Due to the occasional onshore flow, patches tend to make their way to the coast and can lead to some unpleasant days on the beach where visitors have to walk through seaweed and occasionally get a whiff of a smell described as resembling rotten eggs.

The distinctive "smell" often associated with the Florida Keys is primarily caused by decomposing Sargassum seaweed. This brown macroalgae floats in the Atlantic and washes ashore in large quantities during the spring and summer months. As the seaweed piles up on the beach and begins to rot in the tropical heat, it undergoes a biological breakdown that releases hydrogen sulfide gas. This gas is notorious for its "rotten egg" odor. While the smell can be pungent and occasionally irritating to those with respiratory issues, the seaweed itself is a vital part of the ecosystem, providing a habitat for diverse marine life. In 2026, local authorities in Monroe County employ various management strategies, such as mechanical beach raking in high-traffic tourist areas, to mitigate the odor. However, in more remote or protected parts of the Keys, the smell is a natural, albeit smelly, part of the coastal cycle. The intensity of the scent fluctuates based on the size of the "Great Atlantic Sargassum Belt" and the prevailing wind directions.