Decomposing Coastal Seaweed Releases Toxic Gases Threatening Public Health

Rotting seaweed accumulations on beaches can release harmful gases that have caused deaths in France and illness across Caribbean communities. These blooms result from nutrient pollution and warm waters that trigger excessive seaweed growth, which marine life cannot survive. Removing seaweed before decomposition and repurposing collected material for fertilizer or biogas production represent viable prevention strategies.

Seaweed blooms occur when excess nutrients from agricultural runoff and wastewater enter coastal waters alongside warming temperatures, creating ideal conditions for rapid growth. When ocean movements concentrate these algae masses on shores, decomposition begins releasing multiple sulfur compounds into the air. Research has identified hydrogen sulfide as a primary culprit, alongside dimethyl sulfide, methanethiol, and dimethyl disulfide—each with distinct chemical hazards and odors ranging from rotten eggs to decayed vegetables.

Beach cleanup workers face the greatest exposure risk due to proximity to decomposing material, though even residents in nearby areas may experience respiratory effects. Converting collected seaweed into fertilizer or biogas energy represents a dual benefit: eliminating the hazard while creating usable resources. The intensity of gas emissions appears to vary by species, suggesting that different coastal regions may face different levels of health threat depending on their local seaweed composition.