Missing the Reef for the Corals

A recent paper in the journal Frontiers in Marine Science used readily-available public data to conduct an analysis relating coral cover to a variety of biogeophysical forcings, or threats (such as wave height and marine pollution), with surprising results. Lead author Eric Hochberg, a reef systems ecologist at BIOS, explains that prevailing scientific thought expects coral cover to decrease as forcings increase; however, that wasn’t so for the majority of the forcings in the paper’s analysis. “That means we can’t explain the majority of the variation in coral cover across reefs,” he said. “This is a major problem for understanding how reefs work and for predicting their futures.” Photo by Stacy Peltier.

A new study published in the journal Frontiers in Marine Science has uncovered some unexpected trends in the relationship between coral reefs and their environment, contrary to prevailing scientific expectations and understanding. Authors Eric Hochberg, a BIOS reef systems ecologist, and Michelle Gierach, a scientist at NASA’s Jet Propulsion Laboratory (JPL), used readily-available public data for coral cover (the amount of coral in a given reef area) to conduct a meta-analysis, or an analysis of data from different studies. In this analysis, they statistically related reef condition to a suite of biogeophysical forcing parameters (forcings), such as aragonite saturation state (a figure thought to impact the ability of corals to calcify), significant wave height, number of coral species, and various local threats, among others.

Some of the forcings showed expected relationships with coral cover, which is to say that as the forcings, or threats, increased, the coral cover decreased. These included the 10-year trend for bleaching alert areas (or BAA, a measurement used by NOAA’s Coral Reef Watch to identify potentially harmful levels of heat stress), the number of significant wave height events, local marine pollution threat, and number of coral species. The remaining correlations, however, did not exhibit the expected trends.

All but one of the local threats (marine pollution)—including coastal development, overfishing, and sedimentation—had trends counter to prevailing scientific thought. That is, as the threats increased, there was not an associated decrease in coral cover. Recognizing that the forcings likely work in tandem, as opposed to individually, Hochberg and Gierach also tested models that considered their collective influences. Surprisingly, coral cover was poorly predicted by these models, as well.

“The biogeophysical forcings together only explain 43% of the variance in coral cover,” Hochberg said. “That means we can’t explain the majority of the variation in coral cover across reefs. This is a major problem for understanding how reefs work and for predicting their futures.”

The authors concluded that, rather than being an issue of understanding reef ecology, the results highlight the problem of using sparse, disparate data to synthesize a global model of reefs.

“Virtually all reef assessments and monitoring are conducted using SCUBA,” Hochberg said. “Those Herculean efforts produce extremely useful data, but they focus on very small areas. They simply cannot capture the nature of an entire reef.”

The solution to this problem lies in remote sensing approaches, which can offer uniform, high-density data sets at the ecosystem scale across reef regions, rather than at smaller local scales. However, in order to successfully measure reef condition at the ecosystem scale, the underlying data need to be able to spectrally discriminate, or tell the difference, between the fundamental reef benthic types (i.e., live coral from algae or sand). Several satellite missions are under development that may meet these requirements, such as the NASA Surface Biology and Geology (SBG) mission.

Airborne remote sensing offers another viable alternative, one example being the NASA Earth Venture Suborbital-2 (EVS-2) COral Reef Airborne Laboratory (CORAL) mission. During 2016-17 CORAL used the NASA JPL Portable Remote Imaging SpectroMeter (PRISM), a state-of-the-art airborne sensor, to map benthic cover for ~1% of the global reef area in sections of Florida, the Great Barrier Reef, main Hawaiian Islands, Marianas, and Palau. With these data, CORAL scientists quantified benthic cover and modeled primary production and calcification at the reef scale, with the aim of understanding the relationship between reef ecosystems and their environment. Results are forthcoming after final data and analysis quality assurances.

“The fundamental, unanswered question facing reef science is ‘How much coral is there?’” Hochberg said. “There are global reef mapping efforts underway, but they only identify the locations of reefs, and they don’t address that fundamental question. Right now, the only way to get coral cover for the world’s reefs is through remote sensing using advanced imaging spectrometers like PRISM and the one being planned for SBG. CORAL is the first step toward that global picture, and we’re currently taking the next steps to ensure missions like SBG can successfully be applied to coral reefs, in addition to terrestrial environments.”