Plough clearing up seaweed on a beach in Yucatan, Mexico. Since 2011, a brown algae called sargassum has increasingly piled up on Mexican coastlines. This algae?™s growth is cyclical and beneficial in normal amounts, providing a floating habitat for sea animals. Researchers blame multiple factors for the increased levels of sargassum, including rising sea temperatures and fertilisers running off into the ocean. Decomposing sargassum releases hydrogen sulfide gas which smells like rotten eggs and can irritate the eyes, nose and throat.