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 (pCO(2)) in the Tigris River were investigated. The alkalinity concentrations of the Tigris River ranged from 1718 to 4995 mu q/L with an average of 3064 mu q/L (median: 2877 mu eq/L), 23 times the global median value of 1256 mu eq/L Higher alkalinity values occurred in autumn and winter months that were due to carbonate rock weathering, pCO(2) values varied from 95.6 to 16,001.8 mu atm with a total average of 1277 +/- 1883 mu atm. >80% of samples had pCO(2) above atmospheric equilibrium (390 mu atm). The highest levels of pCO(2) occurred at stations Diyarbakir, Bismil and Cizre, which are categorized as polluted stations. The lowest levels of pCO(2) 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 39 mol/m(2)/y. Thus, 0.15 Tg C/y as CO2 released from the river to the atmosphere, about 0.6 fold the riverine DIC flux (0.25 Tg C/y). Correlative analyses demonstrated that population density and air temperature increased pCO(2), while slope degree decreased pCO(2). pCO(2) 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. (C) 2016 Elsevier B.V. All rights reserved.