Mote Marine Laboratory

“Baby” corals of at least some species are vulnerable to Deepwater Horizon oil and are especially likely to die when exposed to dispersants used during a spill, according to a lab-based study by Mote Marine Laboratory scientists that was be published online Jan. 9 in the peer-reviewed journal PLOS ONE.

• The complete paper is available from the PLOS ONE Web site.

Mountainous star coral (Montastraea faveolata) spawns, releasing sperm and eggs that will combine to produce babies, or larvae, which in turn will settle to form new corals. (Photo by the National Oceanic and Atmospheric Administration)

This study is the first controlled test of how Deepwater Horizon oil and the dispersant Corexit® 9500 affect coral larvae — drifting offspring of corals that must settle and grow to maintain and expand reefs. While the study focused on two coral species from the Florida Keys — an area not directly impacted by the spill — the results highlight concerns about corals nearer to the spill site and provide new insights for mitigating oil spills near reefs.

The Deepwater Horizon rig spilled more than 200 million gallons of oil into the Gulf of Mexico, and responders used nearly 2 million gallons of dispersant — a chemical that can break up oil slicks into small droplets that diffuse in the water column. Dispersants help keep oil slicks from reaching shore, but they may change and even exacerbate the threats from oil toxins in the underwater environment.

Scientists from Mote, a world-class marine research institution based in Sarasota, Fla., have been studying the impacts of Deepwater Horizon on multiple species of marine life since the early days of the spill.

In this study, they focused on coral larvae to learn how oil and dispersant might affect fragile reefs in the Florida Keys, home to the world’s third-largest barrier reef. It’s important to note that oil from the Deepwater Horizon never reached the Keys reefs, but these studies illustrate what could happen if a spill ever affects that area. It also sheds light on how deep-water corals closer to the spill could have been affected.

Scientists from Mote’s main Lab in Sarasota and Mote’s Tropical Research Laboratory on Summerland Key, Fla. tested larvae from two coral species widespread in the Florida Keys — mustard hill coral (Porites astreoides) and mountainous star coral (Montastraea faveolata) — in water containing the dissolved components of Deepwater Horizon oil from the source, weathered oil, the dispersant Corexit® 9500 and the combined oil and dispersant. They monitored the coral larvae for 72 hours at different concentrations of each solution, and they also tested how the mountainous star coral larvae fared in solutions that were slowly diluted over 96 hours.

The oil and dispersant levels and exposures in this experiment were designed to represent the levels of oil and dispersant coral larvae might encounter following a spill in the wild. They were based in part on recommendations from Ecological Risk Assessment workshops held by U.S. federal agencies, along with findings from a working group called CROSERF involving government, academia and industry experts.

Larvae exposed to oil components died sooner and settled less than control larvae given only seawater. Mustard hill coral larvae were significantly less likely to survive and settle amid high concentrations of oil components (0.62 parts per million) and mountainous star coral had significantly lower survival rates even at the lowest oil concentration (0.49 ppm diluted over time). Larvae exposed to weathered crude oil had significantly lowered survival rates and stopped settling after 72 hours, while the control larvae continued to settle through 96 hours.

Both species were highly vulnerable to Corexit® 9500 — no mountainous star coral larvae settled or survived at the medium and high concentrations (50 and 100 ppm) and no mustard hill coral larvae settled or survived at the high concentration (100 ppm). Both species of coral larvae were significantly less likely to survive and settle amid medium concentrations (4.28 ppm for mustard hill coral and 18.56 ppm for mountainous star coral) or high concentrations (30.99 for mustard hill and 35.76 ppm for mountainous star) of oil mixed with dispersant. Even at a low concentration (0.86 ppm) of oil-dispersant mixture diluted over 96 hours, most of the mountainous star coral did not survive.

“Dispersant, and the mixture of oil and dispersant, may be highly toxic to coral larvae and prevent them from building new parts of the reef,” said Dr. Kim Ritchie, principal investigator on the emergency Protect Our Reefs grant supporting this study and manager of the Marine Microbiology Program at Mote. “In addition, our results support the growing knowledge that certain coral species may fare worse than others during oil spills.”

Ritchie carried out the study with co-investigators Dr. Dana Wetzel, manager of Mote’s Environmental Laboratory for Forensics, and Dr. Gretchen Goodbody-Gringley, former Mote postdoctoral researcher who is now an instructor at Bermuda Institute of Ocean Sciences.

Wetzel noted that the study helps broaden the scientific understanding of oil spill effects. “While we have knowledge about the toxicity of oil and dispersants on fish and shellfish, there has been limited information available so far on the lethal effects of oil on coral larvae.”

Said Ritchie: “We hope our results will be useful to resource managers as they work to prepare for and preserve coral reefs in the wake of future potential oil spills and many other man-made impacts.”

The coral species in this study were from Florida Keys waters not directly affected by the Deepwater Horizon spill, and they are also known to occur in the Flower Garden Banks National Marine Sanctuary — an area of the Northwestern Gulf of Mexico that is closer to the spill site but also received no direct impacts. The two species are common reef builders in the Gulf and Caribbean — an area that has lost roughly 80 percent of its coral cover since 1975, and faces threats from climate change, ocean acidification, coral disease outbreaks and pollution. Corals in the Florida Keys may face new challenges as oil exploration proceeds in neighboring waters off Cuba.

“To understand how oil and dispersant could affect wild corals, more research is needed on their complex natural life cycles,” Ritchie said. “Coral larvae seem to settle with help from landing pads called ‘biofilms’ that are formed by microbes like marine bacteria. This delicate natural process might be interrupted by dispersant and its mixture with oil, so it’s important to know how it works in detail.”

Mote scientists and collaborators are actively investigating how coral larvae settle, and their recent research has shown that beneficial bacteria and genetic packets produced by these bacteria seem to help larvae settle.