A strange algae plume has turned the normally crystal clear Caribbean Sea around the Virgin Islands green down to a depth of roughly 80 feet (25 metres) and sharply decreased visibility in these popular dive waters. How and if the plume will have any long-term effect on the region’s marine life remains unknown.

Tyler Smith, assistant professor at the Center for Marine and Environmental Studies at the University of the Virgin Islands, said that when he went diving Tuesday the visibility inside the plume was no more than 10 feet (3 metres). Below 80 feet, the water was just as clear as normally.

The reason behind the extraordinary plume can be found in South America, in the Orinoco River which flows through Venezuela before reaching the Atlantic Ocean. When the Orinoco outflow is larger than normal, the vast amounts of nutrient-rich freshwater from Venezuela cause a major algae bloom in the nearby ocean. Mixed fresh- and saltwater is lighter than seawater and will therefore rise to the top of the water column.

“It’s very stable, so it just sits there,” Smith explains.

Carried by currents, the algae plume has now spread from the South American east-coast to the Caribbean Sea and can currently be seen not only off the British and U.S. Virgin Islands but in Puerto Rican waters as well. The first patch was noticed by Smith and his colleagues in the waters off St. Croix on April 9.

When the amount of photosynthesising alga increases in a region, it attracts all sorts of organisms that feed on algae and make it possible for these populations to boom as well. The algae plume around the Virgin Islands supports an entire food chain of marine life, including plankton, jellyfish, crustaceans and fish. It is not dangerous to swim or scuba dive in, but some people might dislike the high density of jellyfish.

“This is an event that occurs every year, but we haven’t seen it come this far north,” says Trika Gerard, marine ecologist with the U.S. National Oceanic and Atmospheric Administration (NOAA). In a stroke of good luck, a NOAA research vessel was scheduled to research reef fish in these waters from April 7 to April 20 – right at the peak of the unexpected plume.

To find out more about how the plume effects marine life, the Caribbean Fisheries Management Council is urging anyone who goes out fishing in the green plume to report their location, target species and success rate of each trip. According to local fishermen the fishing is always awfully bad when the water is green, but this has not been scientifically researched yet and all data is of interest.

You can reach the Caribbean Fisheries Management Council by calling (787) 766-5927. Their website is http://www.caribbeanfmc.com.