A decree to protect the Mediterranean’s underwater forests

The Spanish government has taken an important step to protect underwater meadows of Posidonia oceanica and Cymodocea nodosa, two plant species essential for Mediterranean biodiversity and CO₂ absorption. Although some regions, such as the Balearic Islands, had already introduced their own legislation, this new national decree extends protection to all seagrass meadows across the Spanish Mediterranean.

Anchoring boats on these underwater “forests” is now definitively prohibited. To guide skippers, the decree also requires regular updates to the cartographic boundaries of these meadows.

The regulation further establishes the conditions under which ecological mooring areas can be created, using buoys that do not damage these habitats.

From now on, any works in these underwater meadows are banned, including the laying of cables, installation of discharge pipelines, gas or oil pipelines, port infrastructure, dredging or sand extraction, renewable energy installations and beach regeneration below the high-tide line.

Restoration of washed-up seagrass

One of the most notable aspects of the decree is the protection of accumulations of dry seagrass that gather along the shore, which play a key role in preventing sand loss during storms. These deposits may only be removed with administrative authorisation and exclusively during the summer tourist season. In such cases, the material must be stored and preserved properly before being returned to its original location.

The decree, now published in the Boletín Oficial del Estado (BOE), also establishes the need for regular monitoring of the conservation status of Posidonia oceanica and Cymodocea nodosa meadows, tracking both their natural conditions and their expansion or decline.

Failure to comply with these protection measures may result in penalties under Spain’s Natural Heritage and Biodiversity Law, with fines ranging from €100 to €2 million depending on the severity of the offence.

A key ecosystem under serious threat

Both Posidonia oceanica and Cymodocea nodosa are marine flowering plants (not algae, as is often believed) that provide habitat and breeding grounds for numerous fish species and other marine organisms. They are also highly effective at capturing atmospheric CO₂, making them true underwater forests.

However, these meadows face three major threats: indiscriminate anchoring—particularly by recreational boats—poorly treated wastewater discharges, and the exceptional warming of the Mediterranean Sea. Experts warn that these pressures are causing a worrying decline in their extent, compounded by their extremely slow recovery rate.

“Coastal and marine ecosystems are, and have always been, a source of multiple ecosystem services—not only providing food (fishing and aquaculture) and raw materials, but also regulating climate, protecting coastlines and offering cultural services such as tourism and heritage”, states a recent study by researchers from the Autonomous University of Madrid and King Juan Carlos University.

Seagrass species are highly sensitive to temperature, with mortality rates increasing significantly above 28°C. Climate projections are concerning: models suggest that Mediterranean sea surface temperatures could rise by an average of 3.4°C by the end of the century. From around 2050 onwards, temperatures above 28°C could become the norm every summer in the western Mediterranean.

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