Ocean acidification impacts the world’s marine ecosystem


Ocean acidification is an issue we do not hear about as much as climate change, but it is nevertheless as important. We asked our Zintro experts to explain the impact of ocean acidification on the health of marine ecosystems

Angelina Souren, a marine biogeochemist, says that the ocean acidification you hear about in the news is the result of increasing carbon dioxide levels in the atmosphere. Carbon dioxide, or CO2, is a greenhouse gas of which emissions have increased greatly since the industrial revolution. It is also emitted by the planet itself and can, for instance, occur in rocks deep in the earth. But such emissions are negligible relative to the increase in anthropogenic emissions. “The increase in atmospheric CO2 results in more CO2 going into the upper layers of the oceans, and that is causing the acidity to go up. Increased uptake of CO2 by phytoplankton plays a role in ocean acidification as well,” explains Souren.

Souren tells us that ocean acidity is often expressed as pH. An increase in pH translates into less acidic and a decrease in pH means more acidic. “The ocean’s surface pH was stable for millions of years, but has already decreased slightly since the industrial revolution when we started emitting more and more CO2,” she says. “If the atmospheric CO2 levels keep increasing the way they are, the ocean’s pH will drop significantly, exceeding the range of natural variations. This is irreversible, from a human point of view. A decreasing pH – increasing acidity – has a wide range of consequences.”

Souren says that for some organisms, it becomes more difficult to build and maintain a shell or a skeleton, but other organisms may do better. Increased acidity also impacts how organisms take their nutrients from the water. “Change is part of nature and many organisms may be able to adapt, but this man-induced change is much faster than what marine organisms have had to cope with in the past. Such rapid changes, whatever their cause, can lead to mass extinctions. The search for key indicators of ocean acidification stress is underway,” she says.

The surface waters of polar and subpolar regions, such as the Southern Ocean that surrounds Antarctica, are the locations where the shell minerals aragonite and calcite will first become chemically unstable, Souren says. “Chemically, this means that they will start to dissolve. We don’t know yet what exactly that is going to mean in real life, but it will change those ecosystems. Other ecosystems that are likely to be impacted quite early on by ocean acidification are coral reefs,” she says. “Between 1988 and 2003, a decline of 21 percent was seen for the calcification rate of Porites corals at two locations in the Great Barrier Reef. This may already be a result of global warming, ocean acidification, or both. However, it is essential to understand that the planet we live on – its biology, chemistry, physics and geology – is not a patchwork of isolated occurrences. It is an intricate network with so many connections and feedback loops that it is not even possible to identify them all. Therefore, if ocean acidification continues at the present pace, it will most certainly influence local ecosystems and will affect the entire planet, the umbrella ecosystem. Among other things, it will also impact how much more CO2 the oceans can absorb from the atmosphere.”

Leonard Sonnenschein, president of the World Aquarium and Conservation for the Oceans Foundation says that climate change, rising atmospheric carbon dioxide, excess nutrient inputs, pollution made toxic by temperature changes, chemical solubility differences, and concomitant microfaunal alterations, are fundamentally altering the chemistry of the ocean, often on a global scale and, in some cases, at rates greatly exceeding those in the historical and recent geological record. “Major observed trends include a shift in the acid-base chemistry of seawater, reduced subsurface oxygen both in near-shore coastal water and in the open ocean, rising coastal nitrogen levels, and widespread increase in mercury and persistent organic pollutants,” explains Sonnenschein. “Most of these perturbations, tied either directly or indirectly to human fossil fuel combustion, fertilizer use, and industrial activity, are projected to grow in coming decades, resulting in increased negative impacts on ocean biota and marine resources.”

Sonnenschein says these changes have affected the melting of the ice caps, which affects the salinity of the water. “The normal pH for freshwater is 7.0. The normal pH for saltwater is 8.0-8.4. As the salinity decreases (more freshwater in the seawater) so does the pH. The acidification affects coral reefs and any carbonate process significantly,” he says. “The carbonate ions are necessary for marine calcifying organisms, such as corals, molluscs, echinoderms, and crustaceans, to produce their calcium carbonate shells and skeletons. Therefore, a significant amount of coral reefs and oceans are dying due to the changes in salinity.”

These changes in salinity also affect the plankton, phytoplankton and bacteria, notes Sonnenschein. “Phytoplankton species are very susceptible to loss of salinity and toxicity. Even more so are the very sensitive bacteria that allow for the nitrogen cycle to complete the change from nitrite to nitrate to elemental nitrogen and oxygen,” he says. “Part of the increase of the hypoxic zones is related to not only the increase in fertilizer runoff, but also the increased inability for the bacteria to survive that are necessary for the nitrogen to be removed, which causes hypoxic zones. Anthropogenic actions can be taken to positively affect these situations, such as sustainable management of fisheries, individual and corporate energy efficiency, judicious land use, public education programs, and thorough research and data collection. Engagement at every level of society is needed.”

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