The pH of seawater represents a fundamental characteristic of the global ocean. This measurement indicates the acidity or alkalinity of ocean waters, playing a significant role in the health and function of marine ecosystems. Maintaining a stable pH is a delicate balance, as even slight shifts can profoundly influence the diverse life forms inhabiting the ocean.
Understanding Seawater pH
The term pH quantifies the concentration of hydrogen ions in a solution, indicating its acidity or alkalinity on a scale ranging from 0 to 14. A pH value of 7 is considered neutral; values below 7 indicate increasing acidity, and values above 7 indicate increasing alkalinity. Seawater is naturally slightly alkaline, with a pH ranging between 7.9 and 8.3, though the average global surface ocean pH currently stands around 8.1.
The ocean possesses a natural buffering capacity, primarily through the carbonate system, which helps stabilize its pH. This system involves chemical reactions where dissolved carbon dioxide reacts with water to form carbonic acid, which then dissociates into bicarbonate and carbonate ions. These ions can absorb or release hydrogen ions, resisting large changes in pH. This chemical balance has historically maintained ocean pH within a narrow range, supporting diverse marine life.
Natural Influences on Seawater pH
Natural processes contribute to slight, localized variations in seawater pH across different oceanic regions and depths. Temperature influences the solubility of carbon dioxide in water; colder waters absorb more CO2, which can slightly lower pH. Salinity also affects seawater’s buffering capacity.
Water depth influences pH due to variations in pressure, dissolved gases, and organic matter. Deeper waters, less exposed to atmospheric exchange and accumulating decaying organic material, tend to be slightly more acidic than surface waters. Biological processes also influence pH; during photosynthesis, marine algae and plants consume CO2, which can temporarily increase pH in surface waters. Conversely, respiration by marine organisms releases CO2, lowering pH in localized areas.
Human Impact and Ocean Acidification
Human activities have introduced a rapid change to the ocean’s pH balance, primarily through the absorption of excess atmospheric carbon dioxide (CO2). Since the Industrial Revolution, the ocean has absorbed approximately 30% of the CO2 released into the atmosphere from burning fossil fuels and land-use changes. This absorption is a direct consequence of the increasing atmospheric CO2 concentration, which has risen from about 280 parts per million (ppm) pre-industrial levels to over 420 ppm today.
When CO2 dissolves in seawater, it initiates chemical reactions leading to ocean acidification. Carbon dioxide reacts with water to form carbonic acid (H2CO3), a weak acid. This carbonic acid then dissociates, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in hydrogen ions directly lowers the seawater’s pH. This process also reduces the concentration of carbonate ions (CO3^2-), which are important for many marine organisms.
The rate at which the ocean’s pH is decreasing is unprecedented in geological history, occurring approximately 100 times faster than any change experienced in the last 50 million years. This rapid acidification far exceeds the ocean’s natural buffering capacity, posing a significant challenge to marine life. The average pH of the surface ocean has already decreased by about 0.1 units since pre-industrial times, representing roughly a 30% increase in acidity.
Consequences for Marine Ecosystems
The decreasing pH of seawater poses challenges for marine ecosystems, particularly for organisms that rely on calcium carbonate to build their shells and skeletons. Calcifying organisms, such as corals, shellfish (e.g., oysters and clams), and plankton (e.g., pteropods and foraminifera), extract carbonate ions from seawater to form their calcium carbonate structures. Ocean acidification reduces the availability of these carbonate ions, making it more difficult for these organisms to grow and maintain their structures.
For example, pteropods, small marine snails at the base of many marine food webs, experience shell dissolution when exposed to acidified conditions. Coral reefs, formed by polyps that secrete calcium carbonate skeletons, face reduced calcification rates, leading to slower growth and increased vulnerability to erosion and damage. This impacts entire reef ecosystems, which provide habitat and food for countless other species.
Beyond calcifiers, ocean acidification can also affect the behavior and physiology of other marine life. Some studies suggest that increased acidity can impair the sensory abilities of certain fish species, affecting their ability to navigate, find food, and detect predators. Reproductive success and growth rates in various marine organisms may also be compromised. These widespread impacts can disrupt marine food webs, potentially leading to shifts in species distribution, reduced biodiversity, and ecological consequences throughout the global ocean.
References
National Oceanic and Atmospheric Administration. (n.d.). What is Ocean Acidification? [Online]. Available: https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
The Ocean Foundation. (n.d.). Ocean Acidification. [Online]. Available: https://oceanfdn.org/ocean-acidification/
NASA. (n.d.). Carbon Dioxide. [Online]. Available: https://climate.nasa.gov/vital-signs/carbon-dioxide/