| Summary: | 5th International Symposium on the Ocean in a High CO2 World, 13-16 September, Lima, Perú The oceanic uptake of atmospheric CO2 is driving changes in seawater chemistry that result in the decrease of both pH and the concentration of the carbonate ion, which is essential for marine calcifiers to build their hard structures and shells. Reduction in the availability of carbonate ions lowers the saturation state (Ω) for the calcium carbonate mineral aragonite in the ocean. When concentration of aragonite changes from oversaturation (Ω>1) to undersaturation (Ω<1), biogenic calcification is not supported, with severe impacts on marine biodiversity and ecosystem functioning. In this work, we used 17 years (2005–2022) of biogeochemical measurements collected at the Gibraltar Fixed Time series (GIFT) located at the Strait of Gibraltar to estimate temporal trends of seawater pH and Ωaragonite in major Mediterranean and Atlantic water masses that exchange in the area, i.e. the Western Mediterranean Deep Water (WMDW), the Levantine Intermediate Water (LIW) and the North Atlantic Central Water (NACW). Our analysis shows a gradual decline in pH in all water masses that is accompanied by a noticeable temporal drop of Ωaragonite. Decreasing trends in both parameters were more and less pronounced in the NACW and LIW, respectively. Estimated long-term changes of Ω under future increases in atmospheric CO2 according to the CO2 business-as-usual scenario (Shared Socioeconomical Pathway SSP5-8.5) indicate that critical conditions for calcifiers will be reached in Mediterranean and Atlantic waters before the end of the current century, with corrosive environment (undersaturation of carbonate) expected within the next 200 years No
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