A lotic ecosystem is considered a source of carbon dioxide (CO₂) in the atmosphere where it becomes supersaturated with CO₂, which contributes to the global carbon cycle. To enhance our comprehension of the roles of CO₂ in rivers, an outdoor experiment was designed with controlled carbon source inputs to investigate the roles of the dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the phytoplankton community. Plastic enclosures were installed in the Tigris River within Baghdad for that goal. Samples were collected on the first day, as well as on the 5^th and the 12^th days from 14 enclosures. The enclosures were treated by artificial glucose (C₆H₁₂O₆) (10, 20, 30mg/ l) as DOC sources, while sodium bicarbonate (NaHCO₃) (10, 20, 30µM) was used as a DIC source. The results showed that the concentration of nitrate (NO₃^-) and phosphate (PO₄^3-) changed over time and weren't affected by the treatments. On the other hand pH, DOC, and CO₂ concentrations were affected by treatments. Moreover, our results indicated that DOC and DIC treatments had a direct impact on phytoplankton biomass growth via increasing chlorophyll (Chl) concentration. Overall, it was concluded that different carbon sources (DOC and CO₂) could be essential factors that shape river ecosystems function through influencing the base of food webs.