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CSI: Death on a Coral Reef

Posted: June 4, 2019
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One by one, divers dropped into the water from the M/V Fling, a live-aboard charter boat that specializes in recreational diving trips to the Flower Garden Banks National Marine Sanctuary — one of the healthiest reefs in the world.

But what they found when they descended to the East Flower Garden Bank reef at mooring buoy No. 4 on July 25, 2016, was anything but a healthy reef. The usually crystal clear water was green and hazy. The seafloor was littered with dead benthic critters — brittle stars and urchins, bivalves and crustaceans. White bacterial mats covered sponges and corals. Portions of stony coral heads were white and dead, drained of their beneficial algae.

Just two days earlier, on July 23, the Fling had been to that same location to install the No. 4 mooring buoy; it was a short trip, but the dive masters noticed nothing unusual on the reef that day.

The Flower Garden Banks before the localized mortality event. Photo courtesy FGBNMS.

The localized mortality the divers witnessed on the 25th became even more puzzling when, at 2 p.m., they reported the dead and dying coral and bottom-dwelling animals to a crew from the Flower Garden Banks National Marine Sanctuary (FGBNMS) and the Bureau of Ocean Energy Management (BOEM) who were conducting an annual reef monitoring survey 900 feet away at mooring buoy No. 2. The divers from BOEM and FGBNMS saw nothing unusual where they were — the corals were healthy, fish were abundant, and the animals on the seafloor were fine.

Heavy rains and thunderstorms kept the Sanctuary team from further investigating that day; but when they returned to the scene on July 27, they confirmed the Fling’s reports and visually estimated that about 20 percent of the corals near buoy No. 4 were dead. (More exact photographic surveys conducted later put the official count at 17 percent.)

“It was disturbing — we’d never seen anything like this before,” said G.P. Schmahl, Superintendent of the FGBNMS. “The dead areas appeared very stratified. Corals that were 2 to 3 meters (6 to 10 feet) tall were dead from the seafloor up to about a meter (just over 3 feet). Corals were actively sloughing off. Brittle stars and mollusks were just falling apart. There were white bacterial mats covering some of the coral heads and sponges and it looked like mollusks had just rained down from the coral overhangs to litter the seafloor. And, unlike bleaching events where the corals have the ability to come back as conditions improve, these corals were dead and aren’t coming back.”

The Flower Garden Banks before the localized mortality event. Photo courtesy FGBNMS.

The scientists were left with an environmental mystery as puzzling as any plot on CSI: What killed the corals and other organisms on the seafloor? Why was it such a localized event that affected only the area near buoy No. 4? Why was there such a clear line of demarcation between healthy and dead areas on single coral heads, and why did the dead portions appear to start from the bottom up?

Ultimately, the mystery would attract about 40 scientists — including some 25 “first responders” — from numerous universities, agencies and disciplines, including physical oceanographers, chemical oceanographers, biologists, ecologists and modelers. On Feb. 27-28, 2018, they came together in Galveston, Texas, during a symposium sponsored by the FGBNMS, the Gulf of Mexico Coastal Ocean Observing System (GCOOS), the U.S. Integrated Ocean Observing System (IOOS), the Marine Biodiversity Observation Network (MBON), NOAA Deep Sea Coral Research and Technology Program and the National Ocean Service.

Call it CSI: Coral Reef Mortality. The investigation team’s goal? Bring their diverse backgrounds and datasets together to try to unravel the mystery of what happened on the East Flower Garden Bank.

One of the World’s Healthiest Reefs

Worldwide, corals are dying — affected by the one-two punch of environmental degradation and warming seas.

The 56-square mile Flower Garden Banks National Marine Sanctuary, located 70 to 115 miles off the coasts of Texas and Louisiana in the northwestern Gulf of Mexico, includes three separate areas: East Flower Garden Bank, West Flower Garden Bank and Stetson Bank. It’s also situated in the middle of one of the most highly developed oil and gas fields in the world.

