Is seaweed causing tropical beaches to disappear?
July 29, 2000
OFF to the tropics for your holidays? You might have trouble finding the beach, and pollution from increased tourism could be to blame. Researchers in Florida have found evidence that effluent is preventing marine algae from replenishing sand at Negril, one of Jamaica's main resorts. They believe that other beaches, including some in Barbados, could be disappearing for the same reason.
Halimeda, a thick, crusty alga, produces 80 per cent of the sand on Negril's beaches. Like corals, the algae build a skeleton of calcium carbonate. Eventually this skeleton disintegrates to form sand. Halimeda and related algae supply varying amounts of sand to tropical beaches around the world.
But during the past decade, Halimeda has run into trouble in the shallow waters off Negril. Several species of seaweed that are normally only present in small amounts have overrun corals and the sand-forming algae. At the same time, the area began experiencing massive beach erosion, with some sections receding by up to 20 metres.
"It's only been during the past five years that the erosion problem has become a panic situation," says Katy Thacker, executive director of the Negril Coral Reef Preservation Society. To combat the erosion, the town has been trucking in sand from a nearby river and piling up sandbags along the beaches. Prior to that "there was a point where people actually had their lawn chairs in the water", Thacker says.
Brian Lapointe of the Harbor Branch Oceanographic Institution in Fort Pierce, Florida, and his colleagues have previously shown in experiments that the quick-growing seaweeds flourish in high levels of nitrogen and phosphorus, as found in polluted waters. This could allow the seaweeds to dominate the widespread but slow-growing calcifying algae, which are adapted to the low-nutrient conditions typical of tropical reefs.
Lapointe and his team decided to investigate whether this phenomenon is causing the erosion at Negril, where a growth in tourism and agriculture has led to increasing levels of pollution. They found that nitrogen isotope levels in most seaweed matched those in fertilisers used on nearby sugar cane fields. Seaweed from one site contained nitrogen levels that matched those in sewage, and in peat removed through land clearing.
Lapointe believes this shows that pollution from runoff and groundwater seepage has caused the growth of the seaweed. This, in turn, has pushed out Halimeda, and prevented it from replenishing beaches eroded by storms. "I think this is an entirely reasonable conclusion," says Robert Ginsburg, a marine geologist at the University of Miami in Florida.
Previously, scientists had thought that overfishing caused the erosion, by reducing the numbers of fish eating and controlling the seaweed. But without the high nutrient levels in the pollution, Lapointe says, the seaweed could never have taken over. He also believes that recent hurricanes and increasing numbers of sea urchins are making the problem worse by damaging the reefs so they form less of a barrier to wave erosion.
Lapointe says that pollution could be having similar effects in other areas with nearshore reefs, and may be the cause of the beach erosion occurring now in parts of Barbados. "It's really a global issue," he says. Ginsburg agrees that researchers need to explore the possibility that the problem extends beyond Jamaica.
Mark Schrope
From New Scientist magazine, 29 July 2000.