Asian algae: from a threat to the bioversity of the Costa del Sol to a friend of cereal crop farmers?

Its presence in the Mediterranean is nothing new, but its proliferation does not stop giving headaches to the municipalities along the Costa del Sol as to its removal from their beaches. Now the abundant presence of this invasive algae of Asian origin (Rugulopteryx okamurae) could have a viable use by being transformed into fertiliser products for agricultural use, reducing the environmental impact and generating added value.

To this end the Andalusian institute for agricultural, fisheries, food and organic production research and training (Ifapa) and recycling and waste management specialist company Valoriza have signed a collaborative agreement to test the incorporation of this plant waste into the industrial process of sewage sludge composting with the idea of using it later as a fertiliser for rain-fed cereal crops.

The presence of this, at the time unknown, algae species was detected 20 years ago in a French saltwater lagoon where a propagule (an offcut of the plant suitable for propagation) may have arrived with the Japanese clams that were cultivated there. At that time, its development did not go beyond that point. It was in 2015 when the alarm bells began to sound when different-looking, massive brown algae clumps were spotted along the coast of Ceuta.

At the time, it was not known whether it was a native algae that had grown too large or whether it was an invasive algae. Unfortunately, after several years of trying to deal with such a species that was, and still is, multiplying rapidly and pushing the native species aside and colonising the Mediterranean coastline at great speed, the latter has now been confirmed.

Originally from the Pacific, everything suggests that the algae reached the Mediterranean as a stowaway in the ballast waters of large ships that, on arriving in port, empty their compartments and let the water escape with all the microorganisms from other parts of the planet. Most of them do not survive the new environmental conditions, but others, such as Rugulopteryx okamurae, have not only demonstrated their ability to adapt, but have ended up dominating the local habitat. "In fact, ships' ballast water is considered to be one of the most important factors for the movement and incorporation of exotic species to other areas where they were not," explained Javier Urra, a biologist at Malaga's oceanographic centre in the port area, who works together with José Rueda on programmes to monitor the ecology of benthic communities in the sea (that is, the groups of organisms that live and interact with each other on, near or within the seabed).

They formed a team that continues today. Already by 2021, when they started making investigation dives from Tarifa to Cabo de Gata, they saw how the rocky areas of the seabed were dominated by this single species. "Over the years we have seen how the sampling points we have in the provinces of Cadiz, but above all in Granada and Almeria, which are the furthest from the Strait of Gibraltar, have been proliferating so that, of the 300 linear metres where we were diving to see what was there, at the beginning there were half of them with Rugulopteryx, after two years 70% and after four years 85%." These researchers have seen how this species has progressively gained ground over the native species, "so that nowadays if you dive with goggles and a snorkel on any rocky bottom from five metres down from Tarifa to Cabo de Gata, you have a high probability of seeing that everything is occupied by Rugulopteryx."

What are the consequences for marine communities? Firstly, the amount of algae native to the Mediterranean decreases and the invading algae ends up dominating the entire seabed, homogenising it and reducing the complexity of the habitat. In turn, some of the flora and fauna that used to live on these seabeds move to other areas because they can no longer find food, "as this algae is not part of their diet." This is partly because it has a substance that gives it a bad taste and scares them away. It therefore lives safe from predators. "With no other animals feeding on it, it has found the perfect conditions to spread, dominate and cause the marine environments we knew in the Alboran Sea [the westernmost part of the Mediterranean] to be greatly transformed."

"Rugulopteryx okamurae belongs to the class of brown algae that are quite primitive organisms and of which there were already very similar species on the Andalusian coast. It is very difficult to differentiate from other species of the same family, which are the dictyotales." It can live fixed to the rock, at depths of between half a metre up to 40 metres, but it can also form surface clusters that float ashore.

Its reproductive capacity is extraordinary as a single plant can produce thousands of specimens. This, together with the currents and maritime traffic, favours a dispersion that is currently uncontrollable and is having a major impact on the fishing industry, with fishing grounds being so invaded by this algae that it can even break fishing gear, but it also impacts the tourist industry. Sometimes so much algae reaches the coast that it is difficult to remove the unseemly organic remains that wash up on pristine beaches.

The massive arrival this summer has kept the town councils along the Costa del Sol on alert. In Marbella alone so far this year the town hall has spent more than 800,000 euros to remove 75 tonnes of invasive algae from the beaches every day. Moreover, it is not only an aesthetic problem, but also a health problem, as researchers advise, "because when the seaweed dies, it smells bad and begins to decompose a series of compounds that favour the growth of fungi and bacteria that can then cause infections in bathers", said Rueda, who recommends its careful removal. "If you leave a small piece of this algae or propagule behind, it will reproduce."

In the case of Estepona, the specific work for the collection of this algae represents an annual cost overrun of more than 1,000,000 euros. During last year the town hall collected a total of 3,415 tonnes of seaweed, with the highest figure being reached in 2022, when 5,331 tonnes had to be removed from the beaches. The scenario has been repeated in Mijas, Benalmádena and other coastal municipalities that are learning to live with a visitor that nobody wants.

The presence of this species was included at the end of 2020 in the Spanish Catalogue of Invasive Alien Species, and has particularly affected the beaches of San Pedro Alcántara, Puerto Banús, Fontanilla and Cabopino this summer, where the piles of algae washed ashore reached a height of over one metre. In addition, since 2022 this algae has been on the 'List of invasive alien species of Union concern' by the European Commission.

The Spanish government's Ministry for Ecological Transition launched a monitoring strategy that year, but without much success. "What is being done nowadays are monitoring programmes to be able to evaluate the marine environment," said Rueda.