What is Ocean Acidification (OA)?
The ongoing carbon dioxide (CO2) concentration results in direct consequence in the atmosphere. The ocean absorbs over 25% of all anthropogenic (environmental pollution and pollutants) originating in human activity) emissions from the atmosphere each year, and when the CO2 dissolves in seawater, it forms carbonic acid which decreases the pH level of the Earth’s oceans → This process is known as ocean acidification. Average global ocean pH fell to 8.1 from 8.2 of pre-industrial value, after a rise in acidity of about 30% over the last few hundred years. It is expected to be around 7.8-7.9 by 2100, a value of doubled acidity. This is faster than any known change in ocean chemistry in the past 50 million years, and full recovery of OA could take tens to hundreds of millennia.
*The pH scale is a scale that measures the inverse concentration of hydrogen ion from 0 to 14, indicating the acidity or basicity of a solution. Anything below 7 is acidic, 7 is neutral, and anything higher than 7 is basic. It is important to know that acidity increases as the pH decreases.
Emission of CO2 in the atmosphere causes and enhances OA, after the burning of fossil fuels (coal, oil, and gas) or the deforestation that results in fewer gas absorbed by trees. OA can also be caused by other chemical additions or subtractions from the ocean. Acidity can be increased in areas where human activities and its impacts (acid rain and nutrient runoff) are more severe.
What is the process?
CO2 exists in the air by nature, but human activities release more CO2 than what is needed. Leftover CO2 is then collected in the atmosphere and because it absorbs heat from the sun, it creates an extra layer around the planet. Then, approximately 30% of the CO2 is dissolved into seawater and this breaks down into molecules to recombine with various chemicals.
The absorption of CO2 is largely caused by the dissolution of gas into the upper layers of the ocean from the atmosphere.
CO2 is also brought into the oceans through photosynthesis and respiration. Algae and other marine photosynthesizers take in CO2 and store it in their tissues as carbon. Carbon is then passed to zooplankton and other organisms through the food chain, and these organisms can release CO2 to the oceans through respiration. In addition, when marine organisms die and fall to the ocean floor, CO2 is released through the process of decomposition.
OA increases the concentrations of carbon dioxide (CO2), hydrogen ions (H+), and bicarbonate ions (HCO3−) and decreases the concentrations of carbonate ions (CO32−).
What are the dangers?
OA also modifies seawater carbonate chemistry; since carbonate ions that provide chemical building for marine organisms’ shells and skeletons are less available, acid seawater brings significant problems for both small and large marine organisms.
Less accessible carbonate ions impair the ability to build a shell and even dissolves existing shells if the pH gets too low. It then requires the marine organisms to spend more time building energy and maintaining their shells or skeletons, which leaves less energy available for other biological processes like growing, reproducing, or responding to other stresses (enzyme activities and photosynthesis, which then affects primary production).
A lot of species that form shells are incredibly sensitive to changes in pH levels and carbonate ion concentrations. Impacts of OA might be very stressful to those organisms, especially for corals, bivalves (such as oysters, clams, and mussels), pteropods (free-swimming snails), and certain phytoplankton species.
The biological impacts of OA vary for each species since different groups have a wide range of sensitivities to changing seawater chemistry.
At any life stage, a species’ ability to grow or recover from losses can be reduced after OA.
Although OA does not kill all ocean life forms, it will bring changes in the number and abundance of marine organisms. Many ecosystems may be populated by different, and potentially fewer, species in the future. It is unclear whether these biological impacts will be reversible.
Scientists expect OA to eventually impact all primary producers, from microscopic phytoplankton to giant kelp forests, as well as higher trophic levels such as coral reefs, shellfish, and fish.
Even though strategies to deal with fluctuating pH may have evolved, the long-term decrease in pH can eventually surpass the tolerance limits of marine species living in coastal waters.
Increased acidity and temperature in oceans will have direct impacts on the physiology of marine organisms and influence the geographical distribution of species. The most vulnerable areas from OA are regions where there is natural upwelling of deep water onto the continental shelves (West coast of North America), regions near the poles where lower temperatures allow seawater to absorb more CO2, and regions that receive freshwater discharge.
OA rinks to other climate-related problems, such as absorption of atmospheric CO2 in the ocean, global carbon cycle, ocean warming, and deoxygenation. This entire set puts pressures on the marine environment by causing heat, acidity, and oxygen loss. And the interaction between different problems result in bigger and more severe consequences.
Not only the marine organisms, but all marine-dependant communities will be impacted by OA. Changes in species growth, reproduction, and structural and functional alterations in ecosystems can threaten the flow of goods and services. Food and water security, fishing industries, and natural shoreline protection would get harmed. OA would also increase the danger of inundation and erosion in low areas by restricting climate change adaptation and disaster risk reduction efforts.
What species are impacted?
Reef-building corals craft their own homes from calcium carbonate, forming complex reefs that house the coral animals themselves and provide habitat for many other organisms. Acidification may limit coral growth by corroding pre-existing coral skeletons while simultaneously slowing the growth of new ones, and the weaker reefs that result will be more vulnerable to erosion. This erosion will come not only from storm waves, but also from animals that drill into or eat coral. Additionally, acidification may also impact corals before they even begin constructing their homes.
Oysters, Mussels, Urchins, And Starfish
Shelled animals like oysters, mussels, urchins, and starfish are the most vulnerable species to OA. As introduced above, carbonate ions prevent their shells to form normally, increasing the chance of being crushed or eaten. This would then impact the whole food chain as those species are food and habitat for other animals.
Relatively small species, zooplanktons, are also responsive to OA. The 2 types of zooplanktons that build shells made of calcium carbonate are foraminifera and pteropods, and play a major role in the food webs, as almost all larger life eats them. However, because the planktons do not handle high acidity very well, their shells dissolve very quickly.
While large animals like fishes and squid do not have shells, they may also feel the effects of increasing acidity as carbonic acid concentrations rise in their blood. Called acidosis, this condition leads to problems of respiration, growth, and reproduction. Furthermore, more acidic water may also affect fishes’ minds so that they cannot flee threatening noise or have trouble coming back home because of their inability to navigate. A slight shift in dominant fish species can also have significant impacts on the food web and even on human fisheries.
What can be done?
Blue carbon: CO2 from atmosphere or seawater that are captured and held by organic materials such as sea grasses, mangroves, and salt marshes are being investigated as a means to locally offset CO2 levels. However, blue carbon would only work for a small amount of time since the root cause of OA is the global atmospheric CO2 emissions.
Scientists all around the globe are examining different ways to delay and limit CO2 emissions. The long-term goals to solve OA are similar to that of climate change, further actions and research are crucial.