Scientists listen in to determine coral reef health

Coral reefs are some of the most diverse and productive ecosystems on the planet, but their health is being threatened by human activity and climate change, causing ocean acidification, disease outbreaks, overgrowth of algae and other repercussions that result in changes in reef structure and habitat loss.

One measure of coral reef health is the rate of the system’s photosynthesis, or the process by which plants use sunlight to generate sugars used for food. Decreases in reef photosynthesis are correlated with decreased daily productivity, coral bleaching and disease.

The photosynthetic producers in coral reefs range from scleractinian, or hard, corals with endosymbiotic algae that live within coral cells in a mutually symbiotic relationship, to algaes and and microphytobenthos, or small, photosynthetic organisms, that live within the upper sediment layer of the reef. One good way to measure photosynthetic activity is through reef bubble formation, when the rate of photosynthetic oxygen production exceeds the rate of oxygen dissolution into the water. While limited, oxygen bubble formation has been used by marine biologists in the past to assess reef photosynthesis. The primary challenge of this method is finding automated and accurate ways of quantifying bubble formation.

To address this issue, a group of researchers from Xiamen University used acoustic techniques to assess the rate of reef photosynthesis, and therefore health, over time. The team leveraged the fact that oxygen bubbles generate short acoustic pulses when they detach from algae and symbiotic coral surfaces and rise to the water surface.

The researchers published their study on February 27 in the journal Ocean-Land-Atmosphere Research.

“Our goal was… to determine whether passive acoustic monitoring could detect and quantify these photosynthesis-related bubble sounds in situ and to investigate their temporal patterns across seasons and throughout the day. By combining long-term underwater recordings with machine learning techniques, we sought to reveal how these sounds reflect reef metabolic activity and environmental conditions,” said Fei Zhang, Ph.D. candidate at Xiamen University in Xiamen, China and first author of the research paper.

Specifically, the researchers used passive acoustic technology to identify the specific acoustic pulse created when a photosynthetic oxygen bubble detaches from an organism in the coral reefs on Dongshan Island.

“[T]he rate of these [pulses] varies systematically with seasons and environmental conditions. In particular, the signal rate was highest in summer and lowest in winter, reflecting changes in biological productivity,” said Zhongchang Song, associate professor at Xiamen University and corresponding author of the study.

Importantly, these results indicated that reef photosynthesis and productivity could be passively monitored acoustically, offering a non-invasive and continuous way to observe ecosystem metabolism. This approach could also complement traditional methods of reef tracking, including advanced imaging, and help scientists better track the health and productivity of coral reef systems.

While acoustic measurements are helpful in establishing reef health, combining acoustic with other measurements or assessments can provide a more comprehensive picture of reef status. Ideally, the team will integrate environmental and biological measurements, such as light levels, nutrient dynamics and benthic community composition, to improve the accuracy and ecological interpretation of photosynthetic acoustic signals.

Ultimately, the researchers would like to expand monitoring to different reef habitats and geographic regions to determine how broadly this acoustic indicator can be applied. “Our goal is to develop a real-time acoustic monitoring system for coral reef metabolism and health. By continuously listening to reefs, we hope to detect early changes in ecosystem productivity that may signal stress from climate change, pollution or other disturbances. Such an approach could provide a rapid, scalable and non-invasive tool for long-term reef monitoring and conservation management,” said Zhang.

Yingnan Su, Wenzhan Ou and Yu Zhang from the Key Laboratory of Underwater Acoustic Communication and Marine Information Technology of the Ministry of Education in the College of Ocean and Earth Sciences at Xiamen University in Xiamen, China and the State Key Laboratory of Marine Environmental Science in the College of Ocean and Earth Sciences at Xiamen University in Xiamen, China; Chichi Liu and Xuwen Fang from Dongshan Swire Marine Station at Xiamen University in Zhangzhou, China; Shengyao Sun from the State Key Laboratory of Marine Environmental Science in the College of Ocean and Earth Science at Xiamen University in Xiamen, China and Dongshan Swire Marine Station and the National Observation and Research Station for the Taiwan Strait Marine Ecosystem at Xiamen University in Zhangzhou, China; Wupeng Xiao from the State Key Laboratory of Marine Environmental Science in the College of the Environment and Ecology at Xiamen University in Xiamen, China and the Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies in the College of the Environment and Ecology at Xiamen University in Xiamen, China also contributed to this research.