Seaweed strandings cause a stink—here's why they harm your health

Seaweed accumulation on beaches may appear innocuous, but as it decomposes, it can emit hazardous gases. Recent incidents have resulted in fatalities and reported health issues in France and the Caribbean. Seaweeds contribute positively to coastal ecosystems, sequestering carbon and serving as a sustainable food source, however, unchecked growth can become problematic.

Blooms are often triggered by nutrient runoff from sewage or farmland and intensified by warm weather. When these patches collide due to ocean currents and wind, they can blanket coastlines, negatively impacting marine life, fisheries, tourism, and coastal communities. To prevent catastrophic outcomes, it is essential to remove seaweed before it amasses and generates harmful gases.

Workers involved in removal are at the greatest risk of gas inhalation. Low-level exposure to nearby strandings also poses a concern. Scientists have limited knowledge regarding the composition of gases emitted by decaying seaweed, but hydrogen sulfide and ammonia are identified as primary culprits. These gases can cause respiratory problems and death when concentrations exceed 5-20 parts per million, as per European safety guidelines.

Our research, conducted with students and scientists, identified hydrogen sulfide and ammonia as significant contributors to gas emissions from two seaweed species in the U.K. Other toxic gases, such as hydrogen sulfide, dimethyl sulfide, methanethiol, and dimethyl disulfide, were also detected, each with distinct odors ranging from rotten eggs to garlic.

Sulfur emissions were notably higher in Ulva intestinalis compared to Sargassum muticum, indicating that different species may possess varying capacities for causing illness. Nutrient-rich runoff, including fertilizers and sewage pollution, can stimulate seaweed growth. However, harmful bacteria present in sewage may accumulate in seaweed strandings, potentially leading to infections.

We discovered various bacteria associated with the human intestine and feces on decaying seaweed, some of which were antibiotic-resistant. This suggests that seaweed strandings could facilitate the spread of treatment-resistant infections. Climate change is likely to exacerbate seaweed strandings, further heightening health risks.

Preventing the buildup of seaweed strandings is crucial, especially as nuisance species expand into new habitats. Removing seaweed is challenging, particularly in inaccessible locations, and can be costly, as demonstrated by a Chinese incident where over 10,000 workers and $100 million were spent. Adding fresh water to decaying seaweed can modify the decomposition process, reducing hazardous gas emissions by up to 99%.

However, freshwater may not always be accessible, particularly in small Caribbean islands experiencing frequent sargassum seaweed inundations. Exploring the use of reservoirs or rivers to spray water onto seaweed strandings before they become hazardous is a potential mitigation strategy worth investigating.