In coastal environments, acidification and eutrophication affect the physiology of marine macroalgae. We investigated the responses of Ulva pertusa Kjellman (Ulvales, Chlorophyta) under such conditions. Samples were cultured at two different pH settings (low, 7.5; high, 8.0) and at three different ammonium levels (low, 4; medium, 60; high, 120 mu M NH4 (+)). Our objective was to analyze the influence that elevated CO2 and NH4 (+) might have on pH, oxygen evolution, rates of nutrient uptake, chlorophyll fluorescence, growth, and C/N ratio of that organism. Variability in pH value was enhanced under low pH/high NH4 (+) and was significantly different (p < 0.05) from changes measured when the high pH/low NH4 (+) combination was applied. Rates of NH4 (+) uptake and relative growth rates by U. pertusa were increased under low pH/high NH4 (+) conditions and that response was significantly different (p < 0.05) from the other treatments. The rate of photosynthetic oxygen evolution and chlorophyll fluorescence were increased under elevated NH4 (+) concentrations (p < 0.05). However, the C/N ratio of U. pertusa was not affected by higher concentrations of CO2 and NH4 (+) (p > 0.05). Our results indicated that the physiological reactions of this alga were heightened when exposed to either the elevated combination of CO2/NH4 (+) or even when only the level of NH4 (+) was raised. Although such excessive growth can lead to bloom formations in coastal areas, this species also has greater capacity for taking up nutrients and dissolved inorganic carbon.