Scientists agree that coral reefs are in an alarming global state of decline. However, determining the main cause or causes of this decline has proven a much more contentious issue. In the current edition of the Journal of Experimental Marine Biology and Ecology (JEMBE), Harbor Branch marine scientist Dr.Brian Lapointe and colleagues present new evidence they hope will help settle one major debate: whether pollution or overfishing is the main cause of the coral-smothering spread of seaweed on many reefs. The research suggests that pollution from such sources as sewage and agricultural runoff is the main culprit, a conclusion that has major repercussions for managers working to end the decline of reefs in South Florida and around the world.
When seaweed, or macroalgae, spreads over coral reefs, a problem becoming increasingly common, it can smother coral and prevent important reef inhabitants such as fish and lobster from finding the food and shelter they require. The reef that remains is transformed into a dull mound with little of its original vibrant life and color. The two main explanations for such overgrowth are that nutrients in pollution fuel rapid, explosive seaweed growth, or that overfishing and other problems remove key grazers such as fish or sea urchins that would normally feed on the seaweed, keeping its growth and spread in check.
"The reason this issue is so important is that we're losing our coral reefs at a very accelerated rate," says Lapointe. "These systems are basically in catastrophic decline in many parts of the globe, and South Florida is probably losing coral even faster than other parts of the world. This research, I believe, makes it clear that one of the key problems is pollution from land-based sources."
Lapointe has been studying this problem for decades and has found much evidence to support the theory that nutrients, mainly nitrogen, are the key controlling factor in seaweed spread. But the main focus of this at times controversial research has been monitoring case studies at locations with high nutrient levels from pollution such as Negril, Jamaica and the Florida Keys. For the new study, though, he and colleagues set out to conduct a definitive experiment to determine the effects of nutrient enrichment while simultaneously studying the role of grazers. To do this, they turned to a unique study site in the Bahamas called Norman's Pond Cay.
At the island, water from an inland pond with relatively high nitrogen levels flows through a natural mangrove channel and into the ocean. This outflow creates a natural nitrogen gradient as it mixes with seawater, with high concentrations near the channel mouth diluting within about 300 feet to the very low levels typical of surrounding waters.
The team found extremely high amounts of seaweed on reefs near the mouth of the channel, and less farther away as nutrient levels decreased. With the exception of one site, reefs far enough away from the channel were healthy with little or no seaweed overgrowth. The exception was a reef in 80 feet of water near the island. There the group found an abundance of seaweed, but also high levels of nutrients they attributed to groundwater seepage because of similarities between the chemical signature of nitrogen at the site and from groundwater samples taken from deep inside a submerged cave on the island.
Study results supported Lapointe's past research that revealed a critical threshold for nitrogen of about 14 parts per billion, above which damaging seaweed spread is supported and below which it is generally prevented. By comparison, raw sewage is about 40,000 parts per billion nitrogen, while pristine oceanic waters would be about 1 part per billion.
Lapointe says that an even more important aspect of the work was the group's findings on the interactive effects of nutrients and grazers on reefs. The team worked with conch and sea hares, which are grazers naturally found in the area, as well as parrotfish. At each study site they set up cages where the number of grazers was controlled. Some had no grazers, others normal numbers, others extra grazers.
The group found that the grazers ate significantly more seaweed as the nutrient concentrations in surrounding waters and in the algae increased. Furthermore, they found that the number of grazers in a given cage had little effect on the overall abundance of seaweed. Instead, the experiments showed that grazers were selective in the kinds of seaweed they would eat, so the number of grazers mainly controlled the relative abundance of different types of seaweed, not the total abundance of seaweed, which determines the impact on a given reef. Grazers found certain types more palatable and ate mainly those, allowing the less palatable species to proliferate even more. A related study led by Harbor Branch's Dr. Peter Barile found that grazers prefer algae with higher nitrogen concentrations in their tissue as occurs when water nitrogen concentrations increase, likely due in part to a resulting change in flavor.
"To my knowledge, this is the first time anyone has ever shown that with increased nutrients, you get increased grazing in coral reef environments," says Lapointe, "That finding kind of turns the whole field over."
Lapointe says the group's results make it difficult to accept the view held by some coral biologists that reduction of grazers by overfishing is the main driving force behind the damaging spread of algae on reefs, a view known as "top-down" control. "Basically, I think the top-down view has been a matter of looking at the problem through the wrong end of the microscope," says Lapointe. "There has been a lot of denial about the importance of nutrients because they are hard to deal with and hard to measure, so some people would like to just ignore them."
The relative impacts of pollution vs. overfishing have, of course, been explored by other researchers, but with mixed results that Lapointe believes stemmed from such problems as poor choice of study sites. Some research teams, he says, have inadvertently studied the effects of experimentally increased nutrients in locations where nutrient levels were already so high that this would have little effect. In some cases, improper measurement of key nutrients and misinterpretation of nutrient data have also been problems, he says.
While Lapointe says there are likely extreme cases where loss of grazers had a more significant impact than observed during the study, that the nutrient, or "bottom-up," control view supported by the new study appears most widely applicable. As further evidence, Lapointe points to the results of other researchers who have found that overfishing on unpolluted reefs does not cause seaweed overgrowth, but rather the spread of smaller, less troublesome forms of algae. Likewise, Lapointe has found through two decades monitoring severe seaweed overgrowth at reefs in the Looe Key National Marine Sanctuary in the Florida Keys that strict enforcement of fishing bans does not prevent seaweed overgrowth. Consequently, he says that while limiting overfishing is important for various reasons, that reducing pollution in coral reef areas should be the most important goal for managers working to save or restore seaweed-overgrown reefs. New results from the Looe Key research will be published in a forthcoming issue of JEMBE.
Lapointe believes application of his research results is desperately needed in Florida's Everglades, because water from the Everglades with high nitrogen concentrations pours out into Florida Bay, then flows out to numerous coral reefs. Nonetheless, current Everglades restoration plans, which call for a dramatic increase in the amount of this water released, do not call for reduction in nitrogen concentrations. Lapointe believes this could lead to even more devastation of South Florida reefs. "Based on this and other studies, it's clear we're going to have to reduce nitrogen inputs to Florida Bay if we're going to save downstream reefs in the Keys."
The above story is based on materials provided by Harbor Branch Oceanographic Institution. Note: Materials may be edited for content and length.
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