays a crucial role in the development of marine and coastal ecosystems.
Indeed, seagrass meadows are central in the production of raw materials and food, the protection of the coastline from erosion and the purification of water. This small diagram will help you to better understand Posidonia’s virtuous circle.
If Posidonia had not existed, would the Mediterranean coast be composed of so many small fishing ports? Would there be so many culinary specialties based on fish products? Would snorkeling be as spectacular?
Additionally, posidonia has carbon storage capacities and thus represents an important factor for the depollution of our environments.
When the plants die, we can identify a short-term carbon sink (mineralization mechanisms occurring between 2 to 6 years after the burial of the leaves), which is also observed in other species of Magnoliophytes, but also a longer-term “well” with sequestration within the matte.
Taking into account the average matte’s thickness (from 1 to 4 m), the carbon stored in the Mediterranean by the Posidonia meadows would represent from 11 to 42% of the carbon dioxide emissions produced by the Mediterranean countries, since the beginning of the industrial revolution. Imagine if it were to disappear…
Posidonia meadows develop very slowly and the oldest ones observed are several hundred thousand years old.
In the Mediterranean, between Formentera and Ibiza, a seagrass meadow was estimated to be 80,000 to 200,000 years old, making it the oldest living organism known to date on our planet. Biologists consider that 3 to 9% of posidonia meadows are multi-centennial or millennia old. Growing very slowly, these plants can live a very long time.
The threats to Posidonia and how they impact it
Global warming has a real impact on these plants. Although they are resistant to many hardships, including changes in the pH of water or its pollution, they seem to suffer from the increase of water temperatures and from the recurrence of extreme weather events. Thus, other species that are more resilient to temperature changes may supplant them, and the average rise of the sea level is fatal to deep seagrass meadows.
The only threats we can deal with immediately are the ones that are linked to our activities. Every year, when boats anchor, they pull out large quantities of posidonia in only a few minutes, which took several centuries or even several millennia to grow. Anchoring bans have been implemented in some natural reserves and national parks, but the best impact we can have is to encourage boaters to adopt anchoring practices that respect the posidonia meadows.
Some fishing trawls are also responsible for the destruction of vast seagrass meadows by scraping the seabed or by casting their nets in areas richly populated with this species.
Finally, a great communication effort must be made towards communes and swimmers, because we need to stop removing banks of posidonia while cleaning beaches as their role is essential for biodiversity as well as for the maintenance of beaches.
Help us protect Posidonia, step by step
For all the above reasons, it is important to protect our seagrass meadows, as it plays a crucial role in the balance and development of our environment while being very fragile and long to develop. Posidonia is not like a forest that you could regrow in a few years. Once you’ve destroyed a meadow, assuming it is protected, it will take hundreds of years to grow and take back its role in the ecosystem.
This is why on Navily we are working to better protect these aquatic ecosystems by informing boaters about the presence of such meadows at anchorages, giving them the opportunity to report meadows presence directly on Navily. Indeed, you can now report a meadow on the app that will appear clearly in the “type of seabed” of the anchorage page with a dedicated section. This permanent window on the anchorage page, will give boaters access to more information about Posidonia directly on the app to first inform and then protect!
This is a first step towards protecting our Posidonia meadows and we are counting on you to spread the word in a first and important step of educating people about these rather “unknown” aquatic plants.
Add known meadows on anchorage pages, give precision on its location in the comment section, share this article and help us protect a plant without which, the Mediterranean ecosystems and environment we love, wouldn’t survive.
Fair winds Captain,