pH serves as a measure in chemistry, indicating the acidity or alkalinity of a solution. This scale, ranging from 0 to 14, signifies that values below 7 are acidic, 7 is neutral, and above 7 are alkaline or basic. The pH scale operates logarithmically, meaning each whole unit decrease represents a tenfold increase in acidity. Historically, the ocean has maintained a slightly alkaline state, with an average pH of around 8.2. This consistent balance has provided a stable condition for marine life, offering the necessary chemical environment for countless species to thrive.
Understanding Ocean Acidification
Ocean acidification describes the ongoing decrease in the pH of the Earth’s oceans, driven by the absorption of atmospheric carbon dioxide (CO2). This process begins when CO2 from the atmosphere dissolves into seawater, initiating a series of chemical reactions. Human activities, such as burning fossil fuels and deforestation, have significantly elevated atmospheric CO2 concentrations since the Industrial Revolution.
When CO2 enters the ocean, it reacts with water molecules (H2O) to form carbonic acid (H2CO3). Carbonic acid is a weak acid that then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increased concentration of these hydrogen ions leads to a decrease in the ocean’s pH, making it more acidic. While the ocean remains alkaline with an average surface pH of about 8.1 today, down from approximately 8.2 in pre-industrial times, this shift represents a significant increase in acidity; a 0.1 pH unit drop translates to roughly a 25-30% increase in acidity due to the logarithmic nature of the pH scale.
Bicarbonate ions further dissociate into hydrogen ions and carbonate ions (CO32-). This increased hydrogen ion concentration also reduces the availability of carbonate ions in the seawater. Carbonate ions are a building block for many marine organisms, and their reduced availability is a concern for marine ecosystems. The ocean has absorbed approximately one-third to one-half of the CO2 released by human activities since the mid-1800s, slowing atmospheric warming but changing ocean chemistry.
Consequences for Marine Organisms
The changing ocean chemistry impacts various marine organisms, particularly those that build shells and skeletons. Calcifying organisms, such as corals, shellfish like oysters and mussels, and certain plankton, rely on carbonate ions to form their calcium carbonate structures through calcification. As ocean pH decreases and carbonate ion availability diminishes, these organisms find it difficult to build and maintain their shells and skeletons.
Coral reefs, diverse ecosystems providing habitat for thousands of fish species, are susceptible to ocean acidification. Reduced calcification rates can slow coral growth and weaken reef structures, making them more vulnerable to storm damage. Mollusks, including oysters, mussels, and pteropods (tiny sea snails), experience thinning, deformed, or dissolving shells, which increases their vulnerability to predators. Single-celled organisms like foraminifera produce thinner structures in more acidic waters.
Beyond calcifying species, ocean acidification affects non-calcifying organisms, including fish. While some physiological processes like reproduction and growth in adult fish may show limited direct effects, more complex behaviors and early life stages can be compromised. For example, larvae of some fish species, such as clownfish and damselfish, exhibit impaired sensory function, including a reduced sense of smell. This leads to riskier behaviors like swimming further from shelter and not responding to threats, which can decrease their survival rates due to increased predation. Ocean acidification can also reduce hatching success in certain fish species, like sand lance, by affecting enzymes necessary for embryos to break through their eggshells.
Ripple Effects Across Ecosystems
The consequences for individual marine organisms cascade throughout marine ecosystems, leading to ecological disruptions. When species at the base of the food web, such as calcifying plankton, are impacted, the ripple effects can extend to larger animals that rely on them for food. For example, pteropods, often called “sea butterflies” and a food source for whales and other top predators in Arctic marine food webs, face dissolution of their shells in projected future acidity levels. Population declines or shifts in the distribution of such foundational species can have implications for the entire food web structure.
Coral reefs, as complex habitats, exemplify how the degradation of one type of organism can affect an ecosystem. As acidification weakens coral structures and slows their growth, the habitats they provide for an estimated 25% of all marine life, including over 4,000 fish species, diminish. This loss of habitat can lead to reduced biodiversity and altered community structures, impacting fisheries and coastal protection. Studies at natural CO2 seeps show that acidification can lead to reduced habitat complexity and species richness, with a shift towards non-calcified species like turf algae dominating over coralline algae and soft corals over hard corals.
These ecological changes affect the ecosystem services that humans derive from the ocean. Coastal protection, provided by healthy coral reefs and shellfish beds, can be compromised, increasing vulnerability to storms. Fisheries and aquaculture industries face economic losses due to declines in shellfish populations, such as oysters, mussels, and clams. The simplification of marine food webs and loss of biodiversity can diminish the ocean’s capacity to provide food, regulate climate, and support the livelihoods of millions who depend on marine resources.