Abstract. Studies investigating the effect of increasing CO₂ levels on the phosphorus cycle in natural waters are lacking although phosphorus often controls phytoplankton development in many aquatic systems. The aim of our study was to analyse effects of elevated CO₂ levels on phosphorus pool sizes and uptake. The phosphorus dynamic was followed in a CO₂-manipulation mesocosm experiment in the Storfjärden (western Gulf of Finland, Baltic Sea) in summer 2012 and was also studied in the surrounding fjord water. In all mesocosms as well as in surface waters of Storfjärden, dissolved organic phosphorus (DOP) concentrations of 0.26 ± 0.03 and 0.23 ± 0.04 µmol L⁻¹, respectively, formed the main fraction of the total P-pool (TP), whereas phosphate (PO₄) constituted the lowest fraction with mean concentration of 0.15 ± 0.02 in the mesocosms and 0.17 ± 0.07 µmol L⁻¹ in the fjord. Transformation of PO₄ into DOP appeared to be the main pathway of PO₄ turnover. About 82 % of PO₄ was converted into DOP whereby only 18 % of PO₄ was transformed into particulate phosphorus (PP). PO₄ uptake rates measured in the mesocosms ranged between 0.6 and 3.9 nmol L⁻¹ h⁻¹. About 86 % of them was realized by the size fraction < 3 µm. Adenosine triphosphate (ATP) uptake revealed that additional P was supplied from organic compounds accounting for 25–27 % of P provided by PO₄ only. CO₂ additions did not cause significant changes in phosphorus (P) pool sizes, DOP composition, and uptake of PO₄ and ATP when the whole study period was taken into account. However, significant short-term effects were observed for PO₄ and PP pool sizes in CO₂ treatments > 1000 µatm during periods when phytoplankton biomass increased. In addition, we found significant relationships (e.g., between PP and Chl a) in the untreated mesocosms which were not observed under high fCO₂ conditions. Consequently, it can be hypothesized that the relationship between PP formation and phytoplankton growth changed with CO₂ elevation. It can be deduced from the results, that visible effects of CO₂ on P pools are coupled to phytoplankton growth when the transformation of PO₄ into POP was stimulated. The transformation of PO₄ into DOP on the other hand does not seem to be affected. Additionally, there were some indications that cellular mechanisms of P regulation might be modified under CO₂ elevation changing the relationship between cellular constituents.