Large beds of underwater grass may be just the cure for one of the problems that ails the Chesapeake Bay.
Subaquatic vegetation uses sunlight to produce food from carbon dioxide and water through a process called photosynthesis. It’s such a powerful action that the surrounding water becomes less acidic and crystals of calcium carbonate — the same substance used as an antacid to relieve heartburn, acid indigestion, and upset stomach in people — form on the plants’ leaves, scientists said in a study recently published in the journal Nature.
Those crystals can travel downstream and dissolve, making the water more alkaline. That helps to prevent acidification caused by algae bloom die-offs and man-made pollution.
“Just like people take Tums to neutralize the acids that cause heartburn, SAV beds send carbonate minerals to the lower Bay to neutralize acids there,” the study’s co-author Jeremy Testa said in a statement announcing the findings.
It is another reason to reduce nutrient pollution, researchers from the University of Delaware, University of Maryland Center for Environmental Science, St. Mary’s College, Xiamen University in China and Oregon State University wrote, with grasses both providing oxygen and an alkaline compound that buffers acidification.
The burning of fossil fuels has increased the amount of carbon dioxide in the atmosphere. When carbon dioxide gets into water and dissolves, it adds a proton and makes the water more acidic, Testa said.
Acidification hurts shell-forming animals including larval mussels and oysters, according to a summary of the finds released by the University of Maryland. Adult shell-forming animals can also be harmed.
Photosynthesis, on the other hand, takes carbon dioxide out of the water, Testa said, reversing the acidification process and increasing the water’s pH, the scale which measures how acidic or basic something is.
Testa said they noticed that in the area of the Susquehanna Flats, a large underwater grass bed in the upper Chesapeake, total alkalinity was very low. Alkalinity is a scale that measures the presence of calcium carbonate and other compounds that help to buffer or control water’s pH level.
The reason carbonate was disappearing near the bed was that photosynthesis in the bed caused pH to increase so much that calcium carbonate crystals began precipitating on the grass, Testa said. It wasn’t dissolved in the water anymore.
Farther down the bay, they noticed more alkalinity than expected just from water mixing. They used a computer model, which suggested that crystals from the Susquehanna Flats in the Upper Bay could have reached more acidic and oxygen-deprived waters farther downstream and dissolved, Testa said.
He said they want to design a research project that would allow them to identify where calcium carbonate crystals are being produced in other grass beds, and to answer questions like how dense grass must be to produce calcium carbonate crystals and how much calcium carbonate is being transported downstream.
One factor affecting the Chesapeake is an excess of nutrients, particularly nitrogen and phosphorus. Together, they cause a marked increase in algae blooms, killing off thousands of acres of underwater grasses and lowered dissolved oxygen concentration in bottom waters, suffocating bottom-dwelling organisms.
In recent years, underwater grasses have returned to the Susquehanna Flats area and some of the tributaries farther down the bay.