CaCO3 saturation state and anthropogenic carbon in the Gulf of Cádiz

Resumen

CaCO3 saturation states were studied during twelve oceanographic cruises between March 2014 and December 2016 along transects perpendicular to the coastline among Guadiana river and Cape Trafalgar on the eastern shelf of the Gulf of Cádiz, on board the R/V’s Ángeles Alvariño and Ramón Margalef, and during one cruise in three estuaries of the Gulf Cádiz (Guadiana, Tinto and Odiel and Guadalquivir) in July 2017 on board R/V UCADIZ. pH, total alkalinity (TA) and calcium concentrations (Ca2+) were analysed through potentiometric titration. pH, TA and Ca2+ varied between 7.8 and 8.2, 2304.3 and 2585.6 µmol kg-1, and 10.4 and 11.3 mmol kg-1, respectively in the Gulf of Cádiz. The highest values of TA and Ca2+ were found in deep waters related to the presence of the Mediterranean Outflow Water. In the estuaries, the values of pH, TA and Ca2+ presented a greater variation range, being between 7.5 and 8.3, 1653.7 and 3720.4 µmol kg-1, and 1.0 and 11.7 mmol kg-1, respectively, with an increase of TA and a decrease of Ca2+ through the inner estuaries area. CaCO3 saturation state in the Gulf of Cádiz is highest in the surface waters due to the photosynthetic activity and lowest in deep waters for the decrease of pH, with mean values of 3.8 ± 0.5 and 2.5 ± 0.3 for calcite (Ωc) and aragonite (Ωa) saturation states, respectively. However, Ωc and Ωa reached values of 25 and 15 in the internal areas of the estuaries. Spatio-temporal variations of anthropogenic carbon (CANT) were estimated using TrOCA method (Touratier et al., 2007) in the Gulf of Cádiz, having a mean value of 88.9 ± 13.9 µM, and with the highest values during autumn (101.0 ± 14.6 µM). CANT presented the greater values in surface waters and decreased with the distance to the coast.