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.