A green exotic alga that was first detected in Formentera in 2020 has spread throughout s'Estany des Peix and is now growing directly on Posidonia oceanica, prompting researchers to call for urgent monitoring and management.

The species, Batophora occidentalis, is the subject of a new study by researchers from the Mediterranean Institute for Advanced Studies, IMEDEA, a joint centre of the Spanish National Research Council (CSIC) and the University of the Balearic Islands.

The research, published in the peer-reviewed journal PLOS One, documents a rapid expansion of the species inside the Formentera lagoon and provides the first evidence of it growing directly on the leaves and rhizomes of Posidonia oceanica.

First detected in Formentera in 2020

Batophora occidentalis was first reported in the Mediterranean in May 2020, when it was detected in s'Estany des Peix.

Researchers subsequently followed its expansion through surveys carried out in 2023 and 2024.

By 2024, the alga had been detected at 100% of the surveyed locations within the lagoon.

The study describes the species as an exotic green macroalga and warns that its rapid expansion could create ecological problems if it spreads into other native marine habitats.

Earlier research had already documented the species spreading throughout the lagoon. A 2024 study found that Batophora had colonised all the habitats examined at s'Estany des Peix and had begun appearing outside the lagoon near its entrance, including along the margins of Posidonia meadows.

It is now growing directly on Posidonia

The latest study contains a particularly important finding.

Researchers documented Batophora occidentalis growing epiphytically on the leaves and rhizomes of Posidonia oceanica inside s'Estany des Peix.

According to the researchers, this is the first documented evidence of this particular behaviour by Batophora occidentalis on Posidonia.

During the 2024 surveys, the alga covered an average of around 16% of Posidonia leaves at the studied site inside the lagoon.

In individual cases, however, it covered almost 25% of the surface of affected leaves.

Some algae weighed five times more than the leaf

The researchers also measured the weight of the algae growing on Posidonia leaves.

In the most heavily affected cases, the attached Batophora biomass reached more than five times the dry weight of the individual Posidonia leaf.

The researchers say this could have important consequences for the seagrass.

A heavy layer of algae may reduce the amount of light reaching the plant, while the additional weight could make leaves more vulnerable to bending and breakage.

During fieldwork, researchers observed heavily colonised Posidonia leaves that were bent, folded or partly broken.

However, the scientists are careful not to claim that the alga has already caused large-scale Posidonia decline.

The study states that further research is needed to determine the long-term consequences of the interaction between Batophora occidentalis and Posidonia oceanica.

The alga is already dominant in other habitats

The expansion is not limited to Posidonia.

In 2024, Batophora occidentalis reached average coverage of approximately 30% in the Cymodocea nodosa meadows studied and 26% in Caulerpa prolifera meadows.

It became the most abundant macrophyte in both of those habitats at the studied locations.

Researchers also found the species growing on sand, rocks, natural hard substrates, harbour structures, marine debris, anchor chains and boat hulls.

It was recorded on 11 of 63 boat hulls examined in the lagoon's main marina, equivalent to 17.4%.

This is particularly relevant because boats can potentially provide a mechanism for the species to move between locations.

A worrying sign outside the lagoon

One of the most important findings came from outside s'Estany des Peix.

Researchers found a Batophora occidentalis specimen growing on a living Posidonia oceanica shoot in a nearby meadow outside the lagoon.

Based on the sampling undertaken, the researchers estimated almost 12 colonised Posidonia shoots per square metre in the adjacent meadow.

They describe this finding as a potential signal that the species may be beginning to move beyond the sheltered lagoon environment where it has become established.

That does not mean the alga has already colonised Formentera's Posidonia meadows generally.

The study specifically says that the extent of its presence in other areas around the Pityusic Islands remains undetermined.

Why researchers are concerned

Posidonia oceanica is an important Mediterranean seagrass that provides habitat for marine organisms, helps stabilise sediments and contributes to carbon storage.

The Pityusic Islands are home to some of the most extensive and well-preserved Posidonia meadows in the Mediterranean, and these meadows form part of the Ibiza and Formentera marine environment recognised by UNESCO.

The concern is therefore not simply the presence of another species.

Researchers are worried about what could happen if Batophora occidentalis continues expanding from the relatively sheltered lagoon into native marine habitats where it could establish itself more widely.

The study describes this as a potential ecological threat rather than a proven large-scale impact on Posidonia.

Four years of rapid expansion

The speed of the invasion is one of the reasons scientists are calling for action.

Within roughly four years of its first detection, the species had spread across all surveyed locations within s'Estany des Peix and become highly abundant in several native habitats.

The researchers estimate an average density of around 5,400 Batophora stalks per square metre inside the lagoon.

They say complete eradication may no longer be realistic under the current conditions, particularly given the extent of the invasion within the approximately 2-square-kilometre lagoon.

However, they argue that targeted management could still help slow further spread.

Researchers call for monitoring and action

The scientists recommend increased research and monitoring of Batophora occidentalis in lagoons and marinas around the Balearic Islands.

They also propose measures aimed at reducing human-assisted dispersal, including removing the alga from boat hulls within s'Estany des Peix and targeted removal around the lagoon entrance.

The researchers emphasise that important information about the species, including its reproductive cycle and seasonal growth patterns, is still missing.

The Balearic Government already operates a monitoring network for Posidonia meadows, with fixed sampling stations used to track ecological and biological changes over time.

What is known and what remains uncertain

The latest research provides a clear picture of a rapidly expanding exotic alga inside s'Estany des Peix.

What is confirmed is that Batophora occidentalis has become widespread within the lagoon, dominates several native habitats there, grows directly on Posidonia leaves and rhizomes, and has now been observed on a Posidonia shoot outside the lagoon.

What remains uncertain is how far the species has spread beyond s'Estany des Peix and what its long-term effect will be on Posidonia meadows.

For now, researchers describe the situation as an important moment for early monitoring and management.

The concern is not that Formentera's Posidonia meadows have already been overtaken. It is that an exotic species that has expanded remarkably quickly inside one lagoon has now shown signs that it may be capable of moving into one of the Mediterranean's most important seagrass ecosystems.