All types of reefs have a particular “comfort” zone of ideal environmental conditions, and in the case of the Flower Garden Banks that means:
• Clear water (oligotrophic) with enough light to allow the coral’s algae — zooxanthellae — to photosynthesize and feed the coral polyps;
• Water temperatures between 73 and 84 F (23 and 29 C);
• Salinity levels between 33 and 37 psu;
• pH at 8.0 to 8.3;
• Dissolved oxygen between 7 and 14 mg/L.

The FGBNMS is the only national marine sanctuary in the Gulf of Mexico. It includes at least 26 species of shallow-water coral situated on surface expressions of underlying salt domes that are estimated to have begun forming 10,000 to 15,000 years ago. Fishermen discovered the area in the early 1900s and it was officially designated a sanctuary in 1992 through the strong support of local recreational divers, who wanted the reefs protected from oil and gas development, anchoring and fishing impacts.

Its reefs are something of an anomaly; protected by its remote location, experts estimate that the FGBNMS retains about 50 percent healthy coral cover. Though not an apples-to-apples comparison given the differences in size, the world’s vast and most famous reef, Australia’s Great Barrier Reef, is estimated to have only about an average of 18 percent healthy coral cover.

For recreational divers, the reef’s intrinsic beauty and biological diversity — home to sea turtles, mantas and other spectacular creatures — are reasons enough to worry about such a striking, if localized, mortality event in the Flower Garden Banks. But the importance of the reef goes beyond the aesthetic; the Banks provide critical spawning habitat and refuge areas for recreationally and commercially important fish species in the Gulf such as red snapper, jacks, grouper and wahoo.

“Coral reefs are some of the most beautiful and biologically diverse ecosystems on the planet,” said Sanctuary Superintendent Schmahl. “But we also estimate that the commercial value of coral reefs to U.S. fisheries is more than $100 million. If you add in reefs’ importance to recreational fisheries, it’s probably another $100 million. And the Flower Garden Banks are an important part of that.”

Two of the main reef building corals in the Flower Garden Banks — mountainous star coral (Orbicella faveolata) and boulder star coral (Orbicella franksi) — are also designated as threatened under the Endangered Species Act.

In short: Reefs matter. And in the case of the East Flower Garden Bank localized mortality event, resource managers need to know whether this event was an environmental anomaly or a harbinger of things to come.

“Coral cover in the Flower Garden Banks has been measured annually every year since 1989,” said Dr. Michelle Johnston, Research Biologist with the FGBNMS, who leads the long-term monitoring project. “And so far, it has maintained its health at 50 percent cover or above. We’ve had several bleaching events (in 2005, 2010 and 2016) and the corals have typically recovered. When we saw this event, we knew we needed to document what was going on so we could develop a formal analysis and try to put it into a bigger context. We had some really challenging questions to answer.”

Gathering Evidence

On July 27, 2016, FGBNMS divers confirmed the Fling’s reports and began taking environmental samples of the site. They conducted a video survey, visual benthic and fish surveys, conducted photo transects and collected a handful of samples of affected corals, sponges and benthic animals.

Concerned that the cause of the mortality was unknown and posed a potential health risk for divers, the NOAA Dive Center recommended that the FGBNMS dive team cease dive operations in the affected area. The Sanctuary team moved to the West Flower Garden Bank to see whether it was experiencing a similar mortality event.

It wasn’t.

In the meantime, word was spread to the coral reef science community about the mysterious localized mortality event and a call for assistance was put out by the Sanctuary. Texas A&M University’s (TAMU) Geochemical and Environmental Research Group (GERG) in the Department of Oceanography mounted a rapid response cruise from July 30 to Aug. 2 with Dr. Matthew K. Howard, GCOOS data manager and TAMU oceanographer, acting as chief scientist.

They also collected physical oceanography data from recent trips to the reef and other sources, including an autonomous underwater vehicle (AUV) launched by GERG on Aug. 7 and the Texas Automated Buoy System (TABS). TABS buoys are supported by the Texas General Land Office and stationed off the Texas coast. They continuously gather ocean data on salinity, temperature, water density and currents.

The AUV was equipped with sensors that collected data on dissolved oxygen, chlorophyll, turbidity, dissolved organic matter and more — in all, it collected data on more than 2,100 parameters every second as it patrolled the Sanctuary through mid-September.

