Helping Corals Thrive in Warmer Waters: New Study Explores the Value of Artificial Upwelling

Coral fragments at their reef of origin after the experiment. Corals are attached to a mesh that is fixed to the reef substrate and recovery is currently monitored. Photo by Chloe Carbonne

From sea level rise and changes in salinity to incursions of invasive species, climate change impacts threaten the integrity of marine environments in myriad ways. Often, the damage is slow and subtle, making it easy for the general public to miss. But sometimes the ecological losses are fast and dramatic, causing people to take notice and press for change. Such was the case last year, when the highest sea surface temperatures ever recorded in the Caribbean led to massive bleaching events in many places, with heat stress intense enough to prompt the National Oceanic and Atmospheric Administration’s Coral Reef Watch to add three new alert levels to its bleaching forecast.

Global warming, which causes heat stress, bleaching, and reef degradation, is now considered the biggest threat to coral reefs, and the race is on to find solutions. Last April, ASU BIOS began a National Science Foundation-funded project to explore the effectiveness of an understudied strategy: artificial upwelling.

Coral fragments inside the aquaria where each aquarium is exposed to a different seawater temperature treatment. Emily Fricska, Canadian Associates of BIOS intern, takes measurements of the photosynthetic integrity of corals. Photo by Chloe Carbonne

Worldwide, all reef-building corals have a symbiotic relationship with algae living within their tissues. Stress triggered by increased ocean temperatures causes corals to expel these colorful and essential algae and become “bleached.” While corals can sometimes recover from bleaching (some species better than others), the increased intensity and frequency of recent bleaching events have led to unprecedented levels of reef declines.

Artificial upwelling (AU) is a geoengineering intervention that lifts cooler, deep water to the surface ocean, mimicking natural upwelling that has been found to mitigate coral bleaching. Led by ASU BIOS Assistant Scientist Yvonne Sawall, the three-year study seeks to evaluate the potential of AU and identify AU depth and intensity scenarios that mitigate coral bleaching effectively with minimal risk of unwanted side effects. It is focused on three common species: fire coral (Millepora alcicornis), greater star coral (Montastrea cavernosa) and mustard hill coral (Porites astreoides).

In proposing the project, which follows a promising 2020 AU study by Sawall and others, researchers noted that mass bleaching events are expected to occur annually by the second half of this century in many coral reef regions worldwide.

“This calls for unconventional interventions to reduce heat stress-related coral loss and one of those is AU, of which, however, research is so far lacking," Sawall said.

Study results will also increase knowledge about the heat stress responses of various corals and their potential ability to increase their thermal tolerance.

Researchers are pursuing three objectives:

1. Assessing immediate effects of various AU scenarios (depth x duration) on the physiology of corals exposed to heat stress and identifying “ideal” scenarios;
2. Investigating long-term effects of “heat stress-plus-AU treatments” on coral recovery and thermal tolerance; and
3. Testing the effect of AU on reef communities including corals, algae, invertebrates and fishes.

Principal Investigator Sawall and Chloe Carbonne, an ASU BIOS marine biology and ecology post-doc, conducted the project’s initial AU simulation experiment in August and September 2023 in BIOS’s Bermuda Marine Mesocosm Facility. Corals sampled for that experiment were returned to the reef in November and Sawall and her team are monitoring their recovery.

While analysis won’t be complete until early summer, the team found clear signs of bleaching in heat-treated corals within the first two to three weeks of the experiment, with impacts worst in the fire corals, followed by greater star and mustard hill coral, respectively. In the heat treatment-plus-AU experiment, there was less bleaching in all species under the most intense AU scenario (water from 90 meters depth introduced for five hours per day), but milder AU treatments showed little to no effect. These early results pointed to less stress mitigation than was expected based on results from Sawall’s 2020 study.

“This is likely due to the difference in light,” Carbonne said. “The preliminary study was conducted indoors under artificial light using constant light over 12 hours of the day. This experiment was conducted outdoors where light levels varied and had a much higher intensity at midday than the indoor light. This highlights the importance of conducting heat stress experiments in corals under natural light conditions in order to gain more realistic results.”

For next steps, researchers this summer will test two to three more extreme AU scenarios (i.e., exposing corals to more hours of AU each day than previously studied). They will also conduct a comparative study to quantify the effect of indoor versus outdoor lighting, which will be conducted by an ASU graduate student, Sawall said.

Watch a virtual tour of the Bermuda Marine Mesocosm Facility.