Ocean Acidification

March 18, 2010

The Chemical Mysteries of Carbon

In this day and age of green revolution, we are the auditory recipients of buzzwords and environmental vocabulary, one of the most ubiquitous being “carbon” – as in carbon footprint, carbon credits and carbon offsets.

While carbon is key in the intricate chemistry of life, the recent fast and loose talk regarding carbon is due to its ability—in the form of carbon dioxide (CO2), along with other designated greenhouse gases—to fuel furious and consequential changes in global climate patterns. It also causes ocean waters to acidify.

The ocean at Wallis Sands Beach in Rye, New Hampshire.

Ocean acidification is exactly what it sounds like, a phenomenon where saline marine waters drop in their pH (percent hydrogen, a standard measure of acidity). According to a 2005 report released by the UK’s Royal Society, the ocean pH has dropped by .1 units since pre-industrial times.

While this initially seems paltry, the pH scale is logarithmic and the report describes the change as equivalent to an almost 30 percent increase in the concentration of the free hydrogen ions that drive pH levels. With studies suggesting a myriad of potential ecological effects in relation to acidification, an ocean already stressed by issues like overfishing and pollution is facing even more hardship.

Although there is bickering on the international level concerning whether changes in carbon levels are naturally-inclined or human-induced, there is little controversy that carbon is being freed in significant amounts from certain reservoirs it once resided in and incorporated into others.

Ice core data and the infamous Keeling Curve, a continuous data set started in 1958 and measured at the Mauna Loa Observatory in Hawaii, document the continual and unprecedented creep of atmospheric CO2 in recent times, but levels would be unequivocally higher were it not for the ocean’s ability to soak up a significant portion of that.

The “other inconvenient truth”

In present times, the ocean is serving as a net sink for carbon, confining a huge fraction of the world’s carbon in its briny reaches. However, the resilient marine system is finally becoming overwhelmed.

We often associate carbon with climate change, glacier melt and sea level rise, but it is also responsible for the equally sinister yet often lower profile issue of ocean acidification. In some circles this threat is being dubbed the “other inconvenient truth”.

The acidification process is largely driven by the multiple sources and forms of carbon which enter marine waters and quickly surrender to the complex chemistry of the oceans, often reacting with water and forming some version of a molecule containing carbonate (CO3). These reactions sometimes free up hydrogen ions which lower pH.

But, here’s where the chemistry provides a reprieve – the reactions are reversible and form the ocean’s natural buffering system (for more details on the buffering system, see the “Chem toons” link in the Online Resources at the end of the article).

A buffering system allows a liquid to resist changes in pH when either acids or bases are added, but only to a point. At some level, the system becomes overwhelmed and can no longer resist radical changes in pH, which is representative of the current state of things in many of the world’s oceans.

The flux of carbon dioxide from the atmosphere is thought to be one of the main instigators of acidification on a global-wide level. Carbon dioxide has a natural tendency to dissolve in the ocean; the quantity is controlled by temperature (colder water is more amenable to dissolving gaseous substances than warmer water) and by the other forms of carbon already present. The intensification of atmospheric levels of CO2 over the latter part of the 20th century has driven more carbon into receiving seas. But exploring the issue of acidification on a regional scale, such as in the Gulf of Maine, may be significantly more complex than just the aforementioned movement of carbon between atmosphere and ocean.

The influence of rivers

Other processes are likely working in synergy with the air-sea exchange to affect the acidity levels within the Gulf of Maine. A study completed in 2009 by Salisbury et al, and published in the journal Estuarine, Coastal and Shelf Science, suggests the influence of rivers may be important, especially in the coastal waters which support a large variety of flourishing biological life. The rivers seeping into the Gulf of Maine tend to be more acidic relative to marine waters and when mixed well into the water column, will influence the carbon chemistry.

This river effect on the Gulf’s acidity is most significant during the spring freshet, a period of higher riverine discharge due to melting snow and increased precipitation. According to Joe Salisbury, research assistant professor with the Ocean Process Analysis Laboratory at the University of New Hampshire, interestingly, over the last half century, river discharge has also increased substantially during the months of June through August, an event which coincides with the spawning of many commercially important species of shellfish.

The tiny larvae need to incorporate aragonite, a crystallized form of calcium carbonate (CaCO3), to begin the formation of their shells. When the supply of carbonate is limited, which happens in increasingly acidic conditions, these small juvenile creatures may be unable to properly build their hard exteriors. In extreme circumstances, their shells may even begin to corrode. The influence of rivers on the ocean can also favor eutrophication, a process that consumes oxygen and creates more acidic bottom waters.


