Investigation of East Flower Garden Bank coral die-off continues amid new coral bleaching event

September 2016

divers examining and photographing coral in east flower garden banks
Scientists documenting the bleaching event underway in Flower Garden Banks National Marine Sanctuary. Photo: G.P. Schmahl/NOAA.

Scientists continue to explore a broad spectrum of possible causes for July's mass mortality event at East Flower Garden Bank within Flower Garden Banks National Marine Sanctuary, including environmental conditions and human activities. Meanwhile, their efforts are now complicated by a new but unrelated coral bleaching event, which can happen this time of year when corals are stressed by warmer water temperatures.

A preliminary review of vessel traffic data revealed a lack of vessels in the vicinity just before the mortality event. This effectively rules out the possibility of an acute discharge of pollutants as the cause of the die-off. Earlier analyses indicated that no hydrocarbons were present in the area of the mortality, which further reduces the likelihood that the event was due to some sort of petroleum-based contamination.

Ongoing response efforts are now focused on additional analysis of oceanographic dynamics (see animation below) and water quality. NOAA research divers conducting additional response surveys earlier this month observed dead sponges in other locations around East Flower Garden Bank and on a nearby production platform just outside the boundary of Flower Garden Banks National Marine Sanctuary. The die-off and whatever caused it, therefore, may have been more widespread than initially thought.

animation of the salinity level in the gulf of mexico
Scientists are currently unsure of the exact cause of the die-off in the East Flower Garden Bank, marked with a red star. This visualization shows salinity levels in the Gulf of Mexico. White areas indicate low salinity levels, which usually stay near the coastline. Notice the intrusion of freshwater south of the Texas/Louisiana border; this increase in freshwater is a possible contributing factor to the die-off. Image Credit: United States Naval Research Laboratory – Stennis Space Center

The deployment of current meters (instruments that measure the direction and speed of the water currents) around the East Flower Garden Bank coral cap will allow for fine-scale measurement of fluid dynamics. This information may increase scientists' ability to explain the unusual spatial pattern of mortality, including the high concentration of dead animals and plants found in and around sand flats in the center of the bank.

This ability to collect detailed information will be especially important as scientists face the challenge of differentiating the residual effects of the July mortality event from the current bleaching. While bleaching during this time of year is not unheard of due to seasonally warmer water temperatures, the types of corals exhibiting bleaching in the area of the die-off are unusual.

coral head showing the effects of the die-off and the bleaching event

Coral heads at East Flower Garden Bank in early September are a mosaic of healthy coral (green arrows) alongside areas with signs of active bleaching and paling (mild bleaching) (yellow arrows), and areas that previously succumbed to the mortality event (red arrows). The skeletons of coral killed during the July die-off are already being overgrown by algae. Scientists need to recognize these patterns in order to differentiate the impacts of these events. Photo: G.P. Schmahl/NOAA.

Surveys in September found that coral species that normally exhibit resistance to early-stage bleaching were already being impacted on East Flower Garden Bank. It is likely that these colonies were already weakened by the mortality event, making them more sensitive to bleaching now. Scientists also observed bleaching elsewhere on the banks, but only for species that typically bleach during the early phases of such an event.

Flower Garden Banks National Marine Sanctuary is collaborating on the response with academic partners, including experts from Texas A&M University, Rice University, University of North Carolina, Baylor University, University of Texas at Dallas and the University of South Florida. Sanctuary scientists will be conducting additional response cruises during the first two weeks of October.

Meanwhile, with the initial mass mortality event ended, the sanctuary's earlier recommendation to the public to avoid diving, fishing and boating activities in the affected area is no longer in effect. As always, scuba divers and anglers are reminded to use the mooring buoys provided, as anchoring is prohibited, and to adhere to all federal regulations.

To learn more:

Flower Garden Banks National Marine Sanctuary

BOEM Science Notes – Flower Garden Banks National Marine Sanctuary

Long-Term Monitoring at the East and West Flower Garden Banks