Submerged vegetation helps to neutralize ocean acidification 

Carbon dioxide has been changing the chemistry of the ocean for decades. The acidification that results from carbon absorption may have serious impacts on the health of marine organisms and ecosystems.

While many studies have investigated the potential consequences of ocean acidification, the process itself is very complex and not entirely understood. 

In a new study from the University of Delaware, experts have discovered that some coastal areas have their own natural defenses against ocean acidification. The team found that submerged aquatic vegetation (SAV) in the Chesapeake Bay drastically reduces carbon dioxide and acidification during the summer.

After a bay-wide decline of SAV in the 1960s through the early 1980s, efforts to reduce sediment and fertilizer runoff helped to increase SAV cover by 300 percent from 1984 to 2015. 

The recovery of SAV is particularly notable in the Susquehanna Flats, a freshwater region near the mouth of the Susquehanna River at the head of the Chesapeake Bay.

An international team of experts, including Professor Wei-Jun Cai of the University of Delaware, set out to study acidification across the bay.

The researchers determined that heightened photosynthesis by SAV beds at the head of the bay can remove nutrient pollution and generate very high pH levels. 

In the summer, the combination of extra sunlight and nutrients triggers high rates of photosynthesis across the SAV beds. This raises the water’s pH level, making it less acidic.

The higher pH levels increase the concentrations of carbonate ions and the subsequent formation of calcium carbonate minerals. 

Downstream, the calcium carbonate particles dissolve when they reach acidic subsurface waters. The dissolution of the carbonate minerals helps combat acidity and promotes healthier pH levels.

“Just like people take Tums to neutralize the acids that cause heartburn, the idea is that SAV beds send carbonate minerals to the lower Bay to neutralize acids there,” said study co-author Jeremy Testa.

In previous work, Professor Cai showed there was a lot of calcium carbonate dissolution in the lower bay but the source of the carbonate was a mystery.

“We know there is a lot of carbonate dissolution in the lower bay, and we know the upper bay is where the carbonate is formed. So in the paper, we hypothesize that it’s that formation in the SAV bed that gets transported downstream and dissolves and we reproduce this downstream transport with a numerical model,” said Professor Cai. “This carbonate that is transported from upstream actually acted as a way to resist, to buffer the pH of the system.”

The findings highlight the importance of coastal nutrient management and reduction, which not only helps to fight against low oxygen stress but also acidification stress through the recovery of submerged vegetation.

Professor Cai said that while the preliminary results are encouraging, the next steps are to determine if the carbonate particles are really transported by the currents and tides to the lower bay and if so, how fast and under what conditions this happens. 

“This is a very interesting thing,” said Professor Cai. “People talk about ocean acidification and very rarely talk about what resists it, what can buffer the system against ocean acidification. So that’s what we want to find.”

The study is published in the journal Nature Geoscience.

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By Chrissy Sexton, Earth.com Staff Writer