Ocean acidification in the Bay of Biscay.
Supervisors: Guillem Chust, Joana Larreta, Ernesto Villarino (AZTI)
The ocean is transitioning to unprecedented conditions, with an increase in temperatures, an increase in ocean acidification, a decrease in dissolved oxygen and an alteration in primary production, compromising the resilience of marine ecosystems and the communities that depend on it. Ocean pH is a good indicator of ecosystem health; however, efforts including accurate and continuous pH monitoring remain elusive, particularly in the Basque coast (southeastern Bay of Biscay). Here, we examine pH time series to study its variability, changes over time and ocean acidification rates in the Basque coast using a recently deployed Submersible Autonomous Mooring Instrument (SAMI) at 25 m depth, and long-term data collected over the last 20 years from surface down to 100 m depth at three sites. Analysis from the SAMI revealed high pH variability during the day with a marked diel periodicity, presumably driven by community photosynthesis and biological respiration. Analysis of long-term pH trends revealed significant decreases over time in all depth layers (0.5-100 m) at the three coastal sites at rates of 0.039-0.058 pH units per decade from 2002 to 2022. The consistent trends observed in the three sites are therefore associated to global ocean acidification derived from the continuous increase of anthropogenic atmospheric CO2 concentrations, with implications on calcifying marine organisms and the entire ecosystem. Abiotic and biotic factors, including temperature, alkalinity, partial pressure of carbon dioxide, physical and biological pump of the water column may be also influencing the pH variability in the Bay of Biscay’s Basque Coast, hence further research considering such factors might help better understand the underlying factor driving ongoing local changes in pH over time.