Most worrisome is that the dimming of the bay is a whodunit — a mystery that has stoked animus between locals and the business owners, nearly all of whom are originally from the mainland United States, who run tourist trips to the bay. Because bioluminescent bays are seldom studied over long periods, nobody knows what has prompted the dinoflagellates to either leave the bay or diminish their magical bluish-green glow.

And nobody knows if and when the bay’s shimmer will come back to full strength. Some bioluminescent bays in the Caribbean and beyond, including Laguna Grande in Fajardo, P.R., have gone dark temporarily and then resumed full strength (sometimes big storms are to blame for the blackouts). Others come back with more erratic glows. A few have turned off altogether.

Saving Mosquito Bay is critical, scientists and government officials said. Only a handful of prized bioluminescent bays exist in the Caribbean, and a few more lie in the Pacific. Their numbers fluctuate as some die out and others take their place, and many only offer a seasonal glow. For the dinoflagellate to thrive, conditions must be just right. Mosquito Bay is optimal: warm and shallow with stable winds and ringed by red mangroves, which feed the plankton.