Scientists discover the reasons why the beaches of Algarrobo are full of algae in Chile.

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For more than two decades, the beaches of Algarrobo Bay in Chile have been experiencing a proliferation of green algae, a phenomenon that has had a strong impact on activities of relevance to the community such as tourism.

Known as “green tides”, these blooms cause a large part of the beaches and rocks to be covered by a green blanket of algae, causing a bad smell and preventing access to the sea.

In an attempt to minimize their negative effects, the Municipality of Algarrobo has tried to remove them using heavy machinery, but it has been an ineffective measure that has even generated other impacts, as by doing so, large amounts of sand are removed from the beach.

Scientific research on algae in Chile and identified culprits

Although this phenomenon has been known globally for more than 100 years, its frequency has increased in recent decades, causing concern in coastal communities due to the impact on their daily activities, artisanal fishing, and particularly on tourism and recreation.

Since 2021, researchers from various universities and scientific centers in the country, led by the Millennium Institute in Coastal Socio-Ecology (SECOS), the UC Coastal Observatory Center, the UC Coastal Marine Research Station, and the Copas-Coastal center, started a research program to gather scientific evidence on the environmental conditions of Algarrobo Bay and its possible connection with the green tides on the beaches.

Carolina Martínez, director of the UC Coastal Observatory Center, explains that the problem of green tides is especially concentrated in the southern sector of Algarrobo Bay, where due to this problem the population of the commune has seen its recreational and economic activities severely restricted.

“This was one of the concerns we gathered from our work with the communities in this area, affected by a series of complex issues, such as coastal erosion, excessive urbanization, pollution, and a general loss of ecosystems.”

Algae species and contributing factors

After three years of research and several field campaigns, the conclusions of the report, delivered to the municipality and local community organizations at the end of December, eliminate some hypotheses and address possible explanations, although inconclusive, about the factors that have been generating these green tides in the bay for at least 20 years.

One aspect of the research, coordinated by Loretto Contreras, a professor at Andrés Bello University (UNAB) and SECOS researcher, along with Pilar Haye (UCN and SECOS) and an interdisciplinary group of researchers, discovered that there are five species of algae of the Ulva genus present in these green tides. However, the predominant algae corresponds to the species U. stenophylloides, which had not been described for our coasts and could have been introduced to the bay.

According to Loretto Contreras, a researcher at the CAPES Center, the proliferation of these algae would be explained by the levels of nutrients present on the coast of Chile, due to the coastal upwelling phenomenon caused by winds from the south that bring cold and nutrient-rich deep waters. This phenomenon, present in several coastal areas of the country, promotes high growth rates for macroalgae and phytoplankton biomass, favoring a constant and more noticeable growth of these green tides in the summer.

Contreras suspects that these algae may be introduced species, as there were no previous records. However, she admits that further research is needed to identify these species present along Chile, a parallel project that will begin this year.

Impact of coastal erosion and constructions

The study also suggests that the loss of sand from the beaches of Algarrobo due to coastal erosion has impacted the Playa Los Tubos area, transforming permanent sandy bottoms into ephemeral hard bottoms that favor Ulva by being periodically covered by sand, eliminating other species.

However, it is noted that there is not enough information to assess this hypothesis, and more research is needed to define what the sediments were like in that sector over 20 years ago.

Additionally, the potential impact of the closure of the Cofradía Nautical sector due to the jetty that connects the coast with the Pájaro Niño Islet, built in 1978, is mentioned.

Pollution and recommendations

Another factor studied was the contribution of nutrients from the Algarrobo ESVAL Submarine Outfall, which was speculated to influence the phenomenon. However, the conclusions revealed that it would not play a relevant role in the persistence of the green tides in the southern sector of the bay.

The group of authors recommends incorporating a coliform monitoring program in filter organisms in the bay, both in rocky sectors and soft bottoms. This is to better define the area directly impacted by organic matter and fecal coliforms from the outfall and thus establish a safe zone for shellfish harvesting for human consumption.

Finally, the research recommends conducting studies to identify the Ulva species present in other bays in the country, where green tides are also present, such as Coliumo Bay in Biobío.

It is advised to establish baseline studies and set up permanent observation sites through a citizen science program for monitoring green tides in Chile.

“It is also important to establish simple measures to prevent the dispersion of these algae to other bays in Chile, and for that, we are collaborating with the sports clubs in the area,” concludes Sergio Navarrete.

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