The water in seaweed farms is less acidic than the surrounding waters, indicating that the algae can provide refuges to marine organisms vulnerable to ocean acidification caused by climate change, scientists have found.
Researchers analyzed the waters around three species of seaweed at three different farms in China, and found that the kelp known as Saccharina japonica was particularly effective at combating acidity. The team published the findings on Feb. 10 in Science of the Total Environment.
Oceans are growing more acidic as carbon emissions increase. The ocean absorbs about 30% of the carbon dioxide emitted into the atmosphere, according to the U.S. National Oceanic and Atmospheric Administration. As the burning of fossil fuels releases more greenhouse gases, the amount of dissolved carbon dioxide in the ocean increases. The extra carbon dioxide reacts with seawater to form carbonic acid, which breaks into bicarbonate and loose hydrogen ions.
These hydrogen ions lower the pH of seawater, making it more acidic. They also bind with carbonate ions in the water, leaving less for corals, bivalves and other organisms that use the compound to build their hard shells or skeletons.
Since the start of the Industrial Revolution, the average pH of surface ocean waters has dropped from about 8.2 to 8.1, representing an increase in acidity of roughly 26%.
"The ultimate solution to ocean acidification is, as for climate change, the reduction of greenhouse emissions," the researchers wrote in the paper.
But unlike warming, they noted, ocean acidification can be limited in certain spots, particularly by seaweeds and water plants that bind excess carbon dioxide during photosynthesis.
China is the world's largest producer of seaweed aquaculture, with some farms expansive enough to be seen from space. Farms can potentially be placed in areas with vulnerable marine organisms and are easier to expand as needed than wild seaweed habitat.
"In addition, the seaweed farms also provide significant economic and environmental benefits to the coastal farmers," said Xi Xiao, an associate professor of marine science at Zhejiang University and first author of the paper. She and her colleagues have previously found that seaweed farms may remove nutrient pollution from coastal waters.
For the new study, they monitored the water chemistry at three farms along China's eastern coast growing commonly cultivated seaweeds during the weeks before harvesting. Compared with waters beyond the farms, they observed, the seawater in the submerged forests was consistently less acidic, meaning it had higher values on the pH scale.
Additionally, these waters had lower carbon dioxide concentrations and somewhat higher concentrations of oxygen. The researchers also found that conditions were more favorable for calcium carbonate, the key component of the shells of marine organisms, to form rather than dissolve.
Intriguingly, the differences in acidity and carbon dioxide concentrations at seaweed farms relative to adjacent waters didn't differ much between day and night.
"The refugia effects of seaweed farms are exceeding the time window of photosynthesis … and therefore provide a quite stable environment for the marine animals," Xiao said.
The most industrious seaweed seemed to be Saccharina japonica, which raised the seawater pH by about 0.10. Another species, Gracilariopsis lemaneiformis, raised the pH by 0.04, and Porphyra haitanensis by 0.03.
The findings provide evidence that seaweed farming can reduce ocean acidity. However, Xiao points out, her team only looked at three farms and species.
"Therefore, the actual potential of seaweed farms remains largely unknown," she said. The researchers plan to visit more farms in China and other countries to measure how the algae's performance varies by species, location and season.
There are also a number of logistical challenges that must be worked out before seaweed farms can be used at large scale to minimize acidification, including how to make farms that aren't close to sheltered coasts resilient against storms and strong waves.
"There is still a long way to practical application," Xiao said.
The study, "Seaweed farms provide refugia from ocean acidification," published Feb.10 in Science of the Total Environment, was authored by Xi Xiao, Jing Hu, Chao Li, Ke Li, Fangyi Wei, Yitian Lu, Caicai Xu, Yan Yu, Yuzhou Huang, and Jiaping Wu, Zhejiang University; Susana Agustí and Carlos M. Duarte, King Abdullah University of Science and Technology; Weizhou Chen and Zepan Chen, Shantou University; Shengping Liu, Qingdao Agricultural University; and Jiangning Zeng, Key Laboratory of Marine Ecosystem Dynamics.