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Scientists use a variety of tools, from gliders to ships, to measure seawater carbonate chemistry across different spatial and temporal scales.

Tools of the Trade

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There are many different methods that scientists use to measure pH, pCO₂, DIC, and TA. Scientists choose their methodology carefully to help them answer specific questions about ocean acidification.

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Seawater carbonate chemistry measurements can be collected using the following methodologies:
<ul>
	<li>Continuous sensors: These sensors are used to automatically measure seawater carbonate chemistry in situ. Commonly used sensors can measure pH or pCO₂, alongside other water quality characteristics like temperature and salinity. Sensors can be applied to different platforms: 
<ul>
	<li>Moored Continuous Sensors: affixed to buoys or docks, moored sensors measure seawater carbonate chemistry in one place over time</li>
	<li>Underway Cruise: sensors can also be attached to ships, collecting data at the ocean surface as the ship moves along its route</li>
	<li>Continuous Ocean Gliders: ocean gliders are autonomous, underwater vehicles that collect data across large areas and at varying depths as they “swim” through the ocean.</li>
</ul>
</li>
	<li>Discrete sampling: Discrete sampling involves scientists collecting a single data point at a time, either by collecting a water sample or using a handheld probe. This can be done in several ways: 
<ul>
	<li>Discrete Bottle Sampling: water samples are collected, preserved, and then brought back to a lab for analysis. Most methodologies for measuring DIC and TA require benchtop lab equipment, so bottle samples are commonly collected for these two parameters.</li>
	<li>Rosette Sampler: device that makes bottle sampling possible in the open ocean and deep ocean.</li>
	<li>Handheld Probe: handheld sensor that is operated by a user to record discrete measurements.</li>
</ul>
</li>
</ul>

Moored continuous &amp; underway cruise

Susan McLean

Open ocean &amp; deep water sampling

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TA Sites

Total Alkalinity (TA) measures seawater's ability to neutralize acids by accepting hydrogen ions (H⁺), primarily through carbonate ions (HCO₃⁻ and CO₃²⁻) that balance positive charges from other dissolved ions.

Total Alkalinity (TA)

Alkalinity is the measurement of the ability of a solution (i.e. seawater) to accept positively charged ions (H+) and bind them to a negatively charged base ion or molecule, thus neutralizing the acid. Alkalinity is important as it is a measurement of the ocean’s ability to resist acidification.

Janet J. Reimer

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Total Alkalinity (TA) refers to the difference between the negatively (Cl-, SO42-, Br-, and F) and positively (Na+, Mg2+,Ca2+, K+ and Sr2+) charged ions in water. Together these ions have a net positive charge because overall there are more positively charged ions in water. The overall charge of the water is neutralized by the HCO₃⁻ and CO₃²⁻ ions. HCO₃⁻ and CO₃²⁻ are defined as carbonate alkalinity (CA). The ions that influence TA are conservative, meaning that their concentrations are consistent in water. CA is the main factor in determining alkalinity. If the CA is high enough, the HCO₃⁻ and CO₃²⁻ ions can act to buffer the water from dramatic changes in pH accepting hydrogen ions.

Diagram showing seawater carbonate system interactions with chemical species comprising alkalinity.

NASA

The increased forcing of CO₂ (high pCO₂ ) into the water is resulting in lower pH and could alter carbon speciation (see Dissolved Inorganic Carbon section above) which can reduce the water’s ability to buffer changes in pH. In addition, estuaries and other coastal waters typically have lower alkalinity due to dilution from freshwater sources, compared to the open ocean.

pH Sites

pH measures how acidic or alkaline a solution is based on its hydrogen ion (H⁺) concentration. More H⁺ ions mean higher acidity; fewer H⁺ ions indicate a more basic (alkaline) solution.

pH or “potential of hydrogen” is a measurement of the amount of hydrogen ions (H⁺) in a solution. More H⁺ ions result in a more acidic solution, while fewer H⁺ ions result in a more basic or alkaline solution. When CO2 enters the ocean, chemical reactions take place that result in the release of H⁺ ions, leading to a more acidic ocean.

pH is measured on a logarithmic pH scale from 0 (acid) to 14 (base). Strongly acidic solutions can have millions or trillions of times more H⁺ ions than strongly basic solutions, so each unit change on the pH scale corresponds to a 10-fold change in H⁺ concentration. Since the Industrial Revolution, mean surface ocean pH has dropped by 0.1 pH units which equates to a 30% increase in acidity.

pH scale with Hydrogen ion concentrations.
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NOAA PMEL Carbon Program

Time series showing decreasing ocean pH alongside increasing atmospheric CO2 and oceanic pCO2.
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DIC Sites

Dissolved inorganic carbon (DIC) includes four carbon species formed when CO₂ dissolves in water: aqueous CO₂, carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻).

