This is the main conclusion of a new study led by the Institute of Marine Sciences (ICM-CSIC), which for the first time analyzes how this pressure is affecting not only individual species, but also entire communities and ecosystems.
A new international study recently published in the journal Ecology Letters has shown that the acidification of the Mediterranean—caused by rising atmospheric CO₂—is simplifying biodiversity and altering the functioning of coastal ecosystems in our region. Although the effects of this pressure on individual species have been widely studied, few works have examined how entire communities and ecosystems change simultaneously as a result of this increase in atmospheric carbon dioxide.
The study, carried out at natural CO₂ vents in Ischia (Italy)—coastal areas where volcanic CO₂ bubbles naturally lower seawater pH—shows how acidification affects key ocean processes such as calcification, photosynthesis, respiration, and nutrient uptake. The results indicate that these changes lead to simpler, less diverse communities dominated by a few resistant species that outcompete more sensitive ones, especially those forming calcareous structures—essential for building marine habitats, regulating the carbon cycle, and protecting coastlines.
A natural laboratory to understand the ocean’s future
To conduct the study, the scientific team placed and transplanted small substrates at different points along the coast of Ischia where CO₂ released from the seafloor naturally lowers water pH. These sites replicate conditions expected in a future more acidified ocean, allowing researchers to observe how entire marine communities reorganize and how their functioning changes.
Overall, the results reveal a marked ecological transition. As pH decreases, calcifying organisms—such as coralline algae and bryozoans—rapidly disappear, while fast-growing non-calcifying species, especially some algae, expand. The authors emphasize that this shift entails a substantial loss of biodiversity and biomass, simplifying the structure of coastal ecosystems.
Profound changes in ecosystem functioning
The scientific team states that these structural changes significantly alter ecosystem functioning. Under low pH conditions, calcification rates decrease by 50% to 90%, even disappearing entirely in the most extreme cases. In contrast, photosynthesis and nutrient uptake increase markedly, in some cases more than 200-fold when normalized by biomass. Moreover, communities exposed to acidified conditions for nearly 15 years show the same patterns, indicating that these changes remain stable at ecological timescales.
“Our results show that ocean acidification not only weakens ecosystems, but also reorganizes them,” explains Jérémy Carlot, the study’s lead author.
“When calcifying species disappear, fast-growing algae take their place and reconfigure energy and nutrient flows in coastal habitats. These changes will redefine the functioning and resilience of Mediterranean ecosystems under high CO₂ concentrations,” he adds.
Furthermore, the team highlights that although some processes appear to intensify when normalized per unit of biomass, these increases mask a significant reduction in total community biomass. As a result, overall productivity per square meter is expected to decline under more extreme acidification conditions. In addition, experts point out that other climate pressures—such as warming and deoxygenation—will likely intensify species loss and further alter ecosystem functioning.
Overall, the study shows that understanding climate impacts requires looking beyond individual species and assessing the functioning of entire ecosystems. By analyzing natural CO₂ gradients, the researchers provide rare and valuable field evidence that ocean acidification is already driving predictable functional changes in Mediterranean rocky reefs. This underscores the urgent need to reduce carbon emissions and lower our global CO₂ footprint to protect coastal ecosystems.