The green-tide macroalga, Ulva prolifera, was tested in the laboratory to determine its nutrient uptake and photosynthesis under different conditions. In the nutrient concentration experiments U. prolifera showed a saturated uptake for nitrate but an escalating uptake in the tested range for phosphorus. Both N/P and NO3 −/NH4 + ratios influenced nutrient uptake significantly (p < 0.05) while the PSII quantum yield [Y(II)] (p > 0.05) remained unaffected. The maximum N uptake rate (33.9 ± 0.8 μmol g−1 DW h−1) and P uptake rate (11.1 ± 4.7) was detected at N/P ratios of 7.5 and 2.2, respectively. U. prolifera preferred NH4 +-N to NO3 −-N when the NO3 −-N/NH4 +-N ratio was less than 2.2 (p < 0.05). But between ratios of 2.2 and 12.9, the uptake of NO3 −-N surpassed that of NH4 +-N. In the temperature experiments, the highest N uptake rate and [Y(II)] were observed at 20 °C, while the lowest rates were detected at 5 °C. P uptake rates were correlated with increasing temperature.