What Is Ocean Acidification?

Increased atmospheric carbon dioxide (CO2) levels, caused by the burning of fossil fuels and deforestation, is the primary driver of a process termed ocean acidification, where the addition of CO2 to the surface ocean acts to increase seawater acidity and lower pH. Carbon dioxide gas dissolves so readily in seawater that approximately one quarter of human caused CO2 emissions become sequestered in the ocean. Once in the ocean, CO2 combines with water to form a weak acid, resulting in a change in the chemistry of the sea.

The other major change in seawater chemistry involves CO2-driven changes in the solubility of calcium carbonate minerals (CaCO3) used by many marine plants and animals to build their shells and skeletons. The solubility of CaCO3 minerals depend on the amount of dissolved carbonate ions in seawater. More CO2 and lower pH reduces the concentration of carbonate ions, making it more difficult for many organisms to make shell material. You can see a simple graphic depiction of ocean chemistry here.  

Coastal and Estuarine Acidification

While increased atmospheric CO2 is the primary driver of ocean acidification in surface ocean waters, the coastal oceans and estuaries experience additional processes – both natural and anthropogenic (human-caused) – that play a role in changing water chemistry.

Natural input of nutrients to the ocean via rivers, groundwater, and atmospheric deposition promotes algae growth in estuarine waters, making these estuaries areas of high productivity. In a process known as eutrophication, additional input of nutrients to the estuary and coastal ocean (due to use of fertilizer, wastewater treatment, and changing land use) fuels additional growth of algae during the productive spring and summer seasons. If larger animals like fish and shellfish do not consume all of this extra nutrient production, the organic material will be broken down or respired by bacteria leading to conditions of low oxygen called hypoxia. The respiration not only consumes oxygen, but also produces CO2, which results in seasonal decreases in pH in excess of the acidification driven by the atmospheric CO2 which causes ocean acidification. This is a particularly serious issue compounding acidification in the Mid-Atlantic estuaries.   

Multiple Stressors

Ocean acidification is not the only human influence impacting the marine environment. For example, other human-induced stressors such as nutrient-caused oxygen deficiencies (hypoxia) and rising water temperatures can co-occur with normal daily and seasonal marine cycles (e.g., changes in salinity, primary productivity, and tides). The interaction of natural processes and changes in climate can lead to variable, complex, and often unknown ecological responses.

Stressors can be generalized on either global or local scales. Global stressors are usually observed over time in large areas, such as an overall decrease in ocean pH, increase in surface water temperature, or decrease in oxygen within the ocean. Conversely, local stressors are specific in location and variable to each ecosystem and can include hypoxia, toxins, marine debris, overfishing, and coastal shoreline changes.  

How multiple stressors interact to affect individual organisms and entire ecosystems is complex since the impact of co-occurring stressors can be additive or synergetic (meaning that the combined effect is greater than the sum of the individual stressors). For example, increases in both temperature and CO2 concentration have been shown to disproportionately lower the growth rate of some tropical corals.

Changes in ocean circulation and oxygen concentration, bottom disturbances from fishing and sediment movement, and increases in acidification are likely to impact the health and survival of organisms needing calcium carbonate for development, such as shellfish and deep sea corals. Additional monitoring and research is needed to understand how and which co-occurring stressors impact critical marine ecosystems and to identify actions that can mitigate ecological consequences.   

The Need for Information

Acidified ocean waters can negatively affect wild shellfish stocks, as well as aquaculture, restoration, and hatchery industries, including both adult and juvenile shellfish. Adult shellfish become more susceptible to predators and disease as acidified conditions weaken shell strength and structure. Acidified conditions can also cause massive die-offs of juvenile shellfish that are reared in hatcheries and serve as the "seed" for many other aquaculture companies. In a well-studied example, the Whiskey Creek Shellfish Hatchery in Oregon experienced such a die-off in 2007 due to acidified conditions, which left many shellfish growers devastated for years without healthy seed. More acidified conditions cause adult shellfish to have weaker and thinner shells, making them more susceptible to predators and diseases. Hatcheries in the Gulf of Maine and the Mid-Atlantic are working with researchers to watch for changes and ward off massive die-offs like the one experienced at the Whiskey Creek Shellfish Hatchery.

These effects can have serious economic consequences. The Mid-Atlantic oyster, clam, and scallop industries are worth $227 million per year and provide about 35,000 jobs. This does not include the added value of downstream businesses such as processors, distributors, and restaurants. Beyond shellfish, fish stocks such as summer flounder and whole ecosystems such as oyster reef habitat could suffer, complicating local management and restoration efforts. MACAN offers a platform for industry representatives and resource managers to build towards minimizing the effects of acidification. Working together, we can combine scientific expertise, real world experience, and resources to get results. Similar collaborations in the Pacific Northwest have successfully passed new legislation, received funding, and acquired new data for acidification reduction efforts. 

Mid-Atlantic research indicates that there are reasons to be concerned about acidification and that the ocean chemistry is changing and will continue to change, but we are still assessing the level of impact to be expected in this region and what that will mean for its coastal communities. 

What Lies Ahead

In 2009, Congress passed the Federal Ocean Acidification Research and Monitoring (FOARAM) Act, which required an interagency working group to create a Strategic Plan for Federal Research and Monitoring of Ocean Acidification. Regional consortia like MACAN, New England’s NECAN, and the Southeast Atlantic’s SOCAN have formed throughout the country to focus on acidification in their respective territories.

Some Mid-Atlantic states are taking a closer look at acidification’s implications within their own waters. The Maryland task force for example was charged with analyzing the best available science and the potential effects of acidification on ecology to make recommendations for potential strategies to mitigate the effects of acidification in state waters and fisheries. The Maryland task force produced the Task Force to Study the Impact of Ocean Acidification on State Waters Report to the Governor and the Maryland General Assembly calling for monitoring, industry partnerships and collaboration with federal agencies to address the threat. Created in late 2016, New York’s Ocean Acidification Task Force is charged with detailing the causes and factors contributing to ocean acidification and methods to address it in a report to be issued by the end of 2018.

Acidification will continue to influence all areas of the Mid-Atlantic, from its tidal estuaries to deep sea ecosystems. The cooler, less salty waters of the upper Mid-Atlantic are particularly susceptible to ocean acidification, making reductions in the survival, calcification, growth, development, and abundance of marine organisms more likely. The organisms impacted most negatively and directly will likely be calcified algae, corals, mollusks, and echinoderms. Crustaceans, fleshy algae, seagrasses, and diatoms may be less directly affected or may even benefit from acidification. Even still, questions regarding food web impacts that may indirectly negatively impact all species remain.

Combating acidification won’t be easy, but scientists in academia, governments, and NGOs are working to identify a range of approaches that could help stem the tide if applied effectively. They include mitigation, or decreasing the carbon dioxide that cause acidification through behavior changes and technological advances; remediation, steps that can be taken to lessen the acidity of water, such as planting eelgrasses that take up carbon dioxide; and adaptation, or making changes in response to the symptoms of acidification, such as breeding shellfish that are more resistant to acidified waters. Innovative ideas for mitigation, remediation, and adaptation of acidified waters will continue to evolve as research continues. A consortium such as MACAN can help to guide resources for research in mitigation, remediation, and adaptation by collectively identifying the most critical research gaps.  


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Image/Graphic Details

Ocean CO2 and PH from NOAA: Modified after R. A. Feely, Bulletin of the American Meteorological Society, July 2008. Source: NOAA PMEL graphic. Data: Mauna Loa Observatory and Station Aloha.