Biotic and abiotic controls on CO2 partial pressure and CO2 emission in the Tigris River, Turkey

Abstract

Rivers act not only as vectors from the terrestrial biosphere to the ocean but also as significant sources of CO2 to the atmosphere, which plays a key role in the global carbon cycle. In this study, dissolved inorganic carbon (DIC) and carbon dioxide partial pressure (pCO2) in the Tigris River were investigated. The alkalinity concentrations of the Tigris River ranged from 1718 to 4995μeq/L with an average of 3064μeq/L (median: 2877μeq/L), 2.3 times the global median value of 1256μeq/L. Higher alkalinity values occurred in autumn and winter months that were due to carbonate rock weathering. pCO2 values varied from 95.6 to 16,001.8μatm with a total average of 1277±1883μatm. >80% of samples had pCO2 above atmospheric equilibrium (390μatm). The highest levels of pCO2 occurred at stations Diyarbakır, Bismil and Cizre, which are categorized as polluted stations. The lowest levels of pCO2 generally occurred in the wet season (October–April) due to dilution effect of rainfall. The estimated water-to-air CO2 degassing flux from the Tigris River was around 39mol/m2/y. Thus, 0.15Tg C/y as CO2 released from the river to the atmosphere, about 0.6 fold the riverine DIC flux (0.25Tg C/y). Correlative analyses demonstrated that population density and air temperature increased pCO2, while slope degree decreased pCO2. pCO2 also had positive correlations with COD, NH4-N, PO4-P, SO4, Cl, phytoplankton abundance and number of phytoplankton taxa, while negative correlations with DO saturation and pH.