Determining plant growth response to CO2 availability, ger-mane both because CO2 concentrations vary so widely, and because different species show varying sensitivities to CO2enrichment,typically involves growing plants in pots in laboratory or green-house environments. A specific methodological issue that has beenaddressed for terrestrial plants but largely overlooked for sub-mersed plants is the influence of pot size. Meta-analysis by Poorteret al. (2012) reveals that increasing pot size generally leads toincreased terrestrial plant growth. Variations in pot size, however,may or may not influence effects of CO2enrichment on photo-synthetic capacity and growth (Arp, 1991; McConnaughay et al.,1996).The primary objective of this study was to evaluate the impor-tance of pot size for determining responses of a submersed vascularplant to CO2enrichment. Our null hypothesis was that effects ofCO2enrichment on growth and biomass allocation are indepen-dent of pot size, in contrast to the alternative hypothesis that pot