Have you ever felt a shiver of unease while swimming above dark, mysterious depths, unsure of what lies beneath you? While its dense underwater “forests” may seem intimidating because of their depth and the unknown, Posidonia oceanica is actually the foundation of life in the Adriatic Sea.
If we truly understood everything it does for us and for our seas, that sense of unease might be replaced by gratitude and awe for this extraordinary plant. Posidonia is not a threat, it is an ecosystem builder, a habitat creator, and a guardian of our waters.
A hotspot of biodiversity in the Mediterranean
Often called seagrass, tapeweed, or Neptune grass, Posidonia forms meadows that support countless species whose survival, shelter, and reproduction depend on it. These meadows provide a home for over 400 plant species and thousands of animal species, making them a true hotspot of Mediterranean biodiversity.
Beyond supporting marine life, Posidonia stabilizes the seabed, improves water clarity, and serves as an indicator of environmental health. Its meadows act as oxygen factories, buffer the force of waves, and protect coastlines from erosion. Yet, its ecological services go even further (or deeper) than we can imagine.
The oldest and perhaps most sensitive Mediterranean endemic
Frequently mistaken for algae, P. oceanica is actually a flowering marine plant, one of four seagrass species in the Adriatic alongside Cymodocea nodosa, Zostera marina, and Zostera noltii.
Like terrestrial plants, it has roots, stems, leaves, and flowers, but it has evolved to thrive underwater. As a Mediterranean endemic, it grows only in this region, nowhere else in the world. Its unique adaptations have allowed it to survive for millennia and build vast underwater meadows in a narrow coastal belt, from 1 to 30 meters deep. Its proximity to the shore also makes it extremely vulnerable and insufficiently protected.
The largest known meadow, stretching between Formentera and Ibiza, spans eight kilometers and is an astonishing 100,000 years old, making Posidonia one of the oldest living organisms in the Mediterranean.
Posidonia produces olive-like fruits that float on the surface; when they split, the seeds sink and grow new plants. However, these fruits appear only once every few years, making sexual reproduction a slow process.
The plant also reproduces vegetatively, sprouting new shoots from its root system, which helps regenerate its population more quickly. This form of reproduction allows Posidonia to survive even in conditions where sexual reproduction is not possible, but the process – like sexual reproduction – is extremely slow, making the plant highly sensitive to environmental changes.
Valuable even beyond the sea
Its long, dark-green leaves often fall in autumn, forming wrack deposits (natural accumulations of dead stems and leaves along the shore). While these deposits may appear somewhat untidy and visually unappealing, they play a crucial role in stabilizing the coastline. They act as a natural barrier, protecting beaches from high waves and storms, reducing erosion and sediment loss, and helping maintain coastal stability.
These Posidonia wrack deposits are also rich in species. Studies have shown that this unique habitat regularly hosts a large number of insects.
High ecological value, high losses
In addition to providing shelter and protection for countless marine species, Posidonia directly impacts the health of the sea and the entire planet.
It is estimated that one hectare of Posidonia meadow produces up to five times more oxygen than one hectare of the Amazon rainforest. For this reason, the plant is often called the “lungs of the Mediterranea,,” as it absorbs CO₂ from the atmosphere through photosynthesis and releases oxygen into the seawater, reducing greenhouse gas emissions and helping mitigate climate change. Thanks to its carbon-storing capacity, Posidonia is one of nature’s most important allies in the fight against global warming.
Posidonia meadows stretch across nearly two million hectares along the Mediterranean coast and, despite their ecological value, approximately 34% of these underwater meadows have been lost in the past 50 years. The main causes of this drastic decline are human activities: primarily anchoring by large vessels that physically damage the plant, along with pollution from industrial and urban sources and the increasing effects of climate change, such as rising sea temperatures. The disappearance of Posidonia seriously threatens the Mediterranean’s ecological balance, highlighting the urgent need to protect this precious ecosystem.
Protection requires cooperation
Aware of its irreplaceable role, Posidonia is a strictly protected species in Croatia under the Nature Protection Act (NN 80/13, 15/18, 14/19, 127/19). At the European level, it is safeguarded by the Habitats Directive (92/43/EEC; 92/43/EEZ) as a priority habitat – an ecosystem of exceptional importance for the conservation of biodiversity, which is threatened, rare, at risk of disappearing, or critical for the protection of specific species. Posidonia habitats are recognized as particularly vulnerable and require urgent protection and preservation.
To strengthen its protection, Sunce has been monitoring Posidonia for over a decade at various locations, including Kornati National Park, Telašćica Nature Park, Lastovo Archipelago Nature Park, Mljet National Park, and Badija.
Conservation activities include research and data collection on the health of the meadows, as well as awareness-raising campaigns. Through our latest campaign, And where do you anchor?, run for the second consecutive year as part of the Interreg BIOPRESSADRIA project, we aim to educate the public on the importance of Posidonia and reduce human impact on these fragile ecosystems.
The campaign’s goal is not only to answer the question, “And where do you anchor?”, but also to call on sailors and all sea lovers to join us in promoting environmentally responsible anchoring, protecting seagrass and sensitive habitats, discussing the challenges and practices of sustainable navigation and anchoring, and sharing local knowledge, experiences, and stories from the sea. Without collaboration, we cannot effectively protect Posidonia.
The scientific community is well aware of this, which is why experts and institutions involved in the conservation and restoration of this species collaborate through the Mediterranean Posidonia Network (MPN). Sunce is a member of this network as well. This membership enables us the exchange of knowledge and experiences with other members and contribution to a coordinated approach to Posidonia protection across all Mediterranean countries.
‘Lipo li je, lipo li je, na lažini suvoj ležat’
Thanks to its role in maintaining biodiversity and sea health, Posidonia oceanica affects not just marine life but the balance of life on Earth. For thousands of years, it has shaped, nurtured, and protected our seas. Preserving Posidonia means preserving the sea and the coastal way of life. Its dried leaves, washed ashore, have long served as natural beds for fishermen and travelers.
This ancient plant is not only a refuge for marine life and a source of human benefits, but also an inspiration. As the legendary Oliver Dragojević sang: “Lipo li je, lipo li je, na lažini suvoj ležat” (eng. “How lovely it is, how lovely it is, to lie on dry seagrass.”).
If Posidonia disappears, we will lose not just a precious marine ecosystem, but stories, memories, and ways of life that have shaped us for centuries. Knowing its irreplaceable role, can we allow something so vital, yet almost invisible, to vanish? What would the sea look like without this green guardian?
Now it is our duty to protect it because without Posidonia, the Adriatic will never be the same.
The information for this article was collected from the Nature Protection Act, publications of National Geographic magazine, the Mediterranean Posidonia Network platform and ResearchGate, as well as from EU documents on the protection of Europe’s biological diversity (Natura 2000), and from the Handbook for the Inventory and Monitoring of Marine Habitats.