- Neptune grass is generally regarded as the most ecologically important seagrass and shallow-water habitat in the Mediterranean Sea, where it is endemic. But the species has been in decline for many decades.
- A new study found that following the introduction of stronger environmental regulations and practices in the mid-to-late 1980s, Neptune grass (Posidonia oceanica) repopulated sampled sections of the waters off Marseille, France, over the ensuing four decades at rates that experts called “exceptional” and “remarkable.”
- The lead author said the study shows the value of passive restoration: letting seagrass meadows regrow on their own after removing the human-caused drivers of decline, rather than focusing on replanting or transplanting the species.
Neptune grass is generally regarded as the most ecologically important seagrass and shallow-water habitat in the Mediterranean Sea. It suffered a severe decline during the 20th century, and there have been myriad efforts to actively restore it via replanting schemes. A new study points to the merits of a different approach: Remove the human-caused drivers of the decline and let the meadows regrow on their own.
The study, published March 5 in the journal Marine Environmental Research, found that following the introduction of stronger environmental regulations and practices in France in the mid-to-late 1980s, Neptune grass (Posidonia oceanica) repopulated sampled sections of the waters off the city of Marseille over the ensuing four decades.
“We observed exceptional recovering of the meadow in the Bay of Marseille,” Patrick Astruch, a research engineer at GIS Posidonie and the study’s lead author, told Mongabay. GIS Posidonie is a nonprofit marine research group based on the Aix-Marseille University campus.
Astruch called it a “very positive trend” and a lesson in the value of passive restoration, which involves letting seagrass meadows regrow naturally after reducing pollution and other threats.
Pollution and progress
Neptune grass, named after the trident-wielding god of the sea in Roman mythology, hosts up to a quarter of marine species in the Mediterranean, which is the only place it grows. It’s the sea’s “most important ecosystem engineer,” Astruch and his co-authors write. Neptune grass meadows serve as the basis for food webs that the fishing industry relies on, and the species is, as Astruch called it, the “world champion” of carbon sequestration, mitigating the effects of human emissions as well as or better than mangroves or tropical forests.
Yet Neptune grass grows very slowly and has proven vulnerable to pollution and other threats, including bottom trawling, anchoring and coastal development projects. Climate change and invasive species are also factors in its decline. Roughly 7% of Neptune grass biomass was lost annually in the half-century prior to 2009, according to a 2014 study in the journal Biological Conservation.
The historical damage was particularly severe off Marseille, a city of roughly 2 million people on the northwestern Mediterranean coast. The city dumped its untreated wastewater into the Bay of Marseille, while industrial and urban waste entered the bay via coastal rivers. Coastal development and infrastructure projects also had an effect. In the 1970s, while the city was constructing a subway system, it used much of the excavated material from digging tunnels to build artificial breakwaters and beaches. This directly covered some Neptune grass meadows and added sediment to the water, rendering it more opaque and less ideal for the species’ growth, Astruch said.
“The situation was so bad,” Astruch said, calling the 1980s the likely low point for Neptune grass around Marseille.
However, in 1987, the city began operating a wastewater treatment plant that removed most contaminants, and the French government instituted key environmental regulations during that period. These included a law limiting coastal development and a decree that designated Neptune grass as a protected marine plant species whose destruction was prohibited. The seagrass was later protected under European Union laws and regulations.
To gauge the effects of these changes, Astruch and his team took advantage of earlier work. In 1986, researchers had laid down concrete markers on the seabed not far from the shore, forming two 36-square-meter (388-square-foot) areas called quadrats in which the amount of Neptune grass coverage could be measured. Measurements were taken until 1999, but then stopped due to lack of funding, with no data for the next quarter-century.
The team, mostly from GIS Posidonie, took hundreds of underwater photographs of the quadrat areas and used software to map the Neptune grass meadow. In one quadrat, coverage increased from 47% in 1986 to 94% in 2025. In the other, it jumped from 6% to 81%.
“This landmark 40-year study from Marseille offers compelling evidence that Posidonia oceanica meadows can achieve near-full recovery once anthropogenic pressures are decisively removed,” Maria Salomidi, a marine ecologist and researcher at the Institute of Oceanography of the Hellenic Centre for Marine Research in Greece, told Mongabay in an email, calling the recovery “remarkable.” Salomidi wasn’t involved in the Marseille study.
Astruch, who’s worked at GIS Posidonie since 2007, said he’d long wanted to find and study the quadrats but hadn’t had the funding until recently. He said his research was possible because of funding from the city of Marseille, which since 2020 has been led by a left-wing coalition that pushed Neptune grass conservation.
Promoting the passive
Active restoration of seagrass usually involves planting seeds or transplanting vegetation, including sometimes clods of meadows. Benjamin Jones, chief conservation officer of Project Seagrass, a U.K.-based NGO, told Mongabay that most, but not all, of the 61 projects listed in their SeagrassRestorer database are active.
“Small-scale passive projects are quite numerous, generally associated with schemes to avoid anchoring, or installation of advanced mooring systems or ‘eco-moorings,'” he told Mongabay in an email. “The larger passive restoration projects like the study, are usually watershed focused, but often the scope is on improving water quality (with added benefits for seagrass).”
Astruch said that active projects tend to receive more funding and more attention than passive restoration, but aren’t necessarily cost-effective — or effective at all.
“Active restoration of P. oceanica meadows has been tested and implemented for more than half a century, but has most often resulted in limited success in terms of plant survival and meadow recovery,” he and his co-authors write.
He said removing human-caused drivers of degradation is the key, and this study shows that.
Astruch said that while this study was about seagrasses, the same principle could be applied to coral reefs, mangroves and other ecosystems. (Some have even applied it to terrestrial forests.)
“We cannot just play God and plant what we destroy,” he said. “We just need to calm down on the way we are using nature, and we need to respect nature.”
Salomidi also said an important takeaway was that, for Neptune grass populations, “passive restoration must always precede any active intervention as it is the most cost-effective and ecologically sound approach available.”
“Having documented similar recovery patterns following anthropogenic disturbance in Greek waters, we continue to advocate for the immediate removal of pressures, whether from coastal development, pollution, unregulated anchoring, or aquaculture infrastructure, as the single most urgent priority for the protection and restoration of Posidonia meadows in Greece and the wider Mediterranean,” she added.
Banner image: A fan mussel (Pinna nobilis) in a Neptune grass meadow. Image by Arnaud Abadie via Wikimedia Commons (CC BY 2.0).
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Citations:
Astruch, P., Boudouresque, C.-F., Ferrando-Demange, J., Belloni, B., Charbonnel, É., Gravez, V., & Schohn, T. (2026). Passive restoration of a Mediterranean Posidonia oceanica seagrass meadow: A 40-year survey. Marine Environmental Research, 217, 107965. doi:10.1016/j.marenvres.2026.107965
Marbà, N., Díaz-Almela, E., & Duarte, C. M. (2014). Mediterranean seagrass (Posidonia oceanica) loss between 1842 and 2009. Biological Conservation, 176, 183-190. doi:10.1016/j.biocon.2014.05.024
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