Aparently very little comes from the Sargasso Sea. The latest theory is that it is caused by the nutrient rich polution flowing out of the Orinoco and Amazon rivers. I can't see how Orinoco sargassum could reach the BVI but the Amazon feeds the North Brazillian Current and the North Equitorial Counter Current where it has plenty of time to grow and mature as it crosses the Atlantic. Some is deposited on west African beaches and the rest recirculates back in the North Equitorial Current. A certain fraction stays in The North Eqitorial Current and is carried north to the Leewards and the BVI but the majority transfers back into the North Brazillian Current which flows through the Windwards so the problem is much worse down there.