A tropical algae thriving on fertilisers from hotel golf courses and badly treated sewage is killing one of Costa Rica's most important coastal reefs, scientists say.

The green, feather-like algae is spreading along the reefs of Culebra Bay in Costa Rica's north-western Gulf of Papagayo, a popular scuba diving spot and home to a rare species of coral. The algae blocks the sunlight and suffocates the reefs.

A tourism and construction boom along the palm tree-lined beaches is creating nitrogen- and phosphate-rich waste that feeds the algae, known as Caulerpa sertularioides, and Costa Rica is only just becoming aware of the problem.

"It's an ecological disaster," said Cindy Fernandez, a marine biologist with the nonprofit MarViva Association, who alerted the Costa Rican government to the threat, which is now being taken on by the state-run University of Costa Rica.

Scientists say about 80 percent of the reef area, which stretches for about two kilometres along the coast line, is covered in the algae.

The aggressive algae spreads when even the smallest sliver comes loose, from the likes of strong currents or dive boats dropping anchor, to root itself in another part of the reef.

Even the sweep of a diver's hand or the kick of a diver's fin can send a fragment swirling away to start another patch.

That means experts cannot pull it up like weeds.

"If you pull it up it will reproduce faster," said Jenny Asch, coordinator of the government's marine conservation program, who is leading efforts to find a way to eradicate the algae.

If left unchecked, the algae could also severely damage the ecosystem of the bay, allowing non-native species of fish to come in and displace the native species.

The highly invasive Mediterranean strain of the algae, Caulerpa taxifolia, was discovered in Southern California in June 2000, where scientists have used solid chlorine blocks to eradicate the pest.

Costa Rican scientists do not yet know if similar eradication techniques will work on Caulerpa sertularioides.

The algae is the latest challenge facing Costa Rican authorities as the Central American country struggles with conserving its unique tropical biodiversity while attracting tourists and marketing itself as an ecotourism paradise.