Highlights
- We examined pH trends over time across depth in the Basque coast upon 21733 observations during 2002-2022.
- Significant pH decreases over time (0.022 to 0.041 units decade-1) across 0-100m likely driven by the global increase of atmospheric CO2.
- Marked seasonality and higher ocean acidification rates in deeper relative to surface layers tied to environmental factors and the biological activity.
- Importance of pH monitoring in coastal areas to warn on the effect of ocean acidification on marine ecosystems.
Abstract
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 to 0.041 units decade-1), presumably driven by the global increase of atmospheric CO2. Across depth, the pH trends observed also showed significantly higher ocean acidification rates with depth. Seasonally, observed 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 to warn on the effect of ocean acidification on marine ecosystems and the services they provide to society.
Villarino E., Essel D. A., Citores L., Larreta J., Lopez A., Borja A., Flecha S., González M., Cuesta L., Orue-Echevarria D., Esteban X. & Chust G., 2025. Long-term pH trends across depth in coastal areas of the southeastern Bay of Biscay. Continental Shelf Research: 105532. doi: 10.1016/j.csr.2025.105532. Article (subscription required).
