A Semi-Annual Publication of Manomet

Litmus Test: How we're changing the chemistry of our oceans

By: John Hagan and Liza LePage

Look at the ocean today. It probably looks the same as it did when you were a kid, but as the old adage warns, looks can be deceiving. You start to get some clues from our shorelines, with evidence of erosion from higher tides. However, it is beneath the surface, within the shells of shellfish, that a different story is unfolding: ocean acidification.


Since the industrial revolution, the ocean's pH has decreased from 8.2 to 8.1. The number 8.1 doesn't seem much different from 8.2. Not on a linear scale. But the difference between a pH of 8.2 and a pH of 8.1 is quite significant. pH, a measure of the acidity of a liquid, is measured on a log scale, ranging from 0 (acidic) to 14 (alkaline). Through a century of combusting fossil fuels, we have managed to raise our oceans' acidity by 26%.

Although ocean acidification has only become an important research topic in the last 20 years, the basic chemistry to illustrate the phenomenon was well established over a century ago. When the concentration of CO2 (the primary greenhouse gas from burning fossil fuels) increases in the atmosphere, CO2 is absorbed by the ocean at a predictable rate. Since the dawn of the industrial age, oceans have absorbed about one-third of the CO2 that humans have emitted into the atmosphere. Climate change would be happening much faster without this help from the oceans.

That the oceans provide this ecosystem service might be a good thing for humans, but not for marine organisms. When carbon dioxide (CO2) is absorbed by the ocean, it reacts with water (H2O) to form carbonic acid (H2CO3). Carbonic acid then breaks down and forms bicarbonate (HCO3-) and hydrogen ion (H+). The addition of hydrogen ions is what makes something more acidic. So, putting more CO2 into the atmosphere leads to a higher concentration of H+ in the ocean and thus a lower pH score and a more acidic ocean. For marine organisms with shells or structures made of calcite (e.g., clams, mussels, crabs, lobsters, corals), this is a troubling chemical equation.

These animals depend on combining calcium (Ca, which is abundant in seawater) with carbonate (CO3-) to make calcite (calcium carbonate, [CaCO3]), the hard crystalline material that makes up their shells. Increasing the CO2 in seawater leaves less carbonate for sea creatures to combine with calcium to make shells. The hydrogen ions produced in the chemical equation result in lower pH, but it's really the theft of that carbonate molecule by the hydrogen atoms that causes the problem for shell-making organisms. (See Figure 1).

Figure 1: CO2 enters the ocean at a predefined rate depending on how much CO2 is in the atmosphere and the temperature of the water. When CO2 enters the ocean, it reacts wtih water to form a weak acid, carbonic acid. Since the acid is weak, it easily breaks down into bicarbonate and hydrogen ions. The increase of hydrogen ions decreases the amount of available carbonate ions, which shellfish need to construct their shells.

In the last century, humans have increased the concentration of CO2 in the atmosphere by about 44%. The concentration of CO2 in the atmosphere has not been this high in the last 800,000 years. The same is true for ocean acidity. The rate of change in acidity may be greater now than at any time in the last 300 million years.

How is changing the pH of the oceans relevant to humans?

Mark Duffy

Figure 2: Two photos of a clam taken after differing exposures to the acidic mud. The first photo was taken while the clam was being reared in a controlled hatchery, the second was after the clam spent two weeks in the acidic mud flat. Both photos were taken under the same magnification. Clam shows growth and obvious corrosion.
Photos from Mike Doan, Research Associate, Friends of Casco Bay.

We wouldn't know or feel the difference in seawater of pH 8.2 and 8.1, or even 6.0 because our skeletons aren't in contact with seawater, unlike shellfish, which experience the change in acidity in their 'bones.'

The real concern for humans is how ocean acidification will affect the marine ecosystems we rely on. Worldwide, fisheries supply 15% of the animal protein consumed by humans, provide employment for 200 million people, and contribute $230 billion to the global economy. Fish and fisheries are a big deal for both our economy and our dinner tables.

A recent study by United Nation experts estimated that by 2100, ocean acidification will cost the global economy over a trillion dollars. This figure only reflects the potential economic loss linked to coral reefs, which are considered to be the most vulnerable species to ocean acidification. Coral reefs provide diverse ecosystem services (such as shoreline protection), food services for millions of people, tourism revenue for many countries, and raw materials for a diverse array of other industries such as the pharmaceutical industry. If coral reefs dissolve, humans across the globe will feel the impact.

