Mesoscale eddies carry algae to shores, cause coastal choking

Pelagic Sargassum, a type of brown macroalgae that forms floating rafts at the ocean surface, can serve as a habitat for marine fauna and an important carbon sink.

However, since 2011, the rafts of algae have frequently shown up on Caribbean beaches. They contain high levels of arsenic and heavy metals and negatively impact seagrass communities, corals, and water quality. The impacts are widespread, from an increase in sea turtle and fish mortality to a disruption in tourism and local economies.

Andrade-Cano et al. identified mesoscale eddies —vortices or swirls in the Caribbean Sea approximately 50 kilometers in radius— as the carriers of Sargassum and a mechanism for the resultant coastal choking.

The team discovered the eddies with satellite altimetry and constructed a model of the algae rafts based on networks of elastically connected buoyant particles.

“Mesoscale eddies that are capable of transporting Sargassum are very special,” said author Francisco Javier Beron-Vera. “They have material (i.e., fluid) boundaries that defy the exponential stretching an arbitrary material loop typically will experience in turbulent flow, like the ocean. However, they have the capacity of sucking in Sargassum rafts, as they possess finite-time attractors for elastic networks.”

The eddies transport Sargassum to the west due to the variation with latitude of the local angular velocity of the planet. At shallow depths, thermal instabilities destabilize the eddies and spread their contents toward the shore.

Monitoring eddies with satellite altimetry data can inform future coastal choking events, but a forecasting system is necessary to predict exact timing and location.

“Our group is planning to develop such a capability, which requires one to take into account the influence of wind, particularly near the shoreline,” said Beron-Vera.

Source: “Carriers of Sargassum and mechanism for coastal inundationin the Caribbean Sea,” by Fernando Andrade-Cano, Francisco Javier Beron-Vera, Gustavo Jorge Goni, Daniel Karrasch, Maria Josefina Olascoaga, and Joaquin Trinanes, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0079055 .