Still other scientists turned their attention to the satellites trained over the Sanctuary. Satellites help scientists remotely reveal what is happening on the ocean’s surface — they can detect sea surface temperatures and changes in sea level; they can also help scientists determine whether there are high levels of chlorophyll that could indicate an algal bloom or a plume of nutrient-laced water from rivers.

In the Flower Garden Banks, the Sanctuary also has bottom sensors that continuously monitor temperature and salinity at various water depths.

The FGBNMS divers and partners from Rice, University of Houston and University of Boston returned to the site Aug. 5-7 to take additional samples and conduct additional benthic and fish surveys.

Analyzing the Evidence

In February, about 40 scientists from FGBNMS, BOEM, TAMU, the Rosenstiel School of Marine & Atmospheric Science at the University of Miami, University of Houston, Boston University, Rice University, University of Delaware, University of Southern Mississippi, the National Weather Service (NWS), Texas General Land Office, National Oceanic and Atmospheric Administration (NOAA), University of South Florida and the U.S. Geological Survey (USGS) convened in Galveston to share their data and consider potential reasons for the deaths of the corals and other animals on the reef at East Bank — to find an environmental “motive,” if you will.

Among the evidence discussed:
• A preliminary review of vessel traffic showed that there were no commercial ships near the East Flower Garden Bank before the mortality event — indicating that the mortality was not caused by a contaminant spill.
• There were fewer fish near the mortality area than outside of it or at long-term monitoring stations located in both the East and West banks.
• Sea surface temperatures measured by the TABS buoy near East Bank were higher than normal over a prolonged period of time. Beginning in late June, temperatures rose above 30 C (86 F) and stayed high until late September at the East Bank. While temperatures were also above 30 C at the West bank, they were more variable and occasionally dipped below the 30 C threshold.
• Seawater temperature at depth (24 m/78 feet) at East Flower Garden Bank was above 86 F (30 C) for 36 days from late July to early September. At the West Bank, sea water temperature at depth was above 86 F for 21 days.
• Surface salinity on the East Bank started dropping around mid-May, reaching a low point of about 13 psu (practical salinity units) on July 29. Typical salinity range for seawater is 33-37 psu. Surface salinity on the West Bank also began dropping in mid-May, but it never dipped below 23 psu or so.
• There are no recordings for salinity at depth (24m) on the East Bank because a sensor failed. Salinity at depth (24 m) on the West Bank stayed within the 33-37 psu range.
• Dissolved oxygen (DO) on the reef usually ranges from 5-8 mg/L (milligrams per liter). USGS sensors at nearby sites measured between 4-6 mg/L in mid-July. Around the time of the mortality, DO measured as low as 2 mg/L. No sensors were in place in the mortality area at the time of the event. A sensor was placed there as part of the response.
• There was a large influx of freshwater into the Gulf from extreme flooding in Texas rivers — the Colorado, Brazos, Trinity, Neches and Sabine. There was disagreement during the symposium whether that freshwater plume reached the East/West Banks 70 to 100 miles offshore. Chemical analysis and satellite imagery support the low salinity measurements, but it remains unclear whether the influx of freshwater came from the river plume or was the result of an upwelling caused by surface winds or intense rainfall on the reef.
• Calcium carbonate plays an important role in the health of corals and many other reef-dwelling organisms, including animals like sea urchins and brittle stars. Could an influx of more acidic water — which essentially dissolves calcium carbonate — have affected the reef? Data from the last nine years shows that carbonate levels are temperature controlled, but it’s unclear whether increased acidification had any effect on the reef.
• The microbial communities found in coral and sponge samples at depth following the mortality event were distinctly different between the East and West Banks but it remains unclear whether the differences played a role in the reef mortality or whether the microbial communities changed following the mortality.
• The microbial communities on the corals with lesions were less diverse than on the corals without lesions but there is not yet any clear indication of whether the corals died because of the microbial change or whether the microbial change occurred after the corals died. There was also no clear indication of why parts of some single coral heads were healthy, while portions of the same head closer to the seafloor had died.
• Scientists also looked deeper — at whether there were genetic differences between the healthy coral’s genes and the genes of the deceased coral. Particularly, they looked at the genes more likely to be affected by environmental factors such as pollution, temperature or salinity changes but were able to draw no clear conclusions.

