Aragonite saturation state in a continental shelf (Gulf of Cadiz, SW Iberian Peninsula): Evidences of acidification in the coastal area

摘要

The spatiotemporal variability of aragonite saturation state (ΩAr) has been studied in the eastern shelf of the Gulf of Cadiz (GoC) (SW Iberian Peninsula). The study was carried out during the years 2014 and 2016 aboard twelve oceanographic cruises, along three or five transects, located between Cape Trafalgar and the Guadiana River. The GoC exhibited oversaturated of calcium carbonate with ΩAr mean values of 2.68 ± 0.30 in surface and 2.05 ± 0.15 in deep waters. pH, total alkalinity (TA), calcium concentration (Ca~(2+)) and QAr showed a high variability within the surface mixed layer (SML, z ＜ 100 m). Biological activity seemed to be the main process to determine the ΩAr variability in the SML, revealing a greater importance than temperature, mixing or air-sea processes. The buffer factors of the CO_2 system in the GoC have been estimated in the SML waters, and they are related with the biological activity and the temperature changes. A decrease of pH and QAr in this SML with depth was observed, due to the increase of the respiratory processes. In deep waters (z ＞ 100 m), TA and Ca~(2+) concentration presented a conservative behaviour related to the distribution of the different water masses located in the GoC. The vertical variation of ΩAr also depends on the degree of mineralization of these water masses, obtaining the maximum values in the Subtropical North Atlantic Central Water (100-200 m), minimum values in the Subpolar North Atlantic Central Water (about 400 m), and intermediate values associated to the presence of the Mediterranean Water (＞500 m). Results showed a significative acidification of the coastal areas, for those depths lower than 100 m from 2006 to 2016, with a mean decrease of pH and ΩAr of -0.0089 and -0.0552 yr~(-1), respectively.