While there is growing interest in understanding how marine life will respond to future ocean acidification, many coastal ecosystems currently experience intense acidification in response to upwelling, eutrophication, or riverine discharge. Such acidification can be inhibitory to calcifying animals, but less is known regarding how non-calcifying macroalgae may respond to elevated CO2. Here, we report on experiments performed during summer through fall with North Atlantic populations of Gracilaria and Ulva that were grown in situ within a mesotrophic estuary (Shinnecock Bay, NY, USA) or exposed to normal and elevated, but environmentally realistic, levels of pCO(2) and/or nutrients (nitrogen and phosphorus). In nearly all experiments, the growth rates of Gracilaria were significantly increased by an average of 70% beyond in situ and control conditions when exposed to elevated levels of pCO(2) (p<0.05), but were unaffected by nutrient enrichment. In contrast, the growth response of Ulva was more complex as this alga experienced significantly (p<0.05) increased growth rates in response to both elevated pCO(2) and elevated nutrients and, in two cases, pCO(2) and nutrients interacted to provide a synergistically enhanced growth rate for Ulva. Across all experiments, elevated pCO(2) significantly increased Ulva growth rates by 30% (p<0.05), while the response to nutrients was smaller (p>0.05). The delta C-13 content of both Gracilaria and Ulva decreased two-to-three fold when grown under elevated pCO(2) (p<0.001) and mixing models demonstrated these macroalgae experienced a physiological shift from near exclusive use of HCO3- to primarily CO2 use when exposed to elevated pCO(2). This shift in carbon use coupled with significantly increased growth in response to elevated pCO(2) suggests that photosynthesis of these algae was limited by their inorganic carbon supply. Given that eutrophication can yield elevated levels of pCO(2), this study suggests that the overgrowth of macroalgae in eutrophic estuaries can be directly promoted by acidification, a process that will intensify in the coming decades.