Marine Pollution Impact On Corals Is Harsh, But They Can Recover
November 26, 2013

Marine Pollution Impact On Corals Is Harsh, But They Can Recover

[ Watch the Video: Protection From Marine Pollution Would Help Coral Reefs Recover ]

Brett Smith for - Your Universe Online

A new study on coral reefs from a team of Florida and Oregon researchers has found common marine pollution doubled the rate of disease among corals and more than tripled the amount of coral bleaching, an early sign of reef stress.

According to the team’s report in Global Change Biology, a three-year, controlled exposure of corals to high levels of nitrogen and phosphorus at a study site in the Florida Keys severely impacted reef health. However, the corals were able to stage a significant recovery once exposure to these pollutants was stopped.

"We were shocked to see the rapid increase in disease and bleaching from a level of pollution that's fairly common in areas affected by sewage discharge, or fertilizers from agricultural or urban use," said Rebecca Vega-Thurber, an assistant professor in the College of Science at Oregon State University.

"But what was even more surprising is that corals were able to make a strong recovery within 10 months after the nutrient enrichment was stopped," she added. "The problems disappeared. This provides real evidence that not only can nutrient overload cause coral problems, but programs to reduce or eliminate this pollution should help restore coral health. This is actually very good news."

The researchers said their study was motivated by years of watching the decline of coral reefs that were being exposed to sewage outflows or fertilizers in agricultural runoff. These pollutants have caused an increase in reef nutrients such as nitrogen and phosphorus. The study team said they wanted to see how nutrient overloads affect coral reef decline – if at all.

In the study, the researchers looked at the effect of nutrient loading on over 1,200 corals in study plots near Key Largo, FL. The team checked for signs of coral disease and bleaching. They considered and eliminated confounding factors such as water depth, salinity or temperature.

After regularly exposing corals at the study sites to elevated levels of nitrogen and phosphorus, they observed many of the reef sections become diseased and bleached. The team said they also noticed the prevalence of one particular disease called "dark spot syndrome," which was seen on approximately 50 percent of diseased individual corals. Within one year after nutrient injections were stopped, however, the prevalence of dark spot syndrome had dropped to the same level as untouched control plots.

The researchers said they were unsure why nutrient overload affected the corals, but noted they may feed existing pathogens and allow them to proliferate. The pollution also may be toxic to coral, making them more susceptible to pathogens, the study team said.

"A combination of increased stress and a higher level of pathogens is probably the mechanism that affects coral health," Vega-Thurber said. "What's exciting about this research is the clear experimental evidence that stopping the pollution can lead to coral recovery. A lot of people have been hoping for some news like this.

"Some of the corals left in the world are actually among the species that are most hardy," she added. "The others are already dead. We're desperately trying to save what's left, and cleaning up the water may be one mechanism that has the most promise."