7.4 Ocean chemistry and acidification

Ocean Chemistry

Seawater Composition and Properties

Seawater is slightly alkaline, with a pH that typically ranges from 7.5 to 8.4. That alkalinity comes from dissolved ions and chemical buffer systems that resist sudden changes in pH.

The most important of these buffers is the carbonate system, which consists of four interrelated chemical species:

• Dissolved carbon dioxide ($CO_2$)
• Carbonic acid ($H_2CO_3$)
• Bicarbonate ions ($HCO_3^-$)
• Carbonate ions ($CO_3^{2-}$)

These species shift back and forth depending on conditions, and together they regulate ocean pH and drive carbon sequestration.

Dissolved oxygen is the other key dissolved gas. It's essential for nearly all marine life, and its concentration varies with temperature (colder water holds more), salinity, and the balance between photosynthesis (which produces $O_2$) and respiration (which consumes it).

Salinity averages about 35 parts per thousand (ppt) globally. Beyond affecting water density and circulation patterns, salinity also influences gas solubility: higher salinity generally means lower solubility for gases like $O_2$ and $CO_2$.

Chemical Processes and Interactions

When atmospheric $CO_2$ dissolves in seawater, it doesn't just float around as a gas. It reacts with water to form carbonic acid, which then dissociates. The process looks like this:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$

Those extra hydrogen ions ($H^+$) are what lower the pH. The carbonate buffer system absorbs some of these excess $H^+$ ions, which is why ocean pH doesn't swing wildly with every change in atmospheric $CO_2$. But the buffer has limits, and that's where acidification becomes a problem.

Dissolved oxygen levels depend on three main processes:

• Gas exchange at the ocean surface ($O_2$ moves between atmosphere and water)
• Photosynthesis by phytoplankton and marine plants, which adds $O_2$
• Respiration by marine organisms, which removes $O_2$

Ocean Acidification

Causes and Mechanisms

Ocean acidification is the ongoing decrease in ocean pH caused primarily by the absorption of human-produced $CO_2$. The main sources of this anthropogenic $CO_2$ are fossil fuel combustion and deforestation.

The ocean currently absorbs roughly 30–40% of the $CO_2$ humans emit. This makes it a massive carbon sink, which actually slows the pace of atmospheric warming. The tradeoff, though, is that all that absorbed $CO_2$ drives the chemical reaction described above, steadily increasing $H^+$ concentration and pushing pH downward.

Since the start of the Industrial Revolution, ocean surface pH has dropped by about 0.1 units. That sounds small, but because the pH scale is logarithmic, a 0.1 drop represents roughly a 26% increase in hydrogen ion concentration.

Impacts on Marine Life

Acidification hits hardest for organisms that build shells or skeletons out of calcium carbonate ($CaCO_3$). This includes corals, mollusks (like oysters and clams), sea urchins, and certain plankton species such as pteropods and coccolithophores.

Here's why: as pH drops, more carbonate ions ($CO_3^{2-}$) get used up neutralizing excess $H^+$. That leaves fewer carbonate ions available for organisms to pull out of the water and build their structures.

Aragonite is a specific crystal form of $CaCO_3$ that many corals and shellfish depend on. It becomes less stable in more acidic water. Scientists track this using aragonite saturation state, which measures whether seawater has enough dissolved aragonite for organisms to build with. As acidification progresses, aragonite saturation drops, and below a certain threshold, shells and coral skeletons can actually begin to dissolve. This directly threatens coral reef ecosystems and the enormous biodiversity they support.

Oxygen Depletion

Hypoxia and Dead Zones

Hypoxia occurs when dissolved oxygen drops below about 2 milligrams per liter, a threshold too low for most marine animals to survive. When hypoxia persists over a large area, it creates a dead zone where fish, crabs, and other mobile organisms flee (if they can) and bottom-dwelling organisms often die.

Three main factors drive oxygen depletion:

1. Eutrophication — excess nutrients (nitrogen, phosphorus) fuel massive algal blooms. When the algae die, bacteria decompose them and consume huge amounts of $O_2$ in the process.
2. Stratification — when a warm, less-dense surface layer sits on top of cooler, denser deep water, vertical mixing slows down. Oxygen-rich surface water can't reach the depths.
3. Warming temperatures — warmer water physically holds less dissolved oxygen.

Anthropogenic Influences

Human activity drives oxygen depletion through several connected pathways. Rising atmospheric $CO_2$ warms the ocean, which both reduces oxygen solubility and strengthens stratification. Together, these effects choke off oxygen supply to deeper waters.

Eutrophication is the other major driver, and it's largely a coastal problem. Agricultural runoff carrying fertilizers, along with sewage discharge, floods coastal waters with nutrients. The Gulf of Mexico dead zone is a well-known example: nutrient-laden water from the Mississippi River basin fuels seasonal algal blooms, and the subsequent decomposition creates a hypoxic zone that in some years exceeds 20,000 square kilometers.

These stressors don't act in isolation. Acidification, warming, and oxygen loss often overlap in the same regions, compounding the stress on marine ecosystems. This combination is sometimes called the "deadly trio" of ocean threats driven by $CO_2$ emissions.