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pCO₂ in Water

Saturation Concentration of Water

The solubility of CO₂ in water is a temperature-dependent property. As a gas, CO₂ is more soluble in colder waters with the solubility decreasing as the water warms. This means that the highest concentration CO₂ will occur late at night when the Sun has set and the waters are allowed to reach their lowest temperature. This can be extended to apply to seasonality, as the warmer summer months mean that less CO₂ can be dissolved in the water while colder winter months allow for greater dissolution.

Atmospheric Exchange

A major source of CO₂ in water comes from the atmosphere. As CO₂ is generated on land and released into the atmosphere, it builds a concentration gradient between the air and water. As the gradient builds, CO₂ will partition from the atmosphere into bodies of water and is dictated by Henry's Law. This allows the oceans to act as a natural buffer between the generation of CO₂ by anthropogenic activity and the climatic effects from the build up of the gas.

pCO₂ in Aquatic Ecosystems

Metabolism

As mentioned, solubility is temperature dependent, but heating is not the only way that CO₂ can be removed from the environment. CO₂ can also be removed be aquatic plants taking in the gas for use in photosynthesis which contributes to lower concentration of CO₂ during the day. Additionally, when dissolved in water CO₂ forms a carbonate ion which is of biological importance in aquatic environments. This is especially the case for shell-forming species that must absorb the carbonate that is the basis of their shells.

Measurement

The partial pressure of CO₂ in water can be measured using sensors that measure gas diffusion across a hydrophobic membrane. The partial pressure of CO₂ in water can be measured using sensors that measure gas diffusion across a hydrophobic membrane. Generally, these measurements are to be taken frequently as pCO₂ varies with temperature and therefore the value will change between day and night.

Measurements are also taken at varying points of depth and spatial location as the CO₂ can vary spatially as well. This is especially important to do in cases where microhabitats are likely to form. When the water is well mixed, the levels are normally indistinguishable, however this is not always the case and variations are possible. For instance, in cases with a well-defined thermocline, there CO₂ would not be able to pass the temperature barrier and result in two different concentrations based on depth of measurement.

Trophic State and pCO₂

pCO₂ is not only an oceanographic parameter but also applies to inland streams and lakes where they can be much more susceptible to impact from anthropogenic activities. These anthropogenic activities refer to human actions to modify the local environment that usually harm the area in some form. These modifications can include the removal of riparian vegetation and increased soil erosion, both of which would normally act as natural sinks. In this case, sinks refer to processes and substances that trap or absorb some of the carbon content. . By trapping the carbon it can no longer contribute to the area's pCO₂ which will, in turn, prevent more CO₂ from being introduced to the atmosphere. The CO₂ will at some point escape to the atmosphere should the pCO2 ever go above the pressure of the CO₂ present above the water's surface.

Rivers that pass through more urbanized areas show a larger pCO₂ than other areas due to a decrease in the integrity of the habitat as a result of local modifications made for one reason or another. This will also affect the waters downstream, as the excess CO₂ is carried away from its point of origin and contributes to an increased carbon load being transported out to sea. In terms of the relationship between atmospheric and dissolved CO₂, most rivers on the planet can be supersaturated, as photosynthesis and other sinks in the water cannot keep up with the influx of carbon. This means that many rivers emit CO₂ to the atmosphere in a process called outgassing, rather than absorbing it into the water for storage

Oversaturation of CO₂ in Aquatic Environments

Outgassing can be a minor occurrence that happens relatively frequently, or it can occur in a more singular bulk release of CO₂ and cause a disaster such as what happened at Lake Nyos in Cameroon. Water that became supersaturated with CO₂ from volcanic activity nearby entered the bottom of the lake through springs. These waters become trapped at the bottom of the lake due to stratification. The gas cannot escape because there is no contact with the atmosphere to be outgassed and the stratification prevents mixing to limit its upward mobility Then an unknown event triggered what is known as limnic eruption where the gas was abruptly released to the atmosphere. This was not a one-time occurrence as it also happened two years previously at the nearby Lake Monoun, resulting in the death of 37 people. Other lakes, such as Lake Kivu in East Africa, are being monitored due to their large dissolved CO₂ concentration in an effort to be wary of future potential limnic eruption events. Other prevention efforts include the use of controlled piping techniques to move the CO₂-rich waters from the bottom of the lake to the surface. This would allow the gas to be safely released at a slow constant rate and prevent buildup at the bottom of the lake.

Acidification of Aquatic Environments

CO₂ reacts with the water to form a weak acid that contributes to the overall pH of the water. Therefore, as CO₂ continues to enter the water and form bicarbonate, the pH will move to be more acidic, resulting in ocean acidification.