Posidonia oceanica: The wonder of the Mediterranean

Meanwhile, on the other side of the world, researchers have discovered meadows of Posidonia australis spanning 200 square kilometers and thriving in their environment for 4,500 years. It’s important to note that Posidonia australis and Posidonia oceanica are not the same plant. While both belong to the Posidoniaceae family, they have evolved in different regions, resulting in distinct species. The former grows exclusively in Australian waters, whereas the latter is unique to the Mediterranean Sea, where some of its meadows have been declared UNESCO World Heritage Sites.

Here are some fascinating facts to better understand this guardian of the depths.

What is Posidonia oceanica?

So, what is posidonia? Posidonia oceanica is an aquatic plant that is native to the Mediterranean. Unlike algae, it has roots, stems, and leaves, and it even produces flowers and fruits. This distinction is key, as algae are plant-like organisms without these features. When we talk about a posidonia meadow, we’re referring to one of the most vital marine ecosystems for the planet’s balance—comparable to the role coral reefs play in the Caribbean.

Posidonia meadows are typically found near coastal shorelines, making them particularly vulnerable to threats like illegal boat anchoring in protected areas, habitat degradation, and marine pollution. Protecting these underwater ecosystems requires collective effort and conscious actions. Initiatives such as the PosiGenome project by IMEDEA (Mediterranean Institute for Advanced Studies) to study posidonia and the creation of the first coastal observatory within a hotel are steps in the right direction. Companies like Iberostar actively support these measures, emphasizing sustainability.

You can join this movement during your next vacation by choosing accommodations with a commitment to sustainability. Small actions, like eliminating single-use plastics, choosing seafood from responsible sources, or engaging in beach-friendly activities, can make a big impact in protecting this essential marine ecosystem.

Why Posidonia is not an algae

If you’re wondering why posidonia isn’t classified as algae, the answer lies in its anatomy. Unlike algae, posidonia has the full structure of a true plant: roots, stems, leaves, and even a small fruit known as the "sea olive." Additionally, while most algae attach themselves to hard or rocky surfaces, both Posidonia oceanica and its close relative Posidonia australis thrive in sandy seabeds. As the indie band La Habitación Roja beautifully put it in their song Posidonia, it is like “levitating over the water, immersed in its words.”

Imagine a posidonia meadow as an underwater forest, a submerged green lung with roots firmly embedded in the sand. It’s also a thriving aquatic ecosystem, offering shelter, nourishment, and support to countless species of marine flora and fauna—a vital hub of life beneath the waves.

What role does Posidonia play in ecosystems?

A posidonia meadow is much more than a home for marine animals—it’s also an incredibly efficient carbon sink, capable of absorbing up to fifteen times more CO₂ than the Amazon rainforest. In tropical coastal areas, mangroves perform this critical function. In the Mediterranean, posidonia plays a key role in combating climate change by producing large amounts of oxygen through photosynthesis. It also acts as a natural purifier, filtering suspended particles and nutrients from the water. Even plastics and other debris are trapped by posidonia, which later returns them to the shore, contributing to a cleaner sea.

The dry posidonia that washes ashore isn’t waste to be cleared away like discarded plastic. Instead, it serves as a natural shield against moderate and low-intensity storms. It reduces storm impacts, helps return sand to the beaches, and keeps these areas nutrient-rich and healthy. By stabilizing coastlines and enriching dunes with organic matter, posidonia reduces erosion and promotes long-term stability.

It’s no coincidence that you’ll find posidonia leaves on many of the Mediterranean’s most beautiful beaches. In part, it’s thanks to this remarkable plant that the shorelines remain as stunning and resilient as they are today.

Sea olives: The fruit of Posidonia oceanica

The fruit of Posidonia oceanica, known as the sea olive, serves a reproductive purpose, much like fruits such as mangoes or apples, though its role is less significant. Under normal conditions, mature sea olives are rarely produced. When they do appear, they detach from the plant and float, carried by the waves to new locations. However, the primary method of reproduction for Posidonia oceanica is asexual or vegetative, occurring when underground rhizomes or stems grow and spread across sandy seabeds.

In recent years, researchers have observed that global warming has caused posidonia to produce mature fruits more frequently as a response to environmental stress. While this showcases the plant’s remarkable adaptability, it’s also a potential warning sign. Shifts in reproductive habits and seed maturation patterns can disrupt the natural cycles of Posidonia oceanica, potentially putting the species at risk. This phenomenon mirrors the threats faced by other endangered plants. The presence of mature sea olives doesn’t always indicate fertility—it might reflect rising water temperatures or even the effects of extreme heatwaves, such as the one experienced in the Mediterranean summer of 2022.

Sea olives often wash ashore alongside dry posidonia leaves. You can identify them by their oval shape and smooth, shiny texture, resembling an olive, which gives them their name. Their color ranges from light green to dark brown or black, depending on their level of maturity. If you find one, resist the urge to pick it up—it belongs in its natural environment. As long as posidonia continues to thrive in its underwater realm, “dancing with the waves” as celebrated in songs written in its honor, marine ecosystems will recover more effectively from the impacts of climate change, ensuring a safe haven for the diverse species that call the Mediterranean home.