Oceans are the main responsible for capturing the atmospheric carbon dioxide (CO2) on Earth. Without this action, the intensification of the greenhouse effect would be much greater than current days.
Thus, the climate change consequences that we are already experiencing could be much more devastating.
Therefore, knowing what interferes with the exchange of CO2 at the limit between the ocean and the atmosphere, i.e. the sea-air CO2 flux, is very important for predicting future environmental scenarios.
The sea-air CO2 flux is basically controlled by two components: temperature and biological activity. To understand the relation between temperature and a gas, think of water heating up.
As the water is reaching the boiling point, bubbles start to appear and to be released. That is gas that was dissolved in the water.
Thus when you think in reverse, water cooling, the gas gets better conditions to dissolve into water. In resume, the warmer the water, the easier it is to the gas to be released, while the colder the water, the easier it is to the gas to be dissolved.
If you think on the planet, the closer to the equatorial zone, the warmer the waters, and the oceans will be more prone to liberate CO2 into the atmosphere; while towards the poles, it becomes easier to the atmospheric CO2 be captured by the oceans. In addition to that, we have the biological activity of phytoplankton.
Phytoplankton are microscopic marine plants (also known as microalgae) that contain chlorophyll which is the same pigment as in green terrestrial plants.
Phytoplankton are found mainly close to the coast, where they find better nutritional conditions. However, we also can find highly productive agglomerates (or blooms) of phytoplankton in the open ocean with the right conditions.
Like every plant, phytoplankton perform photosynthesis, which is how plants eat. The photosynthesis is the transformation of carbon dioxide and water into food using sun energy. Thus, the uptake of atmospheric CO2 by phytoplankton is a key component to being considered in the sea-air CO2 flux study.
Temperature and phytoplankton activity are driven by environmental factors, and the oceans are a very dynamic environment, with coastal regions in particularly being the most dynamic areas.
That is because there are many agents influencing the region, such as river discharges, biological community, and vegetation coverage. As a result, some regions, like the equatorial zone, for example, are considered to be sources of CO2 to the atmosphere due to the occurrence of upwelling currents. Upwelling is the process in which deep, cold water rises towards the surface.
It also brings nutrients that enable biological activity and high concentrations of CO2. However, even though we already know all this, there are still a lot to learn and field to cover.
To this purpose, in the work of Lencina-Avila and co-workers (2016), we used data collected during a Brazilian cruise that crossed the South Atlantic Ocean at 35°S latitude during mid-spring and early summer in the Southern Hemisphere.
This cruise also sampled two adjacent coastal regions, the South American, and the African one. Data as temperature, salinity, and chlorophyll helped us to assess physically and biologically these regions, while wind speed and CO2 partial pressure (an indirect parameter to measure CO2) in the seawater and in the atmosphere were used to determined de CO2 flux in the boundary between the ocean and the atmosphere.
We found that during the cruise period the entire cruise region captured atmospheric CO2. However, the factors acting on this CO2 uptake were quite different when considering the regions separately.
In the South American coastal region, the La Plata river discharge had a great impact on sea-air CO2 flux result. Rivers are known for carrying high concentrations of nutrients which can, in turn, promote phytoplankton activity.
Our results showed that a phytoplankton bloom, located in the river discharge area, probably occurred prior to the cruise period. This could mean that sea-air CO2 flux can be more intense, i.e. the ocean is capturing more CO2 during blooms activities or closer to the river’s mouth.
In the open ocean, temperature controlled the uptake of atmospheric CO2, and it was the coolest region of the three.
At this region, we found the predominance of one water mass, which is a body of ocean water with a distinctive narrow range of temperature, salinity, and other parameters.
Moreover, during warm months, there are not many nutrients inputs into the open ocean. Many of it occurs through eddies, which are circular currents that swirl carrying nutrients and organisms. The cruise encountered some of these eddies that seem to have an impact on the sea-air CO2 flux. These data will be subject to another study.
In the African coast, it is found part of one of the most important upwelling ecosystems of the planet, the Benguela Upwelling system. A strong front, i.e. large variations of parameters such as temperature, between the upwelling system and the open ocean was present. In the open ocean portion of the front, sea-air CO2 flux was close to zero whereas, in the coastal portion, CO2 uptake increased considerably.
This was a reflection of the coupled action of temperature and phytoplankton on capturing the CO2. But don’t upwelling regions bring high concentrations of CO2?
True, however, what our data show is that the CO2 upwelled was consumed by phytoplankton, together with the nutrients, to the point that the concentration of CO2 was higher in the atmosphere than in the ocean.
Our next step was to try forecasting the partial pressure of CO2 in the seawater from temperature, salinity, and chlorophyll in the sea surface. If successful, we could use satellite data, for instance, and get a better coverage of the sea-air CO2 flux in the ocean.
Our models were very good, except for the African model. This was our shortest area sampled, added to the complexity of the region, our data was not able to properly estimate the CO2 partial pressure.
The results found in this study contribute to a better knowledge of the sea-air CO2 flux, and the carbon cycle itself, especially in the South American portion. More studies will show if these regions will continue to capture atmospheric CO2. As said before, the ocean is a dynamic environment, and it keeps changing.
Citation: Lencina-Avila J M, Ito R, Garcia C A E , Tavano V M. (2016). Sea-air carbon dioxide fluxes along 35°S in the South Atlantic Ocean. Deep-Sea Research I, 115:175-187. doi: 10.1016/j.dsr.2016.06.004
Figure legend: This Knowridge.com image is credited to Lencina-Avila J M et al. Please do not cite or distribute without author’s permission.
