Posidonia oceanica meadows from satellite view

Posidonia oceanica is a marine plant endemic to the Mediterranean Sea distributed up to 45 meters deep, which is not only important for keeping the waters transparent and protecting the coasts from erosion, but also for its important role in mitigating climate change. . These marine plants can remove CO2 up to 35 times faster than tropical forests, but why is it important to monitor Posidonia oceanica? «This queen of the Mediterranean is the marine plant with the greatest capacity for carbon sequestration on the entire planet. That is, it is the marine plant that eliminates CO2 the fastest with the smallest surface area. It is estimated that, in the last century, we have lost around 15 percent of the total area of ​​marine plants in the Mediterranean, mainly due to the anchoring of boats in grassland areas due to ignorance of their importance, and whose detriment continues to be a trend. , reducing their capacity to mitigate climate change,” says Mar Roca Mora, ICMAN-CSIC researcher.

Let’s put ourselves in context. Since pre-industrial times, atmospheric CO2 levels have risen exponentially, causing planet Earth to warm. Part of the atmospheric CO2 is dissolved in the water column, where marine photosynthetic organisms are responsible for converting and removing a small part of said gas. In the case of seagrasses such as Posidonia oceanica, they convert the carbon available in the water into organic carbon deposited under their roots in conditions of low oxygen availability, which allows low degradation of carbon by bacteria and, if not be altered, remain almost intact for thousands and thousands of years (carbon stock).

Monitoring the extent of these seagrass meadows is essential to know the state of conservation, changes due to impacts, planning conservation measures and evaluating the public policies applied. However, the wide extension of covering the entire Mediterranean Sea represents a challenge in updating the cartography of these ecosystems with traditional underwater methods composed of oceanographic vessels, divers and underwater instrumentation.

Faced with this challenge, high-resolution satellite images can be an excellent ally as a support tool for field methods, allowing larger areas to be covered without having to go to each of the meadows. The optical images from the Sentinel-2 satellite of the European Space Agency’s Copernicus program at 10 meters pixel resolution represent a paradigm shift in the monitoring of our coasts, not only in the emerged part but also up to several tens of meters deep. in good visibility conditions.

In the work published by the ICMAN-CSIC, thanks to the combination of knowledge about the physics of light and water, massive cloud computing techniques to process 3,500 Sentinel-2 images and the use of machine learning algorithms, You can get to know the signal of the seabed and characterize different types of benthic ecosystems. Specifically, the Posidonia oceanica signal has been characterized up to 30 meters deep, estimating its extension (505.6 km2) with an accuracy of 92.5% compared to official data in the Balearic Sea. However, the importance of this study does not lie in updating said cartography in the Balearic Sea, since it is a well-studied region, but rather in using all available data as a preliminary methodological validation step for the development of an open access tool. for the Mediterranean Sea.

Another important aspect lies in depth. This study has developed an open access regional bathymetry by compiling existing information, demonstrating the importance of said variable in the model. This data has made it possible to optimize the detection of the underwater plant in the shallowest areas, as well as to derive its annual carbon sequestration rate at 227 tons of carbon, one more small contribution to the removal of the very high CO2 emissions that today they continue to increase. 12.27±2.1 million tons of organic carbon is the estimated stock under the roots of these plants in the Balearic Sea up to 30 meters deep, so the degradation of the grasslands would be a source of carbon emissions. carbon whose loss could not be balanced for thousands of years. The carbon sequestration maps show us the priority in protecting the shallowest meadows, which are much more exposed to the anchoring of recreational boats, and are also the ones that act the most as carbon sinks due to their greater photosynthetic capacity.

Due to the challenge posed by the scarcity of this updated bathymetric data, satellites can once again be helpful in this task, where satellite-derived bathymetry can provide key information that helps monitor less studied areas such as the African Mediterranean coast or simply where the data is not accessible. However, remote sensing also has limitations, because what we measure is the amount of reflected light, having limitations in the detection of these plants in the shadow of cliffs, very deep areas or murky waters.

«These coastal satellite image computing techniques can be of great help to support the monitoring of Mediterranean seagrasses. The replanting of these grasslands is a relevant action, but it is much more important to update the information to conserve the existing ones, preventing millions of tons of carbon sequestered under their roots from being returned to the water and part of it from continuing its journey into the atmosphere. This fact would transmute their role from mitigating climate change to emitting greenhouse gases,” concludes the ICMAN-CSIC researcher.