Dynamic of CO2, CH4 and N2O in the Guadalquivir estuary

Abstract

The concentration of dissolved CH4 and N2O, as well as the partial pressure of CO2 (pCO2) were studied in the Guadalquivir estuary. Samples were taken in March and April 2018 and 2019, under different rainy and tidal conditions. The available database for summer 2017 (Sierra et al., 2020) was included in the interpretation of the factors that determine the variability of these gases in the Guadalquivir estuary. Two different types of samplings were carried out: a longitudinal transect across the river with salinity values close to zero and another one during two consecutive tidal cycles in the mouth of the estuary. The highest concentrations were found in the upper zone of the estuary and during the low tide. This distribution was related to 4 factors: temperature, salinity, exchange with the atmosphere, and biochemical processes together with the river inputs. Temperature is one of the factors that clearly seems to determine the distribution of gases and fluxes, showing the highest values in the upper zone during the summer of 2017. Intense rains cause a dilution effect of the gas in the water column, this provoked, during the season of spring 2018, an increase in the salinity factor in the distribution of gases in the middle zone. High concentrations of the gases have been linked to production (oxidation of organic matter and nitrification) processes in the water column, as well as to benthic production and lateral inputs. While the gases concentrations at the mouth presented values close to those of the equilibrium with the atmosphere, the fluxes in the upper zone of the estuary reached average values of 89.6 mmol m-2 d-1, 121.7 µmol m-2 d-1 and 59.9 µmol m-2 d-1 for CO2, CH4 and N2O, respectively. Generally, water-atmosphere fluxes are positive through the whole study, which means that the estuary acts as a source of these gasses to the atmosphere.