Dissolved Inorganic Carbon (DIC)

When carbon dioxide enters the ocean, it reacts with water to create four specific chemical forms called species: aqueous carbon dioxide (CO₂(aq)), carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻). Collectively these species are known as dissolved inorganic carbon (DIC).
A series of reactions take place simultaneously until an equilibrium is reached among the four species. When CO₂ is dissolved in water, either from the atmosphere or from respiration from coastal organisms, it reacts with H₂O to produce H₂CO₃.

CO2(aq) + H2O &lt;-&gt; H₂CO₃

H₂CO₃ &lt;-&gt; HCO₃⁻ + H⁺: H₂CO₃ breaks down to release a hydrogen ion

HCO₃⁻ &lt;-&gt; CO₃²⁻ + H⁺: If pCO2 increases, HCO₃⁻ loses another H⁺

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These reactions occur simultaneously until an equilibrium, or balance, is reached. If more CO2 is added (a rise in pCO2) H₂CO₃ breaks down into HCO₃⁻ and CO₃²⁻ and more H⁺ ions get released, lowering the pH and increasing acidification.

DIC consists of aqueous carbon dioxide (CO₂ (aq)), carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻). Related through a series of chemical reactions, these carbonate species exist in equilibrium with one another.

Seacoast Science Center

Bjerrum plots illustrate how carbonate species remain in equilibrium as they go through the reactions described above. Note that changes in carbonate species concentration (y axis) is related to the pH (x axis).

Carly LaRoche

Saturation state (Ω) serves as a measure of whether calcium carbonate mineral forms are more likely to dissolve or precipitate under current seawater conditions.

Aragonite Mineral Saturation State (Ω)

If two of the four carbonate chemistry parameters have been measured in combination with salinity and temperature, calcium carbonate saturation state (Ω) can be derived. Saturation state is a measure of how much calcium carbonate is dissolved in seawater and therefore serves as a key indicator of ocean acidification. 

At equilibrium, Ω = 1

Saturated, Ω &gt; 1: Calcium carbonate will form

Undersaturated, Ω &lt; 1: Calcium carbonate will dissolve.

Saturation state can be computed for different mineral forms of calcium carbonate, including calcite and aragonite. Larvae and juvenile life stages of shelled organisms often use aragonite as their primary shell building material; for this reason, the saturation state of aragonite is particularly important to track. 


Pteropod shell dissolution occurs over the course of 45 days when exposed to predicted 2100 levels of pH and saturation state.

NOAA Pacific Marine Environmental Laboratory

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pCO₂ Sites

pCO₂ measures how much CO₂ is dissolved in seawater. Coastal systems can either emit or absorb CO₂ in order to maintain equilibrium between the pCO₂ of the ocean and the atmosphere.


Partial Pressure of Carbon Dioxide (pCO₂)

Partial Pressure of Carbon Dioxide (pCO₂) refers to the idea that in a mixture of gases, the total pressure exerted by that mixture is equal to the sum of the individual pressures, or partial pressures, of the gases. According to Henry’s law, the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the liquid. As atmospheric CO₂ levels have increased from human activities, the ocean has responded by absorbing CO₂ in order to maintain equilibrium between the air and seawater.

Often, estuaries and coastal areas have very high concentrations of dissolved CO₂ compared to the overlying atmosphere. High estuarine pCO₂ is typically the result of biological activity, in which CO₂ production (through respiration, by heterotrophs) exceeds CO₂ consumption (through photosynthesis, by autotrophs). Located at the land-sea interface, estuaries receive organic matter and nutrients from the land, providing extra fuel for heterotrophs. When an estuary’s pCO₂ is higher than the pCO₂ of the air above it, CO₂ leaves the water and enters the atmosphere.
Think about a can of soda. Before it is opened there is a headspace at the top of the can that is pressurized with CO₂. That high CO₂ partial pressure of the headspace ensures that the CO₂ in the soda remains dissolved. When the can is opened, the CO₂ in the headspace encounters the outside atmosphere (which has a relatively low concentration of CO₂). The result is that the soda fizzes and bubbles float to the surface, releasing previously dissolved CO₂ into the atmosphere.

When pCO₂ concentrations are higher in the atmosphere than the ocean, the ocean will absorb enough CO₂ to bring it into equilibrium with the overlying air.

Visualization of ocean-atmosphere CO₂ exchange from 1861 to 2100 shows where the ocean absorbs CO₂ and where the ocean emits CO₂ (NOAA Science on a Sphere).

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MACAN’s committed to understanding, predicting, and mitigating the impact of acidification on our oceans and coastal ecosystems.

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Increased atmospheric carbon dioxide drives ocean acidification, lowering pH.

Diverse monitoring addresses complex coastal and estuarine acidification drivers.

Factors such as geography, climate, and human activities vary across different regions, impacting the extent and rate of ocean acidification.

Ocean acidification is driven by excess global carbon dioxide and water pollution from land runoff, and there are ways communities can help.

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Ocean acidification changes water chemistry in ways that can hinder how marine species grow, reproduce, and survive.

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Aragonite Mineral Saturation State (Ω)

About

At equilibrium, Ω = 1

Mineral Forms

Photo: NOAA Pacific Marine Environmental Laboratory

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