By John K. Reed, NOAA Cooperative Institute of Ocean Exploration, Research and Technology (CIOERT) - Harbor Branch Oceanographic Institute—Florida Atlantic University (HBOI-FAU)
Most shallow water coral reefs around the world have been impacted by various human activities, such as climate change, pollution, sedimentation from dredging and land runoff, overfishing, and damage from bottom tending fishing gear. Mesophotic coral ecosystems (MCEs), found from 30 to 100 meters depth in the Gulf of Mexico, are thought to be less likely to be impacted by human activities because of their depth and may serve as a refuge for corals and fishes.
We are studying the benthic communities of Pulley Ridge, the deepest known photosynthetic coral reef off the continental U.S., with the hopes of understanding whether mesophotic reefs do indeed serve as a refuge for impacted shallow reef species. Thirty years ago, studies were conducted at Pulley Ridge, so we have a unique opportunity to be able to compare it to the data we collect to see if there has been a change in the health and biodiversity of the Pulley Ridge MCE. The overarching goal of our research is to improve the understanding of the ecological importance of MCEs to enable resource managers to better protect MCEs.
This year will be our fourth and final research cruise to Pulley Ridge as part of this five-year study to investigate the role that the relatively healthy deep, mesophotic reefs of Pulley Ridge may play in replenishing key fish species, such as grouper and snapper, and other organisms in the downstream reefs of the Florida Keys and Dry Tortugas. To better understand the benthic mesophotic communities and fish populations, we use a remotely operated vehicle to characterize these habitats with video and still photographs.
To date, we have completed the analysis of data from the first three research cruises (2012-2014). We have identified 216 species of macro-invertebrates and algae. These consist of 102 species of Porifera (sponges); 27 Scleractinia (hard corals); 19 Octocorals (sea fans); 5 Antipatharia (black corals); and various mobile taxa such as crabs, snails, sea stars, and sea urchins (Figure 1), as well as 31 species of algae (green, red, and brown seaweeds).
Algae are the dominant feature on Pulley Ridge, covering over 50 percent of the bottom. Two species, in particular, dominate the bottom habitat: a calcarous pink algae that encrusts the bottom and a large leafy green algae called Anadyomene menziesii. The latter species is found only at Pulley Ridge and nowhere else in the Gulf of Mexico or the Caribbean.
The dominant corals on Pulley Ridge are species that can tolerate low light levels. Seafloor light measured at southern Pulley Ridge (65-70 meters) is only one to two percent (5–30 microeinsteins per second per square meter) of available surface light. Thus, the corals of Pulley Ridge are primarily plate-shaped corals (Agaricia grahamae, Agaricia fragilis, Helioseris cucullata, and Montastraea cavernosa) which are able to capture the light more efficiently.
What is chilling is the apparent loss of coral cover over the past 10 years on Pulley Ridge. In 2003, a study by the U.S. Geological Survey reported that the average coral cover at southern Pulley Ridge was 11.9 percent, with a maximum of 23.2 percent in the central region of the ridge. By 2013, our study shows the average hard coral cover was 0.85 percent, with a maximum of 5.6 percent. This is a 92.8-percent loss of coral cover in a decade.
In 2014, we discovered a new coral area with the densest cover of Agaricia corals known in the Gulf of Mexico (2.6-4.98 percent cover). This new area is unprotected and outside of the Pulley Ridge Habitat Area of Particular Concern (HAPC), so our team has submitted a proposal to the Gulf of Mexico Fishery Management Council for this new region to be included in the Pulley Ridge HAPC. On a positive note, a large number of these corals are relatively new recruits: 47.7 percent are less than five centimeters in diameter and 35.4 percent are five to nine centimeters. So, it appears that the coral is growing back from whatever die-off occurred after 2003.
The big question remains, what happened?
The level of diseased coral at Pulley Ridge is about four percent, which is not unusual compared to shallow reefs. There is no evidence of extensive damage from fish traps or trawls, although there have been ghost traps observed primarily along the southern drop off of Pulley Ridge.
At depths of 70-80 meters, we do not normally expect damage from hurricanes, but it is a possibility. In 2012, Hurricane Isaac went directly over Pulley Ridge, but our dives in 2013 did not indicate any obvious damage from the hurricane.
Turbidity and smothering are unlikely in this region that is over 100 miles off the Florida coast and the water is usually crystal clear with greater than 20-meter visibility on the bottom.
Could the 2010 Deepwater Horizon oil spill have caused the die off? Not likely. In the summer of 2010 during the oil spill, CIOERT at HBOI-FAU conducted a month-long research cruise with the Johnson-Sea-Link submersible to survey the mesophotic reefs all along the west Florida shelf to see if there was any oil impact to them. We saw no evidence whatsoever. In addition, we saw no recent coral death on Pulley Ridge.
Temperature change is another possibility. Temperatures recorded over the past three years are relatively mild, 20-27°C (68-80 °F). We haven’t seen temperatures warm enough to cause coral bleaching (i.e., temperatures in excess of 30°C (86°F) for several weeks). However, being close to the steep drop off of the Florida Escarpment and the strong Loop Current, the possibility exists for severe cold-water upwelling events. Although cold water events rarely cause coral bleaching or death, it has happened in the Florida Keys from strong winter cold fronts.
So for now, we have no smoking gun for what happened to cause the die-off of corals at Pulley Ridge.
For more detailed information on our findings from 2012-2014, see: