Long-term pH trends across depth in coastal areas of the southeastern Bay of Biscay

Increased CO₂ concentrations in the atmosphere have triggered ocean acidification over the past decades in the global ocean. However, regional efforts of pH monitoring across the southern Bay of Biscay's Basque coast remain elusive, with only a few short-term studies limited to the ocean's surface. Here we examine pH trends over time across the Basque coast using 21733 observations of long-term data collected during 2002–2022 with quarterly CTD casts from surface down to 100 m at three coastal sites. Results revealed significant pH decreases over time in all depth layers (0.5–100 m) at the three coastal sites (0.022–0.041 units decade⁻¹), presumably driven by the global increase of atmospheric CO₂. Across depth, observed pH trends also showed significantly higher ocean acidification rates with depth. Seasonally, observed pH changes ranged from wintertime highs of 8.18 ± 0.07 to summertime lows of 8.14 ± 0.05, with a mean seasonal amplitude of about ∼0.04 pH units. The observed pH seasonality and vertical patterns appeared to be tied to the combined effect of environmental factors alongside the development of the thermocline as well as to differences in the biological activity across the water column. Taken together, these findings highlight the importance of pH monitoring in coastal areas and warn on the effect of ocean acidification on marine ecosystems and the services they provide to society.