Orcas, recognized as apex predators and highly intelligent marine mammals, navigate a complex and rapidly changing ocean environment. The health and future of these animals are linked to the planet’s oceans, which absorb a substantial portion of the carbon dioxide (CO2) released into the atmosphere from human activities. This absorption initiates a cascade of environmental changes that ultimately threaten orca populations, particularly through indirect impacts on their food supply and habitat stability. Understanding the connection between rising atmospheric CO2 and the challenges facing orcas is important for marine conservation efforts globally. The resulting chemical and physical shifts in the water column affect every level of the marine food web, including the orca.
The Chemical Pathway: CO2 and Ocean Acidification
The primary mechanism by which atmospheric carbon dioxide alters the marine environment is ocean acidification. When the ocean absorbs CO2 from the air, the gas dissolves into the seawater and reacts with water molecules. This reaction forms carbonic acid, which is a weak acid that quickly dissociates.
The dissociation of carbonic acid releases hydrogen ions into the water, which increases acidity. This increase in hydrogen ions causes the ocean’s pH level to drop, making the water more corrosive. Since the start of the Industrial Era, the ocean’s average surface pH has dropped by approximately 0.1 units, which represents about a 30% increase in acidity.
A drop in pH also significantly reduces the concentration of available carbonate ions in the water. Carbonate ions are chemically bound by the excess hydrogen ions, forming bicarbonate instead. Many marine organisms rely on carbonate ions to build and maintain their shells and skeletons, which are made of calcium carbonate. This fundamental shift in ocean chemistry directly impacts the base of the food chain.
Cascading Effects on the Orca Food Supply
The most significant threat posed by rising carbon dioxide levels to orcas is the disruption of their food supply, which is profoundly affected by the underlying chemical and temperature changes in the ocean. The foundation of the marine food web is at risk due to ocean acidification. Shell-forming organisms, such as pteropods, struggle to produce and maintain their calcium carbonate shells in water with fewer carbonate ions.
Pteropods, tiny planktonic sea snails, are sometimes called the “potato chips of the sea” due to their importance as a food source for numerous small fish. These organisms are highly susceptible to corrosive conditions, and their shells can even begin to dissolve in more acidic waters. If pteropod populations decline, the species that feed on them, such as juvenile pink salmon, face nutritional stress.
This effect moves up the food chain to the orca’s specialized diet. For example, the endangered Southern Resident orca population primarily relies on Chinook salmon, a species that ultimately depends on a healthy lower trophic level for survival. A decline in the health and abundance of smaller prey fish translates directly into a reduced food supply for the salmon.
The problem is compounded by rising sea temperatures, another consequence of increased atmospheric CO2. Warming waters disrupt the migration and distribution patterns of prey species, forcing them to shift their ranges to cooler areas. This change in distribution makes the prey less predictable and harder for orcas to find and catch.
When orcas face prolonged periods of food shortage, they begin to metabolize their stored blubber for energy. This process releases stored environmental toxins, which they accumulate as apex predators, back into their bloodstream at high concentrations. The resulting nutritional stress and toxin exposure can compromise their reproductive success and immune system function.
Habitat Disruption and Physiological Consequences
Beyond the chemical changes, the rise in atmospheric carbon dioxide drives physical habitat disruption that directly affects the orca’s ability to thrive. Rising global temperatures lead to the melting of sea ice, particularly in polar regions, which opens up new territories to orcas. This forces certain populations, like those in the Arctic, to shift their range further north.
This migration introduces them into ecosystems where they have not historically been present, increasing predation pressure on resident species like belugas and narwhals. The loss of sea ice also removes a platform used by some orca ecotypes, like Antarctic Type A orcas, for specialized hunting techniques to catch seals.
The physical properties of the ocean are also changing in ways that interfere with the orca’s primary sense: sound. Orcas rely on echolocation and acoustic communication to navigate, hunt, and socialize. Ocean acidification reduces the absorption of low-frequency sound waves in seawater due to changes in borate ion chemistry.
This reduced sound absorption means that low-frequency sounds, such as those produced by large commercial ships, can travel farther, potentially twice as far as before. The resulting increase in background noise can mask the orca’s echolocation clicks and calls, reducing their foraging efficiency and ability to communicate over long distances.
Chronic environmental instability, including unpredictable prey availability and temperature fluctuations, leads to elevated physiological stress in orcas. Research on wild populations has shown a correlation between low Chinook salmon availability and increased levels of stress hormones, known as glucocorticoids, in their scat. Sustained high levels of these hormones can ultimately impair their reproductive function and overall health.