Mark Green, a professor and shellfish researcher at St. Joseph’s College in Standish, Maine, has shown that increased eutrophication proves to be quite stressful for shellfish as they settle out of the free-floating larval stage into their permanent homes in the sediment.

The intricate seasonal workings of living things may also induce pH changes within the Gulf of Maine. When conditions are right, throngs of minuscule marine plant-life known as phytoplankton engage in photosynthesis, one of the most elemental of biological processes.

Photosynthesis ultimately proves to be an acid-consuming process, allowing the phytoplankton to grab carbon out of surface waters and incorporate it into their biomass. However, grazers and bacterial microbes are respiring en masse, a process directly at odds with the acid-consuming nature of photosynthesis. Respiration essentially produces acid, serving to increase the number of positive hydrogen ions in the surrounding water. Because both processes will wax and wane in response to the population dynamics of the organisms responsible for the photosynthesizing or respiring, the balance of the two relative to one another may constantly shift.

How these multiple stressors work in concert to affect pH within the Gulf can be hard to characterize. Throw in the temporal variability and things become even more tangled. There has been some basic study on the nature of acidity effects on the living things within the Gulf of Maine, but there are also suggested impacts collected from studies in multiple geographic locations that may be transferable to our local ecology.

Many studies have described the concept of de-calcification, a more formal term for the corrosion of shell material mentioned prior, that in addition to shellfish, can afflict other organisms with hard exteriors including diatoms, pterapods, and corals.

The Center for Ocean Solutions cites acidosis, a build-up of carbonic acid in living tissue, as another ailment striking organisms living in increasingly acidic waters, with research indicating larval stages of certain species of fish are especially susceptible.

Plenty still unknown: Noise?

Despite the few impacts we may be in the process of elucidating, the vastness of what we don’t understand likely far overshadows what we do. However, in the broadest of ecological senses, living things tend to be adapted to certain conditions – specific ranges of salinity, temperature and pH. While changing any of these elements may not always produce appreciable effects in all organisms, it may at the very least cause them to spend increasing amounts of energy maintaining the status quo.

In addition to the possible biological ramifications, a stranger, unexpected result of ocean acidification has recently been suggested by researchers at the Monterey Bay Aquarium Research Institute. Lowering pH may actually change the nature of sound absorption underwater, both inhibiting the communication and altering the behavior of marine organisms such as whales and dolphins, and allowing anthropogenic, or human-produced, noise to travel further.

This may be an especially pertinent issue in the Gulf of Maine as it sees a large volume of clamorous human activity while serving as habitat for noise-sensitive animals, including the very endangered North Atlantic Right Whale (see the Gulf of Maine Times Winter 2009 Article – Sounding The Alarm: The Emerging Threat of Noise Pollution).

Perhaps the most dire of all the potential consequences of ocean acidification is how it may influence the ocean’s ability to store carbon in the future. More acidic waters are less likely to accept incoming carbon and the seas may be starting the slow walk towards becoming a net source of carbon, out-gassing it as they have at other points in the long-standing history of the world.

The addition of more carbon to the atmosphere will serve only to increase climate change trends already in motion locally, as well as globally. At certain times of year the Gulf of Maine is already a carbon source, but the reasons for this are mysterious and poorly understood.

The threat ocean acidification presents is real and present and at some point may provoke damage that will outlast us and many of our future generations. If that happens, the ways in which we relate to our ocean and the many ways in which we thrive from it presence will be altered inextricably and necessarily. As controversial as the concept of climate change is, the constant conversation means people are engaged in the discussion and ultimately the outcome. If there is to be any action to combat the rapid acidifying of the sea’s expanses, ocean acidification needs to become another thread of the current dialogue.

I would like to acknowledge Joe Salisbury, research assistant professor with the Ocean Process Analysis Laboratory, part of the University of New Hampshire’s Institute of Earth and Ocean Science, for his invaluable help educating me on the issues influencing ocean acidification within the Gulf of Maine so I could communicate the science to our readers.
–Carrie Schuman

Online Resources:

Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide –policy report released by the UK’s Royal Society in 2005

Coastal Acidification by Rivers: A Threat to Shellfish?

An Ominous Warning on the Effects of Ocean Acidification – Author and NY Times science writer Carl Zimmer

NOAA’s State of the Science Fact Sheet – Ocean Acidification

Chemtoons: Animations about how acids, bases, and buffers work

Center for Ocean Solutions: Decalcification

Center for Ocean Solutions: Ocean Acidification

NRDC – Ocean Acidification: The Other CO2 Problem

Ocean-Acidification.Net – The Ocean in a High CO2 World


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