Two Seagrass Worlds, One Silent Struggle

Several times a week, I return to the site of Ngouja, in Mayotte.
A simple place, almost still, where time seems to slow beneath the surface.

There, I find the lagoon’s green turtles.
I watch them graze on the short seagrass with a calm, almost meditative rhythm — like silent herds feeding beneath the water. Their movements are slow, deliberate, repeated — an ancient behavior that seems to belong to a different pace than our own.

Beneath them, the seagrass meadows stretch in dense, living patches.
They shape the landscape quietly, without ever demanding attention.
And yet, as I watch them, another image comes to mind.
The Mediterranean.

For years, I have moved above the seagrass beds of Posidonia oceanica. A different kind of landscape — denser, more structured, almost forest-like. Where Ngouja feels open and dynamic, Posidonia evokes stability and time.

Two environments. Two rhythms.
​
But the same question keeps returning:
What do these seagrass ecosystems truly share, beyond their appearance?
And what do they reveal about the state of our oceans today?

Two Worlds, One Function

At first glance, everything seems to separate tropical seagrass meadows from their Mediterranean counterpart.
In Ngouja, seagrass is composed of multiple species. It grows quickly, adapts, and recolonizes. Its dynamics are fluid, responsive — but also fragile.
In the Mediterranean, Posidonia oceanica follows a different tempo. Endemic to this sea, it expands only a few centimeters per year. Over centuries — sometimes millennia — it builds thick underwater structures known as “matte,” creating one of the most stable coastal ecosystems on Earth.
On one side, a fast-growing, adaptive system.

On the other, a slow, long-term builder.
And yet, despite these differences, their role is the same.
Seagrass meadows are among the hidden foundations of coastal oceans.
They act as nurseries for countless species, shelter juvenile fish and invertebrates, feed turtles, stabilize sediments, and help maintain water clarity. They also protect coastlines by absorbing wave energy.
​
Without them, entire ecosystems begin to unravel.

An Invisible Climate Role

But their importance extends far beyond biodiversity.
Beneath the surface, seagrass meadows play a critical role in regulating the global climate.
They capture carbon dioxide — much like terrestrial forests.
But more importantly, they store it.

Over time, dead leaves, roots, and organic matter accumulate in the sediment below, forming a long-term carbon reservoir. This carbon can remain trapped for centuries, even millennia.
On average, seagrass meadows can store up to 140 tonnes of carbon per hectare.
Per unit area, they can be up to 30 to 40 times more efficient than forest soils at storing carbon over the long term.

In the Mediterranean alone, Posidonia oceanica captures an estimated 5.7 million tonnes of CO₂ each year.

A remarkable figure for such a discreet ecosystem.
But this balance is fragile.
When seagrass meadows are damaged — by anchoring, pollution, coastal development, or rising temperatures — the carbon they have stored can be released.
The system reverses.
​
What was once a carbon sink becomes a source.

Resilience and Fragility

In Ngouja, tropical seagrass gives the impression of a living, resilient system. Its fast growth allows it to recover under the right conditions.
But this apparent resilience comes with vulnerability. Increased turbidity, sediment runoff, or human pressure can lead to rapid decline within just a few years.
In contrast, Mediterranean Posidonia tells a different story.
It is slow. Extremely slow.

But it builds over time. It stabilizes the seabed, stores vast amounts of carbon, and creates long-lasting habitats. When destroyed, recovery can take decades — or may not occur at all on a human timescale.

Fast resilience on one side.
Deep resilience on the other.
Yet in both cases, the same conclusion emerges:
These ecosystems are essential and fragile.

A Silent Decline

Globally, seagrass meadows are in decline.
Their disappearance is rarely dramatic. It does not make headlines. It does not burn or collapse suddenly. It happens slowly, underwater, often beyond our awareness.
In the Mediterranean, significant losses have already occurred, particularly near urbanized coastlines, ports, and anchoring zones.
​
In the Indian Ocean, data remains limited, making the situation harder to quantify. But pressures are clear: sedimentation, runoff, and coastal development.
Less visible than coral reefs, seagrass ecosystems suffer from a lack of recognition.
And therefore, a lack of protection.

Protecting the Invisible

​Protecting seagrass does not always require complex solutions.
Sometimes, the answers are simple:
Limiting uncontrolled anchoring in favor of eco-moorings.
Reducing sediment and pollution runoff.
Managing coastal development.
Strengthening marine protected areas.
But all of this depends on one essential step:
Recognizing their value.
Because it is difficult to protect what remains unseen.

Conclusion

From Ngouja to the Mediterranean, seagrass meadows tell the same story.
That of ecosystems both discreet and essential — capable of sustaining life and regulating the climate, while remaining largely invisible.

Two worlds. Two rhythms.
One vital function.

​And perhaps, one shared urgency:
To learn how to see what we have long overlooked.

Seagrass meadows are among the most important coastal ecosystems on Earth. They provide habitat for marine species, act as nurseries, and play a key role in carbon sequestration. In tropical regions like Mayotte, seagrass ecosystems support species such as green turtles and contribute to lagoon stability. In the Mediterranean, Posidonia oceanica forms dense, long-lived meadows that store carbon for centuries and protect coastlines. These blue carbon ecosystems are essential in the fight against climate change, yet they are declining globally due to pollution, coastal development, and anchoring. Protecting seagrass habitats is critical for ocean resilience, biodiversity, and climate regulation.