The emission of large quantities of carbon dioxide (CO₂) in human activities (fossil fuels and other sources) is associated with increasing temperatures and climate change. But not only that, the greater presence of CO₂ in the atmosphere also affects seas and oceans (by absorption of this gas), increasing the acidity of the waters (acidification, reduction in pH).
In more acidic seas and oceans, organisms such as calcareous phytoplankton, crustaceans or corals have serious survival problems.
An international team led by ICTA-UAB experts has now confirmed that acidification is causing alterations in the production of marine plankton shells in the Mediterranean Sea. The decrease in the surface pH of the ocean limits the production of calcium carbonate from marine plankton, a situation that negatively impacts marine ecosystems, the authors state in the results article that has been published in the journal Communications Earth and Environment. (online ed. August 2023)
"Carbon dioxide (CO₂) emissions produced by human activity have increased alarmingly in recent decades," recall the authors of the new study, whose lead author is Belen Martrat. About 25% of anthropogenic CO₂ has been absorbed by the oceans since the Industrial Revolution, modifying water chemistry and lowering pH, a phenomenon known as ocean acidification.
The study, carried out in collaboration with the University of St. Andrews (United Kingdom), the Max Planck Institute of Chemistry (MPIC) in Mainz (Germany) and the Higher Council for Scientific Research (CSIC) in Barcelona, evaluates the impact of changes induced by CO2 in the Mediterranean Sea on foraminifera, a specific group of calcifying planktonic organisms.
To do this, the researchers studied records from the last two thousand years in different enclaves of the Mediterranean: the Alboran Sea, off the coast of Barcelona and the Strait of Sicily. They selected the western Mediterranean as the study area as it is a region especially affected by anthropogenic pressure and climate change. Due to its high alkalinity and the rapid circulation of water masses in the basin, Mediterranean waters are very prone to absorbing anthropogenic CO2, which has caused a decrease in pH by 0.08 units since the Industrial Revolution, thus affecting biogenic calcification of marine plankton.
Foraminifera are a common type of marine calcifying zooplankton that live in the upper part of the ocean, very sensitive to climate and environmental changes. These single-celled organisms build a shell, several hundred micrometers in size, which is made of calcium carbonate. Despite the extreme robustness of the shell, these calcite structures are very sensitive to changes in seawater chemistry, making them an ideal tool to study the long-term impacts of carbon alterations in the sea. marine ecosystems.
“The study shows that the impact of CO2 is already noticeable in the chemistry of the shells of planktonic calcifying organisms,” explains Sven Pallacks, lead author of the study. This is reflected in an accelerated decrease in the weight of shells throughout the 20th century. In contrast, before the Industrial Revolution, the shells of planktonic foraminifera were heavier, without showing great variability in weight over time.
The study found that ocean acidification caused by anthropogenic emissions is the most important driver of the decline in foraminiferal calcite mass, while ocean warming potentially mitigates this effect.
“This demonstrates the alteration, throughout the basin, of the production of marine calcite under the increase in atmospheric concentrations of CO2 and the acidification of surface waters in the Mediterranean Sea,” explains Patrizia Ziveri, oceanographer at ICTA-UAB.
By reconstructing the records, they evaluated the impact of acidification on the calcification of planktonic foraminifera. However, the results can be extrapolated to other calcifying planktonic organisms that live in the surface waters of the Mediterranean, such as coccolithophores or pteropods, which play an important role in modulating atmospheric CO2.
The results of the study indicate that anthropogenic ocean acidification at the surface of the Mediterranean Sea impaired foraminiferal calcification during the 20th century. Given that calcifying plankton is a fundamental component in both the constitution of the marine food web and the biogeochemical cycle, acidification would have negative effects on the services offered by marine ecosystems, including climate regulation, the functioning of ocean ecosystems and food security, which highlights once again the importance of mitigating climate change through a drastic reduction in CO₂ emissions, the authors explain in a note published by the Universitat Autònoma de Barcelona (UAB).
