Toxic Gases from Decaying Seaweed: Health Risks and Coastal Mitigation

Original publisher: The Conversation. This article explains how decaying seaweed releases gases such as hydrogen sulfide and ammonia, creating health hazards for beachgoers and workers, and outlines how removal and management strategies can reduce risk and salvage value from seaweed.

• Rotting seaweed can release toxic gases that may reach hazardous levels if large masses accumulate.
• Hydrogen sulfide is a key contributor and ammonia is discussed in relation to safety guidelines of five to twenty parts per million (ppm).
• Removing seaweed before it piles up is emphasized, with freshwater soaking shown to suppress gas formation by up to 99% in experiments.
• Collected seaweed can be reused as a soil additive or for energy generation, offering waste-to-energy opportunities while protecting coastal economies.

Overview

Seaweed strandings on coastlines can become health hazards as decaying masses release gases. The article cites incidents in Brittany, France, and the Caribbean where exposure to toxic gases has caused headaches, nausea, and even fatalities, illustrating the real-world stakes of seaweed management on beaches.

Why seaweed blooms occur

Excess nutrients from sewage treatment works and farmland runoff stimulate rapid seaweed growth. Warm weather further accelerates biomass accumulation. When ocean currents and winds drive patches together, strandings overwhelm shorelines with dense mats that can disrupt fisheries and tourism and threaten coastal communities dependent on healthy ocean ecosystems.

Composition of the culprits

Gases produced during decomposition are not fully characterized, but hydrogen sulfide is a primary suspect, producing a rotten-egg smell, along with ammonia and other sulfur compounds such as dimethyl sulfide and methanethiol. European safety guidelines suggest keeping exposure to these gases within five to twenty ppm. The author’s UK study quantified gases released from two nuisance seaweed species, Ulva intestinalis and Sargassum muticum, finding hydrogen sulfide as a major contributor and ammonia not liberated in the initial eight days of experiments.

Health and environmental risks

In addition to respiratory irritation, decaying seaweed strandings can harbor bacteria associated with the human gut and sewage, potentially leading to infections, antibiotic resistance, and amplified health risks for swimmers and wildlife. Climate change is likely to worsen strandings, expanding the health and economic footprint on coastal regions.

Removal and spray solutions

The best defense against catastrophic gas release is to remove seaweed before it accumulates. Cleanup is labor-intensive and costly; for instance, one of the largest seaweed strandings in China involved over 10,000 workers and costs above US$100 million. The article notes that freshwater addition to decaying seaweed can alter the decomposition process by reducing sulfate, a substrate used by anaerobic bacteria to generate hydrogen sulfide, achieving up to a 99% reduction in hazardous gases. While freshwater spray may not always be available, reservoirs and rivers could provide a mitigation option for stranded patches.

Potential uses after collection

Collected seaweed can be repurposed as soil additive and fertilizer or used for energy generation through anaerobic digestion to produce methane-rich biogas for heating and electricity, turning a hazard into resource.

Conclusion

Prevention and rapid response to seaweed strandings are crucial to safeguarding public health and coastal economies. As conditions shift with climate change, proactive management and exploration of seaweed-based resources will be essential components of coastal resilience.