Violent ocean motion no magic potion for reefs
Hurricanes bring temporary relief to Florida reefs smothered by invasive seaweed
In August, Harbor Branch scientists began a new survey of Florida coral reefs expecting to document the devastating spread of harmful seaweed that has been progressing now for several years, but hurricane havoc has instead led the team to a surprising find. With their first survey nearly completed, it appears all reefs in the path of hurricanes Frances and Jeanne have been largely scoured free of the menacing seaweed, and many have also been buried in sand. Seaweed overgrowth problems are likely to return, however, and could even be exacerbated by the storms temporary removal. Further south, off areas beyond the brunt of the storms, the team has found alarming concentrations of a cyanobacterium similar to algae killing corals and other reef organisms.
"This is uncharted territory," says Harbor Branch marine ecologist Brian Lapointe, "no one has ever had the chance to study the impacts of natural phenomena like hurricanes on reefs under siege from these harmful algal blooms that we believe are triggered by humans."
For three decades now, Lapointe has been studying the harmful spread of macroalgae, or seaweed, on coral reefs throughout Florida and around the world. Besides smothering and killing coral itself, such harmful algal blooms (HABs) cover the food on which many fish rely, forcing them and their predators away, and HABs can fill ledges and crannies that attract lobster.
Ongoing funding from the Environmental Protection Agency that began in 2003 had allowed Lapointe and his team to study two isolated reefs off Palm Beach County that had become almost completely covered in lawns of an invasive alga called Caulerpa brachypus, among other species. This insidious alien is closely related to Caulerpa taxifolia, which has caused billions of dollars in damage since accidental introduction to the Mediterranean in 1989. South Florida dive operators have reported for years that overgrowth was so bad at some previously popular dive reefs that these locations were no longer worth visiting.
Lapointe believes based on past research that the spread of C. brachypus and other macroalgae species, in Florida and elsewhere, is largely driven by nutrient pollution from land-based sources.
Because so little was known about the extent of the macroalgae problem in South Florida beyond the two reefs already studied, and because of the potential for severe ecological and economic damage, Senator Ken Pruitt pushed for several years to fund a reef survey. In May, the state legislature appropriated $500,000 for the project (http://www.hboi.edu/news/press/jun0804.Html).
The survey, which is being conducted in conjunction with the Florida Fish and Wildlife Research Institute in St. Petersburg, involves detailed studies of 84 reef sites between north Miami and Ft. Pierce, 110 miles to the north. Seventy-seven sites have now been studied, and the rest will be completed once weather permits.
Working before the hurricanes in northern Dade County, the Harbor Branch team found no signs of macroalgae overgrowth, but instead large concentrations of a cyanobacterium (once referred to as blue-green algae) called Lyngbya, which grows in large, stringy clumps and can be as troublesome as HABs. Lyngbya grows over and kills soft and hard corals as well as associated organisms such as sponges. The species has also been tied to the production of compounds that can cause tumors in sea turtles and other animals, and research at other institutions has indicated its spread may also be tied to pollution.
Before the team was able to study reefs farther up the coast, hurricanes Jeanne and Frances pummeled the region, forcing a two-month suspension of the project. Once the work resumed, the team continued to find Lyngbya in parts of Palm Beach, Broward and Dade counties, including new growth since the hurricanes. But they found that in northern Palm Beach County, Martin, and St. Lucie counties, all directly hit by the storms, reefs had been scoured clean of C. brachypus and other macroalgae. Hurricane churning also completely covered many reefs with sediment, and dusted the rest. Just before the hurricanes hit, the team had also begun to see patches of Lyngbya at these reefs, but none was seen after.
Lapointe says that while the removal of macroalgae may be a welcome event for reefs, the reprieve is not likely to last. Small fragments of C. brachypus, for instance, have already been spotted and blooms could re-emerge as the environmental conditions that have fostered its spread and that of other troublesome species have not changed.
"This is a break, not a solution for the reefs," says Lapointe of the hurricanes inadvertent macroalgae cleaning. "Unless we can get a clear understanding of what drives these blooms and control the culprits, its going to be more of the same and well risk losing this incredible ecological and economic resource."
Based on the teams observations it appears that some of the sand covering the reefs may have originated from beach nourishment projects. Sand used for beach nourishment is often dredged from offshore borrow pits, where silty sediment much finer than sand normally found on beaches and around reefs accumulates. A number of Palm Beach County reefs are covered in such silty material. At reefs closer to shore, in water depths of 40 feet or less, the team has been finding coarser beach sand covering reefs.
As buried and scoured reefs begin to recover, macroalgae problems could become even more pronounced, says Lapointe. Previous HABs have occurred in competition with healthy reef populations. Now, scoured reefs have become nearly blank slates. Because invasive species such as C. brachypus have no natural predators, unlike native species, they are fierce competitors that will likely have a significant edge over native species as reef recovery progresses.
During the surveys, the team takes video transects of reefs for later analysis to determine what macroalgae and other organisms are present and in what abundance, and roving divers count the number of grazing fish in the study area. The researchers also collect water and algae samples whose chemical signatures are studied to determine if they more closely match the signatures of natural sources of nitrogen or of land-based pollution sources. This information can help clarify the most likely cause or causes of the HABs.
Mark Schrope | EurekAlert!