Seagrass meadows absorb less CO2 than previously thought

Carbon dioxide (CO₂) emissions are a major factor impacting our climate. According to many scientists, a solution for diminishing the amount of CO₂ from the atmosphere is to restore natural CO₂ reservoirs on the coasts that have been destroyed in the past decades, including mangrove forests, salt marshes, or seagrass meadows. However, a new study led by the Helmholz-Zentrum Hereon has found, in some cases, seagrass meadows absorb far less CO₂ than previously thought.

Seagrasses are aquatic plants thriving in shallow, light-flooded water. They are anchored by their roots to the soft seabed and are vital grazing grounds and nurseries for many marine animal species. In order to develop their tissues, seagrasses absorb CO₂, and when they ultimately die and become deposited in the sediment of the seabed, they act as carbon sinks.

Yet, according to recent findings, fighting climate change by recultivating seagrass meadows is not as simple as many scientists believed. In fact, some seagrass meadows appear to release even more carbon dioxide into the atmosphere than they store. 

“By taking measurements in seagrass meadows off the coast of Florida, we could show that some of these tropical seagrass meadows absorb considerably less carbon dioxide than has long been thought. In contrast, on some coasts, they even emit more CO₂,” said study lead author Bryce Van Dam, a biochemist at the Helmholz-Zentrum.

Van Dam and his colleagues found that carbonates, substances winding up in the sea through the weathering of rocks, play a fundamental role in binding the carbon dioxide from the atmosphere and storing it in the seawater. The more carbonate the water contains, the more CO₂ it can ultimately absorb and store.

In the warm tropical waters, the metabolic processes of the seagrass plants convert the dissolved carbonates to lime, which trickles to the seafloor. “The result is that these seagrass meadows hardly bind any carbon dioxide. On the contrary, they tend to release carbon dioxide through various other biochemical processes,” explained study co-author Helmuth Thomas, a biochemist at the Hereon Institute of Carbon Cycles.

“We cannot rely on blue carbon offsets to counteract the CO₂ that we put into the atmosphere by burning fossil fuels. Rather, we must first reduce CO₂ emissions and then protect these coastal habitats for the many financial and environmental benefits they offer us, which may not always include CO₂ sequestration,” concluded Van Dam.

The study is published in the journal Science Advances.

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By Andrei Ionescu, Earth.com Staff Writer