The pH scale measures the acidity or alkalinity of a substance, ranging from 0 to 14. A pH of 7 is neutral; values below 7 indicate acidity, and values above 7 indicate alkalinity. Saltwater is water with dissolved salts. Understanding its pH is significant because it directly influences the health and stability of marine ecosystems.
The Ocean’s pH Scale
Ocean water is naturally slightly alkaline. The typical pH range for surface ocean water is between 7.8 and 8.2. While this range might seem narrow, the pH scale is logarithmic, meaning each whole number change represents a tenfold difference in acidity or alkalinity. For instance, water with a pH of 6 is ten times more acidic than water with a pH of 7.
The average pH of the global ocean is currently around 8.1. Historically, the pre-industrial surface ocean pH was approximately 8.2. This small decrease of 0.1 pH units represents a significant increase in acidity, about a 26% rise in hydrogen ion concentration. Natural pH variations occur across different ocean regions, such as between surface and deep waters, or coastal and open ocean environments.
Key Influences on Saltwater pH
The ocean possesses a natural buffering capacity, primarily due to bicarbonate and carbonate systems. This system allows the ocean to absorb and release hydrogen ions, resisting large pH fluctuations. Dissolved carbon dioxide from the atmosphere plays a significant role in influencing ocean pH. When carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions, affecting the water’s pH.
Biological processes also influence ocean pH. Photosynthesis, carried out by marine plants and algae, consumes carbon dioxide from the water, increasing pH. Conversely, respiration by marine organisms releases carbon dioxide, decreasing pH. Ocean temperature also affects gas solubility; colder waters can absorb more carbon dioxide, lowering pH.
Why Saltwater pH Matters
The stability of saltwater pH is important for marine life and ecosystems. pH affects various physiological processes in marine organisms, including enzyme function and metabolic rates. Marine organisms have evolved to thrive within specific, often narrow, pH ranges. Even slight deviations from their optimal pH can disrupt their biological functions, impacting growth, reproduction, and overall survival.
Calcifying organisms, such as corals, shellfish, and plankton, are susceptible to pH changes. These organisms rely on calcium carbonate to build their shells and skeletons. As pH decreases, the availability of carbonate ions, a key building block for calcium carbonate structures, is reduced. This makes it more difficult for them to form and strengthen protective structures, potentially leading to weaker shells or impaired growth.
Ocean Acidification: A Modern Challenge
Ocean acidification is an ongoing decrease in the ocean’s pH, primarily driven by human activities. The main cause is the increased absorption of excess atmospheric carbon dioxide by the oceans, largely from burning fossil fuels and land-use changes. The ocean has absorbed approximately 30% of anthropogenic carbon dioxide emissions.
When this excess carbon dioxide dissolves in seawater, it produces carbonic acid and increases hydrogen ion concentration, lowering the ocean’s pH. This process also reduces carbonate ion concentration, making it harder for calcifying marine organisms to build their shells and skeletons. Ocean acidification has broad impacts on ecosystems like coral reefs, slowing coral growth and making their skeletons weaker. It also affects economically important species, particularly shellfish, as their ability to form protective shells is inhibited, which can disrupt marine food chains and impact fisheries.