What Killed the Coral?

Often, the beauty of a made-for-TV police procedural is that moving from death to case-solved takes about an hour (minus time for commercial interruptions); evidence is there for the taking and there are usually witnesses who saw the crime and can help identify the perpetrators.

But real life isn’t a television crime drama. And if any of the fish, mantas or sea turtles on the East Bank witnessed what happened in July 2016, they aren’t talking.

“This is one of the best-studied reefs we have, with some of the longest-term datasets available and we still don’t have all the information we need to determine what happened,” said Dr. Steve DiMarco, Professor of Physical Oceanography, who was the Principal Investigator on TAMU’s rapid-response cruise July 30-Aug. 2.

Emma Hickerson, FGBNMS Research Coordinator, puts it this way: “We would love to be able to say that this is what happened and this is why. But I’m not sure we will ever be able to do that. As many pieces of data that we have, and as many investigators who have come to the table to work on this issue, there is still much information that is just not available.”

Instead, the scientists are working together to publish the next best thing: A list of factors that they believe contributed to the localized mortality event and the tools they need to truly have a better picture of what’s happening on the reef. That way, the next time there’s a similar event on another reef, researchers will be able to make some comparisons and have a roadmap for the next investigation.

For now, “suspects” in the East Bank mortality event include:

• High river outflow
• Low salinity
• High chlorophyll
• High organic matter
• Low oxygen (hypoxia)
• Sustained high water temperatures

“Perhaps what happened was a perfect storm of events,” Johnston said. “If the East bank had been affected by any one of these things at a time, it probably would have been OK. But if many of these things came together at the same time in the same place, that could have been what ultimately caused this localized mortality.”

Ultimately, Symposium participants decided on this consensus statement:

Low dissolved oxygen was the most likely contributing factor of the 2016 highly localized mortality event at East Flower Garden Bank. Instrumentation on and around the reef documented low surface salinity and higher than average seawater temperature. High organic matter was detected by remote sensing, and unusually high levels of freshwater outflow from Gulf Coast rivers were also measured. The linkages between the conditions measured on the reef at the time of the event, and dissolved oxygen factor, are undetermined. The mechanism resulting in the highly localized nature of the mortality event cannot be determined from available data.

So how to move forward?

Dr. Bill Kiene, Regional Science & Policy Analyst, for NOAA’s Southeast Region, said the localized mortality event at the East bank reef is a powerful example of the need for scientists to work together with resource managers and funding decision makers to develop new and better ways of monitoring our oceans. “This event was so symbolic and so contained that it could provide the impetus to justify expanding our capabilities.”

That could mean expanding sentinel monitoring sites, adding new buoys that measure a wider array of ocean parameters at the surface that are also equipped to provide alerts when something anomalous happens; adding more sensors at depth to collect information in places buoys can’t reach.

Researchers also suggested the need to develop an expanded network of citizen scientists who could help collect reef data between the BOEM/FGBNMS regularly scheduled trips. “After all, it was the divers on the Fling who noticed this event and alerted us to what was happening,” Johnston said.

Overall, though, the mortality highlights the need for better ocean observations throughout the Gulf, said Dr. Barbara Kirkpatrick, Executive Director for the Gulf of Mexico Coastal Ocean Observing System (GCOOS). “In 2010, the Deepwater Horizon oil spill provided a stark example of the tools we were missing to respond to man-made and naturally occurring disasters in the Gulf,” she said. “Following that disaster, no money has been allocated for new, sustained observations that provide real-time or near-real time data in the Gulf. Now, this reef mortality — while localized to a much smaller area — is giving us another example of what we’re lacking. It’s not just about the beauty of the reef and its unique ecosystem. The Gulf provides the U.S. with enormous economic benefits and if we don’t understand and protect the ecosystem, what are we all going to lose?”

Read the full meeting report

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