Scientists identified Acid zones in Chesapeake Bay


A group of researchers team found a zone of water in ocean increasing the level of acidity in Chesapeake Bay. The team from university of Delaware led by professor Wei-jun Cai identified and understood the factors causing ocean acidification (OA) changes in water.

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The pH scale ranges from 0-14, with 7 considered neutral examined by U.S. Geological Survey. A pH less than 7 is acidic, while a pH greater than 7 is alkaline. Levels of pH indicates the change in composition of water. The group discovered pH minimum zone that occurs at a depth of approximately 30-50 feet in the Chesapeake Bay. The pH in this zone is roughly 7.4, nearly 10 times higher in acidity found in surface waters. But it contains an average pH of 8.2.

The Chesapeake Bay the largest delta in the United States. Due to amalgamations like acids produced when base water rich in toxic hydrogen sulfide gets mixed upward.

This study shows the oxidation of hydrogen sulfide and ammonia from the bottom waters. A major contributor to lower pH in coastal oceans. This leads to rapid acidification in coastal waters compared to the open ocean, said Cai.

Biogeochemical Model

Past studies have shown that acidification occurs in nutrient-rich coastal water areas with less oxygen and high levels of carbon dioxide near ground level. However, scientists don’t know accurately the amount of OA is occurring in a large bay like the Chesapeake Bay.  Agricultural nutrients entering the water have had a progressive impact on the Bay’s bottom water’s becoming anoxic, or oxygen depleted. During the summer months over the past 50 years.

During research cruises aboard UD’s 146-foot research vessel Hugh R. Sharp in August 2013 and 2014, UD researchers Cai and George Luther collected water samples repeatedly from deep basin of the main Chesapeake Bay. The researchers measured oxygen, hydrogen sulfide, pH, dissolved inorganic carbon and total alkalinity.

From the data they noticed the Bay’s pH to reach a minimum at depths between 10-15 meters. To explain Cai built biogeochemical model to simulate the way oxygen is consumed and inorganic carbon and acids are produced to match the observations measured in the Chesapeake Bay. Using direct hydrogen sulfide measurements collected in the bottom waters. Calculated amount of acid need to be produced to explain this minimum zone.

Eutrophication process

Cai explained that in the coastal ocean, excess nutrients introduced into the ecosystem from land cause plant overgrowth, a process known as Eutrophication. That upsets the water’s natural chemistry and causes the death of marine species. When that organic matter sinks to the bottom sediment it is consumed by bacteria that respire. Generating excess carbon dioxide that mixes upward into the water column.

He compared the outcomes of Chesapeake Bay model to data from the Gulf of Mexico, considered as barrier system able to counteract the changes from OA and keep itself in balance. But in large eutrophic delta like the Chesapeake Bay, the combined environmental and climate change biological agent make the Bay more vulnerable, and the excess nutrients and increase in acidity may take a larger toll.

The team’s research shows that currently dissolving of living shells and non-living minerals like aragonite and calcite provided a self-regulating mechanism to guard or prevent the Chesapeake Bay’s bottom waters from becoming acidic.

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In addition to providing marine environment for tourism and outdoor recreation along the East Coast. It has more importance in the nation’s economy through harvesting seafood including shellfish, like blue crab and oysters.