Science is ramping up fast to test marine animals' ability to cope with increasing acidity. Different species have different resiliencies to acidification that are still poorly understood. Most studies show that juvenile shellfish are especially vulnerable, showing higher mortality in more acidic water.

Mark Green, a marine scientist from Saint Joseph's College in Maine, has been researching the effects of ocean acidification on juvenile soft shell clams for over a decade. His research shows that juvenile clam growth begins to be compromised at a pH of 7.8 and that below 6.8, clam spat and baby clams simply dissolve.2

However, results can be variable. One study showed that the shell mass of several crustaceans, including lobsters, actually increased with acidity. But the net effect is unknown; shell-making requires energy, and that energy could be diverted from other life functions such as growth and reproduction.3

Zooming in to the local level

While atmospheric CO2 increases at an even rate across the world, increasing ocean acidity is not globally uniform. Many marine scientists differentiate ocean acidification from coastal acidification because the way inshore waters become more acidic goes beyond atmospheric deposition.

Due to its cold, fresher, and nutrient rich waters, the Gulf of Maine is considered to be an ocean acidification hotspot. A study led by Woods Hole Oceanographic Institute found that the Gulf of Maine has the lowest acid buffering capacity4 of any region on the eastern seaboard.

Cold water can absorb more gas than warm water. So cold water absorbs more CO2, which makes colder water more acidic. Moreover, Maine's 60 freshwater rivers makes the inshore waters more acidic because freshwater is naturally more acidic. In Maine's more populated areas, like Casco Bay, high nutrient run off from fertilizer and waste water treatment plants, as well as deposition of airborne pollutants, exacerbates local acidity in semi enclosed bays by triggering algal blooms, which decreases oxygen and further increases CO2.

Although Maine is often idealized as a pristine area, the coastal ecosystem and the people who depend on it are vulnerable due to the natural and human induced reasons stated above. Maine's real risk does not come directly from its cold, acidic water, but from the region's economic reliance on the very species that are projected to be affected by it.

Mark Duffy

Figure 3: NOAA's 2013 Annual Commercial Landing Statistics show lobsters contributed 78% of Maine's $475 million dollar fishing revenue, while 89% of the fishing economy came from shellfish. This huge reliance on shell-building organisms makes the potential effects of ocean acidification important to Maine.

The Gulf of Maine once supported a diverse ecosystem, but now, as a result of overharvesting of fish stocks, it is crustacean dominated. The lobster industry in Maine has been booming in the last decade and is home to the highest lobster density in the world. In 2013, 78% ($368 million) of all fisheries revenue in Maine came from lobstering. (See Figure 3).

The Downeast Fisheries Partnership (DFP), which Manomet formed in 2012, with the Penobscot East Resource Center and the Downeast Salmon Federation, is trying to restore diversity to the Downeast Maine aquatic ecosystem so that communities will be able to "fish forever." Dependence on a single shell-bearing species—lobster—in the face of warming waters and acidifying waters, presents a risky and vulnerable future for rural coastal communities.

"Our approach to locally driven management will help Maine's fisheries to adapt to a series of threats, including acidification," said DFP Coordinator Anne Hayden. "Local stakeholders have the most intimate knowledge of how their ecosystems are affected by coastal acidity. Rebuilding diversity in river and coastal ecosystems and engaging fishermen and others in their stewardship will make rural communities more resilient to unforeseen changes."

Ocean acidification is already disrupting some Maine ecosystems. Maine clammers have noticed that flats which were once productive are no longer economically harvestable. A study by Friends of Casco Bay found that in a comparison of productive and unproductive flats, nearly all unproductive flats in the Casco Bay had mud with low pH, while all productive flats had higher pH.5

"We know from research that acidic mud negatively affects clams, and we know that we have low pH mud in our coastal flats," explained Casco Baykeeper Joe Payne. "However, it's hard for us to talk about this issue when clammers are faced with problems that they can see, like invasive green crabs (Carcinus maenas). There are a lot of flats that are no longer productive, but if coastal acidification is the lead culprit, this is something we can manage."

Luckily for Maine, officials have begun to take action to address this issue. In May, the State Legislature formed the groundbreaking Maine Ocean Acidification Study Commission to assess the threat of ocean acidification on the state's commercially harvestable species. The Commission, the first of its kind on the east coast, will provide the government with a report on December 5, which will hopefully open the door for action in Maine and throughout New England.

