Heavy swells blamed for darkening Puerto Rico's glowing lagoon

A popular lagoon that glows at night off Puerto Rico's northeast coast is lighting up once again as scientists wrap up their investigation into why it went dark last month.

The secretary of the Natural Resources Department says a prolonged and heavy swell had swept away microscopic plankton known as dinoflagellates that are found in heavy concentrations in the lagoon and emit light through a chemical reaction when disturbed.

Carmen Guerrero said Friday that there are typically thousands of dinoflagellates per liter (gallon) of water in the lagoon, but those concentrations had dropped to less than 100 in early November as heavy swells dispersed them.

Scientists celebrated that it wasn't a man-made problem, but their investigation also found that the lagoon has high amounts of fecal matter and other bacteria.