Carbon and calcium dynamics in the Guadalquivir river estuary

Resumen

The longitudinal variation of the different species associated with the carbon cycle has been studied during the spring of 2019 in the estuary of the Guadalquivir river. This study has been carried out with two different tidal coefficients. For achieving this purpose, two campaigns were carried out aboard the B/O UCADIZ., the first one was in March from 19th to 22nd and the second one was in April from 9th to 11st.Total alkalinity (TA) and dissolved inorganic carbon (DIC) increase into the estuary, where they reach maximum concentrations of 4034 and 3760 µmolkg-1 respectively. These high values in the river area are due to processes derived from biological activity such as breathing, and mainly the leaching of the carbonated basin and the dissolution processes of CaCO3, as indicated by the values above the unit registered for the TA/DIC ratio. The contribution of organic bases to the TA (AOrg) has maximum values in the most fluvial zone, contributing up to 19% in the TA. The apparent oxygen utilization (AOU) varies between -8,35 and 208,92 µmolkg-1, presenting positive values in most stations. This indicates the predominance of the processes of degradation of organic matter throughout the estuary, especially in the most rivera rea, coinciding with the highest values of DOC (335 µmolkg-1) and nutrients. Equally, the high correlations between the DIC and nutrients indicate an intense remineralization of organic matter in the system, especially in the internal zone. The concentration of Ca2+ has been measured by potentiometric titration, and its value decreases towards the internal estuary area, with a variation range between 2,54 y 10,59 mmolkg-1. On the other hand, the degree of saturation of CaCO3 (Ω) has values greater than unit in all sampling station (ΩAr: 2,08-14,56; ΩCa: 3,23 - 24,99). DOC, DIC and nutrients have the highest concentrations at low tide, which is indicative than the Guadalquivir estuary acts an exporter of these substances to the Gulf of Cadiz. On the othe other hand, the difference in the tidal coefficient between the two campaigns does not seem to have affected the behavior of the different variables studied in the longitudinal or circadian sampling.