The influence of environmental variables on Ulva rigida C. Ag. growth and production

During the last few decades the increase of nutrient load, in addition to the reduced water exchange, has caused an abnormal increase of Ulva growth in the Lagoon of Venice. In order to assess the influence of environmental variables (light, temperature and nutrient concentrations) on Ulva growth and production, experiments in controlled and semi-natural conditions were carried out. Net production, growth and nutrient uptake rates increased with temperature from 10 to 20 °C while they showed a slight reduction at 30 °C. The net production rate increased with light intensity up to about 475 μmol s-1 m-2 and then decreased at higher illumination values. Net production occurred even in very low light (1.9 μmol s-1 m-2) and only a slight inhibition of photosynthesis was caused by light intensities as high as 2280 μmol s-1 m-2. Moreover, the photosynthetic capacity was not severely damaged by dark exposure for a relatively long time. Despite a preference for ammonia as a nitrogen source, nitrate can also be used when ammonia is scarce. Phosphorus availability did not affect the nitrogen uptake rate nor oxygen production, while the growth rate was consistently reduced under phosphorus limitation. The results confirm the wide capacity of adaptation to environmental variations and the high growth potential of this macroalga.