FILE – This April 1, 2005 file photo shows the Fajardo Grand Lagoon at the Nature Reserve of Las Cabezas de San Juan at dawn, about 35 miles east of San Juan, Puerto Rico. Biologists are investigating why the bioluminescent bay off the northeast coast off the Puerto Rican coast has lit up once again, Friday Dec. 6, 2013, as scientists wrap up their investigation into why it went dark last month, officials said Friday Photo: HERMINIO RODRIGUEZ, ASSOCIATED PRESS

By DANICA COTO | Associated Press

SAN JUAN, Puerto Rico (AP) — A popular lagoon that glows at night off Puerto Rico’s northeast coast has lit up once again as scientists wrap up their investigation into why it went dark last month, officials said Friday.

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, said Carmen Guerrero, secretary of the Natural Resources Department.

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, she said.

“The big swell prevented these organisms from staying in the lagoon,” she said.

Scientists found that the swell was eight times bigger than what the lagoon experiences on average.

The lagoon draws hundreds of tourists a year who rent kayaks or boats by night from the nearby city of Fajardo to observe the water emit a greenish light when fish swim by or when they trail an arm through it.

Scientists and residents were concerned that the lagoon stopped glowing because of runoff from the construction of a nearby water and sewer treatment plant, or because of people cutting down mangroves to allow larger boats into the area.

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.

Those findings demonstrate a need for the water and sewer treatment plant currently being built, said Laura Velez, president of the Environmental Quality Board, which monitors such projects.

Government officials said the contamination problem was the result of a nearby community lacking a proper sewage system.

Residents had asked that the plant be moved elsewhere when the lagoon began going dark, and the government temporarily suspended construction of it for two weeks to allow scientists to investigate.

Alberto Lazaro, president of the state Water and Sewer Authority, said the agency is contemplating relocating the plant and that officials will meet with residents before choosing a location.