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Shallow waters and fast currents boost elkhorn coral restoration

To help rare elkhorn corals survive in Florida waters, coral researchers should place them in shallow ocean locations with fast currents, scientists at the University of Miami NOAA Cooperative Institute for Marine and Atmospheric Studies and the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science explained in a paper, "Reef site and habitat influence effectiveness of Acropora palmata restoration and its microbiome in the Florida Keys" on November 11, 2024, in Communications Earth & Environment.

This new insight offers valuable guidance for coral restoration efforts, since elkhorn coral colonies are a key reef-building species throughout Florida and the Caribbean. They play a crucial role in providing structural support to reefs and habitat for many reef species due to their complex branch-like shape.

According to a NOAA assessment published in 2024, the Florida Coral Reef lost 75% of its restored elkhorn coral populations during the Florida marine heat wave in the summer of 2023. Decline of elkhorn corals adversely affects overall ecosystem health and often prompts replacement by other species, such as sponges, macroalgae and other non-reef-building organisms.

A major finding of the study indicates that outplanting of elkhorn coral colonies should prioritize shallower depths with faster currents in low-nutrient environments to promote a healthier microbial community.

Other key findings include:

• Fine-scale differences in environmental conditions between reef habitats are correlated with differences in survivorship. Specifically, factors such as current velocity and shallower reef depth are associated with higher elkhorn survivorship.
• Certain bacteria are correlated with coral survivorship.
• Elevated levels of nitrate, nitrite, and temperature, particularly during summer months, negatively affect elkhorn at the microbial level by potentially contributing 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.

In 2019, the research team documented survivorship rates of four genets on three coral reefs in the upper Florida Keys that are actively being restored. (Genets refers to a genetically unique colony or group of colonies that can be traced back to a single sexual reproductive event.)

The scientists collected habitat data, including the assessment of dissolved inorganic nutrient concentration, ocean current data, remotely sensed satellite sea surface temperature, and chlorophyll-a concentrations derived from ocean color radiometric measurements, coral outplant survivorship data, and microbiome data. The team correlated the data to understand how these factors interact.

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 overfishing.

The success of coral restoration efforts, such as the outplanting of elkhorn coral, can help mitigate reef degradation and help preserve these ecosystems. Understanding the role of genet survivorship, environmental conditions, and the coral microbiome in restoration success allows coral managers to make informed decisions about where and how to restore reefs more effectively.

"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.

"It is important to highlight that this study was conducted during a year of minimal bleaching in the Florida Keys. Understanding how bacteria, along with environmental factors such as currents and depth, interact during periods of severe temperature stress and influence coral survivorship remains a critical area for further investigation," added Rosales.