Brown seaweed washes ashore in Nayarit, but experts say it's not sargassum

A large accumulation of brown seaweed that washed ashore on a popular beach in Mexico's Pacific state of Nayarit has been identified as a harmless macroalgae, easing fears that the region was experiencing its first major sargassum invasion.

As reported by Mexico News Daily, for the past two weeks, residents and visitors in Rincón de Guayabitos have reported thick mats of brown vegetation covering the shoreline.

The unusually large accumulation on Sunday prompted local officials and social media users to describe the event as a "sargassum" invasion, the outlet reports.

However, researchers from the Autonomous University of Nayarit (UAN) say the material is not the Atlantic sargassum that has plagued Mexico's Caribbean coast for years.

Instead, it has been identified as “hypnea,” a naturally-occurring Pacific macroalgae.

Speaking with Radio UAN, marine researchers Ubisha Hernández Almeida and María Alcántara said the seaweed buildup is the result of seasonal shifts in ocean currents, water temperatures and nutrient levels.

Unlike Caribbean sargassum, which can release gases as it decomposes that may cause respiratory irritation and skin discomfort, hypnea is not associated with massive transoceanic blooms and is not considered a health risk.

While it can produce unpleasant odours as it breaks down, experts say it is largely a temporary nuisance rather than an environmental or public health threat.

A severe sargassum season

The clarification comes as Mexico's Caribbean coast continues to grapple with another severe sargassum season.

Officials in Quintana Roo report collecting nearly 79,500 tonnes of sargassum by July 6, with Playa del Carmen and Cancún accounting for about 60 per cent of the total.

Extensive containment and cleanup efforts remain underway across the region.

Researchers note that Pacific events like the one in Guayabitos are typically localized and short-lived.

Residents say smaller accumulations have occurred in previous years, including in 2021, but never on the scale seen this week.

Experts also stressed that the accumulation is not a harmful "red tide," which involves toxic microscopic algae that can contaminate shellfish and trigger health warnings, Mexico News Daily reports. 

Instead, the Guayabitos event consists of larger seaweed mats that may be unsightly and foul-smelling but have not prompted any public health advisories.

Local authorities, volunteers and residents have begun clearing the beaches, while tourism operators remain optimistic the event will be temporary.

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