"Maine's economic future rests on the fragile shoulders of the lobster industry and we have no idea how this resource will fare," remarked Green, one of the leading scientists on the Commission. "One of the major recommendations coming out of the Commission's report will be to address this unknown and other knowledge gaps that exist. We need to expand our ability to monitor pH along the coast so we have a better idea of how conditions are changing."

Clam Flats, like the one pictured here in Georgetown, Maine, are highly vunerable to invasive green crabs and acidic conditions. Manomet is working with shellfish harvesters to help manage these systems.

The Commission's report will also address actions that local communities and stakeholder groups can do to protect their coastal ecosystems from rising acidity.

One of the most effective actions communities can take is to limit nutrient runoff in coastal waters.
Wastewater treatment plants can be a big source of nutrients to coastal waters, which increases acidity. Plants across the state are ill-equipped to handle large storm events and often untreated water is discharged into the ocean when the system is overloaded. By upgrading these systems, we could greatly minimize nutrient inputs to coastal waters.

Nutrient run off can also be mitigated by improving local land-use practices. By limiting the amount of fertilizer and reducing the amount of impervious surfaces found in neighborhoods, agricultural lands, and public recreational areas, we can lower nutrient output and slow down coastal acidification.

Actions can be taken directly to vulnerable coastal waters to reduce acidity. Mark Green discovered that clam flats were two to five times more productive after crushed up shells were added to the mud. Although it was once illegal to add shells to coastal waters because of the chance of disease, it is now a regulated practice. Green hopes to continue to research the effect of this method on large scale projects and make the practice more available to coastal stakeholders.

Seaweed aquaculture is another new tool against local acidification. Ocean Approved, a Portland based aquaculture company that grows kelp grass, is providing a local solution while generating a new economic income for the state. As kelp grows, it absorbs CO2. Ocean Approved takes the product out of the water to be harvested before any decomposition occurs, creating a temporary carbon sink to local ecosystems.

"A major part of combating coastal acidification is getting people to understand what is at risk and how they can help," explained Payne. "These problems don't just affect shellfish harvesters as the economic benefit of each bushel of clams penetrates through the entire economy."

When you consider the value that gets added to a bushel of clams as it moves down the production line from landing to retail sales and the indirect boost it brings to tourism, it becomes clear that healthy and productive marine ecosystems are important to the entire community. From this macro perspective, shellfisheries and aquaculture are valued in excess of a billion dollars annually to Maine—an income the state cannot afford to lose.

Zooming back out

While coastal acidification can be managed through local action, there is only one solution to ocean acidification—reducing human emissions of greenhouse gases. If humans continue to stay on a business-as-usual CO2 emissions pathway, average ocean pH will drop to about 7.7 by 2100, a 150% increase in acidity over the preindustrial ocean and a level proven to be detrimental to some marine organisms and coastal economies.

Ocean acidification might seem like a big deal, and it is, but it is just a small piece of the climate story. Everything we care about—economic, social, and environmental—will be impacted by our carbon emissions. That's why Manomet is working so hard with the business sector on climate change mitigation. There is hope for a resilient future, but we need everyone to be involved. By bringing more people together to join in the effort to address climate change, we will ensure a vibrant, safe and healthy future for generations to come.


1 Secretariat of the Convention on Biological Diversity. 2014. An Updated Synthesis of the Impacts of Ocean Acidification on Marine Biodiversity. Available online at: http://www.cbd.int/doc/publications/cbd-ts-75-en.pdf

2 Green, M. et al. 2009. Death by dissolution: Sediment saturation state as a mortality factor for juvenile bivalves. Limnology and Oceanography. 1037–1047 pp. Available online: http://www.aslo.org/lo/toc/vol_54/issue_4/1037.pdf

3 Ries, J.B., Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology. 1131-1134 pp. Available online: http://intl-geology.geoscienceworld.org/content/37/12/1131.full.pdf+html

4 Gulf of Maine has the lowest alkalinity to dissolved inorganic carbon ratio of any region on the eastern seaboard. Wang, Z.A. 2013. The marine inorganic carbon system along the Gulf of Mexico and Atlantic coasts of the United States: Insights from a transregional coastal carbon study. Limnology and Oceanography. 325-342 pp. Available online: http://www.aslo.org/lo/toc/vol_58/issue_1/0325.pdf

5 Friends of Casco Bay. 2013. The Mystery of the Disappearing Clams. Available online: http://www.cascobay.org/the-mystery-of-the-disappearing-clams/

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