The effect of nutrient concentrations, nutrient ratios and temperature on photosynthesis and nutrient uptake by Ulva prolifera: implications for the explosion in green tides

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 mu mol 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 A degrees C, while the lowest rates were detected at 5 A degrees C. P uptake rates were correlated with increasing temperature.