27 Nov Scientific research links Algarrobo’s “green tides” to natural conditions and environmental degradation: findings were delivered to the Municipality of Algarrobo

The three-year report, carried out by national universities and scientific centers, reveals the presence of an algal species not previously recorded in Chile and analyzes oceanographic, geological, and environmental factors that may explain the persistence of the phenomenon in the bay. The results were delivered this morning to the Municipality of Algarrobo to support local decision-making.

A three-year study conducted by an interdisciplinary team from Chilean universities and research centers—including the Millennium Institute for Coastal Socio-Ecology (SECOS), the UC Coastal Observatory Center, the UC Coastal Marine Research Station (ECIM), the Laboratory of Ecology and Molecular Biology in Algae at Universidad Andrés Bello (LEBMA), and the COPAS-Coastal center—identified the drivers behind the recurrent “green tides” affecting Algarrobo Bay for more than two decades.

The results were presented to the Municipality of Algarrobo and local community organizations on November 25 by Dr. Carolina Martínez and Dr. Sergio Navarrete, SECOS researchers, with the aim of providing scientific evidence to inform decision-making and support integrated coastal management. The meeting, held at the Municipal Environmental Management Department, was attended by Mayor Marco Antonio González and COPAS-Coastal Deputy Director Fabio Tapia.

Proliferation of algae

The report identifies five species of the genus Ulva present in the green tides, but confirms that the predominant species is Ulva stenophylloides, an alga not previously described along Chilean coasts and potentially introduced to the bay. “We believe these may be introduced species, as there were no previous records. However, further collaborative work is needed to determine their distribution throughout the country,” stated Loretto Contreras, director of the Algal Ecology and Molecular Biology Laboratory at Universidad Andrés Bello and associated SECOS researcher.

The study also highlights the role of coastal upwelling—a natural process that brings cold, nutrient-rich deep waters to the surface—stimulating rapid growth of macroalgae and phytoplankton. These conditions amplify the intensity of green tides, particularly during summer.

Sediments and sand loss

The study suggests that coastal erosion and the gradual loss of sand in areas such as Los Tubos Beach may have transformed former sandy bottoms into temporary hard substrates, conditions that facilitate the proliferation of Ulva. However, it cautions that there is still insufficient historical information to fully assess this dynamic.

Researchers also evaluated the impact of the breakwater built in 1978 connecting the coast to Islote Pájaro Niño in the Cofradía Náutica sector. According to the study led by Dr. Matías Gómez of Universidad de Talca, this structure altered sediment transport and reduced the natural dynamics of southern bay beaches. “These changes must be analyzed alongside increased urban development and pressure on the coastline, which have substantially modified sediment inputs,” explained Carolina Martínez, director of the UC Coastal Observatory Center and SECOS researcher.

Environmental monitoring

Regarding the ESVAL Submarine Outfall, researchers recommended implementing a monitoring program of coliforms in filter-feeding organisms—both rocky- and soft-bottom species—to precisely map areas affected by organic matter and establish safe zones for shellfish harvesting for human consumption.

Green tides have affected Algarrobo for years, becoming an ecological and social challenge with significant impacts on tourism and recreational beach use. The results delivered to the municipality provide updated information to better understand this complex phenomenon, shaped by natural processes as well as environmental changes linked to coastal development.

The research team called for expanded monitoring programs, environmental restoration, and integrated coastal management—key measures to address the transformations experienced by the bay over the past two decades. In the second half of 2026, a follow-up report on coastal erosion and sediment transport will be issued to support the municipality in designing restoration measures for sandy shores affected by severe erosion in recent years.

Finally, studies led by Dr. Loretto Contreras are focused on developing additional uses and valorization pathways for algal biomass, framed within Coastal Sustainability and Blue Economy approaches, with the local Algarrobo community at the center of these initiatives.