A team of researchers led by University of Delaware Professor Wei-Jun Cai has discovered a ‘pH minimum zone’ that occurs at a depth of 30-50 feet (10-15 m) in the Chesapeake Bay, the largest estuary in the United States.
Chesapeake Bay near the Rappahannock River Mouth, VA. Image credit: University of Delaware.
pH is a measure of how acidic/basic water is. The range goes from 0 – 14, with 7 being neutral. pHs of less than 7 indicate acidity, while a pH greater than 7 is alkaline (basic).
The pH in the Chesapeake Bay’s ‘acid zone’ is roughly 7.4, nearly 10 times higher in acidity (or a unit lower in pH) than what is found in surface waters, which have an average pH of 8.2.
This zone is suspected to be due to a combination of factors, most importantly, from acids produced when bottom water rich in toxic hydrogen sulfide gets mixed upward.
“This study shows for the first time that the oxidation of hydrogen sulfide and ammonia from the bottom waters could be a major contributor to lower pH in coastal oceans and may lead to more rapid acidification in coastal waters compared to the open ocean,” Professor Cai said.
During cruises aboard the 146-foot research vessel Hugh R. Sharp in August 2013 and 2014, Professor Cai and co-authors collected water samples repeatedly from a deep basin of the main Chesapeake Bay.
The team measured oxygen, hydrogen sulfide, pH, dissolved inorganic carbon and total alkalinity.
As the scientists analyzed the data from these cruises and another in April 2015, they noticed that the Bay’s pH seemed to reach a minimum at depths between 30 and 50 feet.
To explain this, they built a 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, they calculated how much acid would need to be produced to explain this minimum zone.
“In the coastal ocean, in general, there is a synergistic effect on ocean acidification (OA) when 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,” Professor Cai said.
“When that organic matter sinks to the bottom sediment it is consumed by bacteria that respire, creating excess carbon dioxide that mixes upward into the water column.”
“The water is already lower in pH and when you add just a little more carbon dioxide and other acids, it creates a tipping point that leads to a decrease in pH.”
The scientists compared the results of their Chesapeake Bay model to data from the Gulf of Mexico, which is considered a well-buffered system that is able to counteract the changes from OA and keep itself in balance.
But in large eutrophic estuaries like the Chesapeake Bay, the combined environmental and climate change stressors make the Bay more vulnerable, and the excess nutrients and increase in acidity may take a larger toll.
“Given how widespread low-oxygen zones are in coastal waters worldwide, understanding these processes will allow us to predict the acidification of estuaries under expected increases in carbon dioxide and ongoing mitigation of nutrient inputs by management actions,” said co-author Dr. Jeremy Testa, of the University of Maryland Center for Environmental Science.
“These results will allow us to identify where and when shell-forming organisms like oysters will thrive or suffer in the future.”
The study shows that currently the dissolving of living shells and non-living aragonite and calcite minerals has provided a self-regulating mechanism to buffer or prevent the Chesapeake Bay’s bottom waters from becoming acidic.
The findings were published online today in the journal Nature Communications.
_____
Wei-Jun Cai et al. 2017. Redox reactions and weak buffering capacity lead to acidification in the Chesapeake Bay. Nature Communications 8, article number: 369; doi: 10.1038/s41467-017-00417-7
