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The Northeast Pacific coastal ocean, as a typical river-influenced coastal upwelling system, is characterized by significant variability of sea surface partial pressure of carbon dioxide (pCO 2 , <200 to >1000 μatm). This study reviewed the pCO 2 variability and its underlying controlling mechanism in this highly dynamic region by bringing together previous scientific findings and historical data. The large pCO 2 variability reflects the complex interactions between physical processes (riverine input and coastal upwelling) and the biological responses to the nutrient transportation associated with these physical processes, while temperature and air-sea gas exchange play a minor role in affecting pCO 2 . Both the river water and upwelled subsurface water are characterized by higher concentrations of pCO 2 and nutrients when compared to the coastal surface water. The presence of high chlorophyll-a and low pCO 2 in river plumes and areas adjacent to upwelling locations showed the intense biological CO 2 uptake. The influences of riverine input and coastal upwelling thus mainly depend on the competing effect of high background pCO 2 of river water and upwelled subsurface water vs. the biological dropdown of pCO 2 resulting from the riverine-and upwelling-associated nutrient supplies. The strength of upwelling-favorable wind plays an important role in the pCO 2 variability by affecting the intensity of coastal upwelling, with stronger wind speed causing more intense upwelling. The long-term pCO 2 increasing rate in