The sea along the Basque coast is changing quietly. An analysis of more than 21,700 measurements collected between 2002 and 2022 shows that the pH of seawater is steadily decreasing—clear evidence of ocean acidification driven by rising atmospheric carbon dioxide.
According to the study, published in Continental Shelf Research, the pH of waters from the surface down to 100 meters is declining by 0.022 to 0.041 units per decade. “If this trend continues at the same pace, the impacts on the health of marine ecosystems could be significant,” explains Ernesto Villarino, AZTI researcher and lead author of the study.
Conducted by AZTI with the support of the Naturklima Foundation, as part of the Gipuzkoa Marine Climate Change Observatory, and in collaboration with the Institute of Marine Sciences of Andalusia (ICMAN-CSIC), this research analyzes the longest continuous pH monitoring series ever recorded along the Basque coast. The data, provided by the Basque Water Agency (URA), confirm that the Bay of Biscay is also affected by ocean acidification, underlining the need to strengthen mitigation and climate-adaptation strategies.
The sea absorbs CO₂—at a high cost
The average pH of ocean surface waters is around 8.1, slightly basic. Over recent decades, however, a slow but persistent decline has been observed, a process known as ocean acidification.
This shift is a direct consequence of the ocean’s ability to absorb CO₂—a mechanism that helps slow global warming but alters the chemical balance of seawater. In the Cantabrian Sea, this change can affect species reliant on calcium carbonate to build shells or skeletons, such as bivalves, cold-water corals and certain crustaceans. If the trend intensifies, it may also impact economic activities such as aquaculture.
The results also indicate that acidification rates in the Bay of Biscay are slightly higher than the global average and increase with depth. The three monitoring stations analyzed show very similar values, suggesting a remarkably consistent pattern along the coast. This finding concerns the research team, which calls for reinforced monitoring to better understand local impacts of this global phenomenon.
While no drastic effects are expected in the coming decades if the current rate is maintained, the combination with other climate-related stressors—rising sea temperature, declining oxygen levels, sea-level rise and extreme weather events—could accelerate or amplify consequences.
A key indicator of climate change
pH monitoring is one of the Essential Ocean Variables defined by the Global Ocean Observing System (GOOS). The findings of this study confirm that the Bay of Biscay is not immune to global ocean acidification and reinforce the need to maintain long-term observation networks.
Scientists warn that only sustained reductions in CO₂ emissions can slow this process, which—together with warming waters, declining oxygen and rising sea levels—is reshaping the delicate balance of the Cantabrian Sea.
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