The influence of stocking density, light and temperature on the growth, production and nutrient removal capacity of Porphyra dioica (Bangiales, Rhodophyta)

The optimal conditions for growth of Porphyra dioica gametophytes were investigated in the laboratory, focusing on bioremediation potential. Porphyra dioica is one of the most common Porphyra species along the northern coast of Portugal and can be found year-round. The influence of stocking density and photon flux density (PFD) on the growth, production and nutrient removal was tested. Maximum growth rates, up to 33% per day, were recorded with 0.1 g fw l−1 at 150 and 250 μmol photons m−2 s −1 . Growth rate decreased significantly with increasing stocking density. Productivity (g fw week−1 ) had an inverse trend, with more production at the higher stocking densities. At 150 μmol m−2 s −1 and with 1.5 g fw l−1 , 1.4 g fw week−1 were produced. At this PFD, there was no significant difference in production between 0.6 to 1.5 g fw l−1 . Nitrogen (N) content of the seaweeds decreased with increasing stocking densities and PFDs. The maximum N removal was recorded at 150 μmol m−2 s −1 , with 1.5 g fw l−1 stocking density (1.67 mg N day−1 ). However, the N removed by thalli at 50 μmol photons m−2 s −1 was statistically equal to that at 150 and 250 μmol photons m−2 s −1 , at a stocking density of 1.0 g fw l−1 . The influence of temperature and photoperiod on growth and reproduction was also assessed. Growth rates of P. dioica were significantly affected by temperature and photoperiod. In this experiment (with 0.3 g fw l−1 stocking density), the highest growth rate, 27.5% fw day−1 , was recorded at 15 °C and 16: 8¯, L:D¯. Male thalli started to release spermatia 21 days after the beginning of the experiment, in temperatures from 10 to 20 °C and with 10, 12 and 16 h of day length. Unfertilized female-like thalli were observed at 10 to 20 °C, under all photoperiods tested. Growth of these thalli declined after 4 weeks. By then, formation of young bladelets in the basal portion of these thalli was observed. After 7 weeks all biomass produced was solely due to these vegetatively propagated young thalli, growing 22.4% to 26.1% day−1 . The results of this study showed that P. dioica appears to be a candidate as a nutrient scrubber in integrated aquaculture systems.