Coral reef health can be measured by calcium carbonate deposits

Marine scientists may be able to monitor coral reef health around the world using a scheme for measuring calcium carbonate on the ocean floor.

Coral reefs have been negatively affected by climate change and pollution from human activities. Ecologists are seeking to comprehend how these ecosystems are working right now to determine and diminish future impacts.

One way scientists can measure coral reef health is by assessing the amount of calcium carbonate produced over time, thanks to shells and skeletons of marine animals collecting on the ocean floor. Researchers often use special tiles on the seabed on which plants and animals settle on. After a certain amount of time passes, for example a year, they collect the tiles and check how much calcium carbonate has gathered on them and what living species are present.

“To understand how marine ecosystems are changing over space and time we need to be able to compare data collected from different habitats,” says marine scientist Maggie Johnson.

“But researchers employ a range of approaches or measure different variables, meaning that the data might not be directly comparable,” she adds. Because of a variety of approaches or the measurements differing from each other, getting a big picture of marine ecosystem health is not always easy.

In order to have a global idea of how coral reefs are doing, Johnson and her colleagues have come up with a standardised method for making and deploying calcification accretion units (CAUs). CAUs are not unlike settlement tiles, but they are specifically used for measuring calcium carbonate.

The study has been published in British Ecological Society’s Methods in Ecology and Evolution.

The researchers have also prepared a detailed “how to” guide to use calcification accretion units, including instructions for tile construction and CAU assembly, from material suppliers to measurements, as well as placement tips and retrieval suggestions, all of which are there to help other scientists to collect comparable data from coral and oyster reefs on a global scale.

“Coral reefs cover less than one percent of the seafloor, yet provide more than 25 percent of the ocean’s biodiversity,” says Johnson. “Much of this diversity comes from tiny critters living in the unseen cracks and crevices,” she adds.

These “tiny critters” living in unseen cracks and crevices, also known as “cryptic taxa”, cannot be picked up by visual surveys, but the CAUs have a small space for these tiny taxa to inhabit.

“During collection, the tiles must be placed in zipped bags to keep the community of cryptic critters intact all the way to the lab,” says Johnson.

The researchers recommend that their colleagues making use of the CAUs process the samples immediately, while the animals and plants that are part of the units are still alive.  Processing includes taking high resolution photographs that are later utilised in identifying species and microscope observations to count baby coral. The calcium carbonate that has settled on the CAUs can later be removed by a weak acid.

“By weighing the tiles before and after decalcifying them, researchers can calculate how much calcium carbonate was deposited on that tile over a year,” says Johnson. “This figure can help compare reef conditions in different parts of the world or how the health of one reef is changing over time.”

“Habitat degradation continues to escalate in coastal marine habitats, and methods that implement standardised protocols to evaluate spatial and temporal changes in ecosystem functioning are becoming increasingly important,” the authors write.

“This standardised and accessible approach [thanks to the use of CAUs] will help promote collaboration and informed decision-making even when using data from all over the world,” Johnson concludes.

THUMBNAIL IMAGE: CAUs are deployed by a SCUBA diver on a coral reef in the Eastern Tropical Pacific. (Photo by Sean Mattson)

HEADLINE IMAGE: (L) A new CAU ready to be swapped with a unit that was deployed for one year on a coral reef in Belize. (R) CAUs were deployed on PVC poles in soft bottom substrate on an oyster reef in the Indian River Lagoon in Florida, shown at low tide. (Photos by Maggie Johnson)