Ocean acidification refers to the gradual decrease in the pH of the Earth’s oceans, primarily due to the absorption of excess atmospheric carbon dioxide (CO2). Since the Industrial Revolution, the oceans have absorbed about 30% of the CO2 emitted by human activities, leading to significant changes in seawater chemistry. This acidification process poses a profound threat to marine ecosystems, especially to the organisms at the base of the food chain and can have cascading effects through higher trophic levels.
Causes of Ocean Acidification
- Increased CO2 Emissions: The main driver of ocean acidification is the rising concentration of CO2 in the atmosphere, which dissolves in seawater to form carbonic acid. This weak acid dissociates into hydrogen ions, lowering the pH and reducing the availability of carbonate ions.
- Decrease in Carbonate Ions: Carbonate ions (CO3^2-) are critical for calcifying organisms like corals, mollusks, and some plankton to form their calcium carbonate shells and skeletons. As carbonate ions decrease due to acidification, these organisms struggle to maintain their structures.
Impacts on Marine Food Chains
- Plankton and Primary Producers: Phytoplankton are the foundation of most marine food chains, providing energy for a wide range of organisms, from small fish to large marine mammals. Certain phytoplankton species, such as coccolithophores, rely on calcium carbonate to build their shells. Acidification reduces their ability to do so, affecting their survival and reproduction. As plankton populations decline or shift in species composition due to acidification, there could be significant disruptions in the marine food web, as many marine organisms depend on plankton as their primary food source.
- Coral Reefs and Associated Ecosystems: Coral reefs are among the most biodiverse ecosystems on Earth, but they are highly sensitive to changes in water chemistry. Ocean acidification weakens coral skeletons, leading to slower growth rates and increased vulnerability to erosion. Coral reefs provide habitat and protection for numerous marine species. As reefs degrade, many species that rely on them for food, shelter, and breeding grounds may decline, impacting the broader food chain.
- Shellfish and Other Calcifiers: Mollusks, such as clams, oysters, and snails, rely on calcium carbonate for their shells. Ocean acidification reduces the availability of carbonate ions, making it more difficult for these organisms to form shells, leading to thinner, weaker shells or even shell dissolution. Shellfish are a critical food source for both humans and marine predators like starfish, crabs, and certain fish. A decline in shellfish populations could ripple through food chains, affecting both ecological balance and human fisheries.
- Fish and Marine Mammals: While most fish are not directly affected by the changes in carbonate chemistry, their food sources, such as plankton, shellfish, and corals, are. This indirect impact can lead to reduced food availability, causing population declines in fish species. Larger predators, including marine mammals (seals, whales, dolphins), depend on healthy fish populations. A disruption at lower levels of the food chain can lead to declines in these higher trophic species.
- Human Impact: Many human communities, especially coastal populations, depend on marine ecosystems for food, economic activities (fisheries, aquaculture), and tourism. As acidification impacts marine food chains, it could lead to reduced fishery yields, threatening food security and livelihoods. Long-Term Outlook
- Ecosystem Shifts: The ongoing acidification of the oceans is expected to cause shifts in species composition, with some species adapting or moving to new environments while others decline or disappear. This could lead to a restructuring of marine ecosystems, where certain species become dominant, and others, especially calcifiers, become rare.
- Cascading Effects: The collapse of coral reefs, shellfish populations, or key plankton species could lead to a cascade of effects, disrupting the balance of marine ecosystems and affecting everything from fish populations to marine mammals and seabirds.
- Uncertain Resilience: While some species may be more resilient to acidification, the long-term effects on biodiversity and ecosystem function are uncertain. Adaptive capacity varies among species, and the speed at which acidification is occurring may outpace the ability of many organisms to adjust.
- Ocean acidification represents: a significant, long-term threat to marine food chains. It alters the fundamental chemistry of the oceans, affecting the organisms at the base of the food chain and triggering ripple effects throughout marine ecosystems. As acidification continues, the long-term sustainability of marine biodiversity, fisheries, and food security will be challenged, making it critical.
Climate Change Nature, YouTube, 21 September 2024. Video and text.
