Conservation

Rare elkhorn coral thrives in shallows where currents are fastest

Scientists at the University of Miami have discovered that a key secret to the survivorship of rare elkhorn coral colonies rests in the shallower waters and where ocean currents are faster, promoting a "healthier microbial community".

13/01/2025
Words by Rob Hutchins
Photography by Michael Aw
Additional photography by Philip Hamilton

Rare elkhorn corals fighting for survival in the waters of Florida will fare better by being placed in shallow ocean locations where fast currents can promote a “healthier microbial community”, scientists from the University of Miami and University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science have revealed.

The new study offers what has been billed “valuable guidance” for coral restoration efforts, in particular those focused on elkhorn coral colonies – a key reef-building species throughout Florida and the Caribbean. This species of coral plays a crucial role in providing structural support to reefs and habitat for many other reef species, due to their complex branch-like shape.

It’s according to an NOAA assessment published in 2024 that the Florida Coral Reef lost 75% of its restored elkhorn coral populations during the Florida marine heat wave in the summer of 2023. A decline in elkhorn corals has an adverse effect on the health of the overall ecosystem and often prompts replacement by other species, such as sponges, macro algae, and other non-reef building organisms.

A major finding of the study – published in the journal Nature Earth & Environment – indicates that when outplanting elkhorn coral colonies, conservationists should prioritise shallower depths with faster currents in low-nutrient environments to promote a healthier microbial community.

Pouring over the fine-scale differences in the environmental conditions between reef habitats, researchers were able to correlate with differences in survivorship, finding that it was factors such as current velocity and shallower reef depth that were specifically associated with higher rates of survival among elkhorn coral colonies.

The study also found that while certain bacteria can be correlated with coral survivorship, elevated levels of nitrate, nitrite, and temperature – particularly during the summer months – negatively affect elkhorn at a microbial level.

It was found that such factors potentially contribute to the decline of core bacteria and an increase in potential coral pathogen.

“Understanding how coral genetics and the surrounding environment affect restoration success can help guide conservation efforts worldwide,” said Stephanie Rosales, the lead author of the study and an associate scientist at the Rosenstiel School’s Cooperative Institute for Marine and Atmospheric Studies.

Coral reefs are critical ecosystems for the planet that support biodiversity, providing habitat for around 25% of all marine species. They also protect coastlines from erosion, support fisheries, and contribute to tourism economies. However, coral reefs are under severe threat due to increased ocean temperature, ocean acidification, pollution, and over-fishing.

Finding the key to successful coral restoration is therefore, important business. The successful out-planting of elkhorn coral can help mitigate reef degradation and help preserve these ecosystems. Understanding the factors that contribute to positive survivorship will, therefore, help coral managers make more informed decisions about where and how to restore reefs. 

“Local governments, environmental groups, and coastal managers can apply this knowledge to make better decisions about where to focus restoration projects and how to care for existing coral populations, ultimately helping with fisheries and coastline protections,” said Rosales.

The study, Reef site and habitat influence effectiveness of Acropora palmata restoration and its microbiome in the Florida Keys was carried out in 2019, a year of minimal bleaching in the Florida Keys. Its authors now want to understand how bacteria – along with environmental factors such as ocean currents and depth – affect survivorship during periods of severe temperature stress.

Click here for more from the Oceanographic Newsroom.

Words by Rob Hutchins
Photography by Michael Aw
Additional photography by Philip Hamilton

Printed editions

Current issue

Back issues

Enjoy so much more from Oceanographic Magazine by becoming a subscriber.
A range of subscription options are available.