Overview

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.  

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.   

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.