Beneath the Gulf of Papagayo, a mesmerizing field of green quivers with the ocean currents, tilting in to shore, and out, with each passing swell.
Beautiful at first glance, but an ecological disaster waiting to happen, according to a recently released study by marine biologist Cindy Fernández, who spent the past five years monitoring the weed’s rampant advance over the bottom of the Gulf, in the northwestern province of Guanacaste.
It is the oceanic equivalent of a monoculture of teak trees, or African palm, explains Fernández – pretty to look at in its symmetry, perhaps, but almost entirely devoid of life.
“When this weed takes over, what we see is a great decline in the number of fish species using an area. Instead of many species, we might see just one,” she said.
In essence, the ecosystem dies.
The underwater plant, a species of algae known by its scientific name, Caulerpa sertularioides, also smothers coral, slowing its growth and sometimes killing it – which in turn effects the entire food chain – including both plant-eating fish and larger predators who come to the reefs to forage.
It looks and acts like a common backyard weed, said Fernández, who explained that even a small part of it broken loose and carried off on ocean currents, or pinned in the anchor of a sailboat or yacht, can contaminate a far-off region.
Getting rid of these underwater weeds is equally difficult, requiring manual extraction – much like the annoying sprouts in a home garden.
While the weed is native to the region, its exponential growth in recent years is not.
“There has been an enormous increase in the amount of this plant over the past five years,” she said. “It spreads very easily.”
Expansive, two- to four-hectare patches of the weed now cover underwater areas off Playa Penca and Playa Blanca, near the mouth of CulebraBay.
Fernandez fingers irresponsible development, which has boomed along the shores of the Gulf, as the primary reason for the weed’s advance.
“More nutrients from hotels and other development are entering the water, everything from wastewater to fertilizers,” she said, adding that heavy rains wash much of that into the Gulf, in a sort of nutrient waterslide.
The Gulf harbors one of the most important areas of Pacific coast coral reefs –including unique species of coral and fish that are rarely seen elsewhere.
According to Bobby Joe Gibbs, owner of Diving Safaris in Playa Hermosa, the weed is always present – but has yet to effect her business, or her client’s enjoyment.
“It hasn’t overgrown the dive sites yet; we have a lot of variety,” she said.
She said people come to see seahorses, octopus, eels, white-tipped sharks, eagle rays, and southern sting rays, among many other species, and that in her opinion, they’ve yet to be affected by the algae.
Javier Canciani, a dive instructor for Deep Blue Diving Adventures, in Playas del Coco, agreed, though he said he’s worried about the future of the region’s corals.
“Most think its just another pretty plant in the ocean. They don’t realize how much its spread in the past few years,” Canciani told The Tico Times after a dive earlier this week.
Fernández said the effects of the quickly growing algae – and its tendency to create a monoculture – have only begun to be felt.
“If they haven’t seen a decrease in species yet, they will,” she said.
The algae was also discovered recently off Isla de Caño, a prized dive site off the Osa Peninsula in the Southern Zone, an alarmbell for biologists, who worry that it might spread and smother coral reefs elsewhere.
Coral reefs have become a valuable and increasingly rare commodity around the globe, according to a report released in April by the United States Geological Survey (USGS).
“Coral ecosystems worldwide are in decline,” reads the report, which specifically cites nutrient and sediment runoff in tropical areas as one of the key issues.
“In tropical waters worldwide, there have been significant declines in coral cover and an increase in algal cover on reefs,” it reads.
In Costa Rica, these things have become commonplace – on both the Pacific, and the longer expanses of coral along the southern Caribbean coast.
“The mismanagement of tourism –from unsustainable development to bad practices among snorkelers and divers, is contributing to the problem,” Fernández said.