The Acid Truth about Chesapeake Bay

Acid zone in Chesapeake Bay identified

A research team, led by University of Delaware professor Wei-Jun Cai, has identified a zone of water that is increasing in acidity in the Chesapeake Bay.

The team analyzed little studied factors that play a role in ocean acidification (OA)—changes in water chemistry that threaten the ability of shellfish such as oysters, clams and scallops to create and maintain their shells, among other impacts.

The U.S. Geological Survey defines pH as "a measure of how acidic or basic water is." The pH scale ranges from 0-14, with 7 considered neutral. A pH less than 7 is acidic, while a pH greater than 7 is alkaline (basic). Battery acid, for example, might have a pH of 1, while Milk of Magnesia might have a pH of 10.

Changes in pH can tell scientists something about how the water chemistry is changing.

In their research, Cai and his colleagues discovered a "pH minimum zone" that occurs at a depth of approximately 10-15 meters (~30-50 feet) in the Chesapeake Bay. The pH in this 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.

Incidentally, the Bay averages 10 m deep, so this only occurs in the deeper channels of the Bay.

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. The team reported the findings in a paper in Nature Communications on August 28, 2017.

"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," said Cai, the paper's lead author and an expert in marine chemistry and carbon's movement through coastal waters.

Science by press release. We've known about sulfide and ammonia production in the sediments and bottoms waters for years. We've known about the production of acid from sulfide and ammonia an equally long time, if not longer (it's responsible for most acid mine drainage).

What seems to be new here is that Cai connected the actual oxidation of ammonia and sulfide with a local pH minimum, just barely detectable over the noisy pH signal in the Bay.

The good news, is that if the Bay actually gets cleaner, this problem, that I would argue is not really a significant problem, will improve, as sulfide and ammonia production decline along with primary production.