The stench of decaying algae began rising from coastal waterways in southeastern Florida early this month, shutting down businesses and beaches during a critical tourism season. Officials arrived, surveyed the toxic muck and declared states of emergency in four counties. Residents shook their heads, then their fists, organizing rallies and haranguing local officials.

In truth, there was little they could do: The disaster that engulfed the St. Lucie River and its estuary had been building for weeks. In May, a 33-square-mile algal bloom crept over Lake Okeechobee, the vast headwaters of the Everglades. After an unseasonably wet winter, the Army Corps of Engineers was forced to discharge water from the lake to lower water levels, flushing the ooze along channels to the east and west until it coagulated along the shores of the famed Treasure Coast.

The mess in Florida is only the latest in a string of algal blooms that some experts believe are increasing in frequency and in severity. An immense plume of blue-green algae last September covered a 636-mile stretch of the Ohio River. A month earlier, the city of Toledo, Ohio, warned more than 400,000 residents to avoid drinking tap water after toxic algae spread over an intake in Lake Erie. (Indeed, the Lake Erie bloom is now an annual event.)