The calls and queries came from fishermen, scuba divers and charter boat operators. What was that mask of dense green water that muddied the waters around the U.S. and British Virgin Islands this spring?
What some labeled an “algae bloom” was in fact from a large plume from the Orinoco River in South America, explains Dr. Richard Nemeth, director of the Center for Marine and Environmental Studies at the University of the Virgin Islands (UVI).
“As the river discharges its water off of Venezuela, it usually mixes with the ocean water, but occasionally some of the brackish water gets trapped in a gyre (a small body of water circulating independently of the ocean around it). Because the water in the plume is less saline and warmer, it can remain intact for quite a long time and very occasionally this water gets pushed up into the Caribbean by the normal currents that enter the Caribbean near the southeastern Antilles islands. These plumes are usually smaller and rarely reach the Virgin Islands. Instead, they mainly impact southern Caribbean islands like Trinidad and the Grenadines.”
This time, Nemeth said, the plume was huge and remained intact for over a month. And, since the water from the Orinoco is high in nutrients, the microscopic algae or phytoplankton that got mixed in had a tremendous population growth that turned the water green.
Scientists diving through this plume estimated its depth at some 80 feet deep, with normal clear Caribbean water underneath.
“This was a natural event that is not related to pollution,” Nemeth says. However, what effect this type of plume has on fish, fish habitats and coral reefs is something that marine scientists are researching.
Some scientists think the plume may be a source of rare species. For example, 19th century naturalist R.H. Schomburk wrote in 1832, “I have already noticed the calcareous and siliceous deposit on the southern side of Anegada, which I consider to be the drift matter of this current, and very likely a part of the sediment brought down by the Oronoco. This explains the reason why there are many plants to be met with on the island, which do not exist in any of the other Virgin islands, but are peculiar to South America."
There’s some indication, says Dr. David Olsen, chief scientist for the St. Thomas Fishermen’s Association, “that Category 5 hurricanes tend to pass through areas where the plume has been.”
UVI’s Nemeth says, “Some think that the nutrients in the plume may harm corals by allowing other macroalgae or seaweed to grow over the corals. We have not seen this happening yet. Still others think that the high concentration of plankton might be good for corals since coral also feed on small plankton at night. We are not sure how it affects the fish, but we did notice while diving that fish which normally were feeding way up in the water column were staying below the plume.”