Relationships between critical photon irradiance for growth and daily compensation point of juvenile Sargassum macrocarpum

This study was designed to determine the critical photon irradiance for gro wth and daily compensation point of juvenile Sargassum macrocarpum. Samplin g and measurement of natural light conditions were carried out in the S. ma crocarpum population at a depth of 8 m off Kiwado in Fukawa Bay, Yamaguchi Prefecture, Japan, from April to June 1998. Photosynthesis and respiration of the juvenile thalli, and diurnal changes in solar irradiance were measur ed for the same period. The critical photon irradiance for growth of the ju venile thalli observed on the population floor was 1.0-1.5% on the sea surf ace. The photosynthetic rate of leaf of juvenile thalli increased linearly with increasing photon irradiance when light levels were lower than 50 muM/ m(2) per s. The respiratory rate and light compensation point of the juveni le thalli were 2.49 muL O-2/cm(2) per h and 4.98 muM/m(2) per s, respective ly. The daily compensation point was estimated with a mathematical model ba sed on photosynthesis-light equations and diurnal changes in solar irradian ce. For a day of average solar irradiance over the period of the present st udy, the estimated daily compensation point of the juvenile thalli was 1.3% on the sea surface. This value agreed well with the observed critical phot on irradiance for growth of juvenile S, macrocarpum on the population floor . The results of the study confirm that the mathematical model is effective for estimating the daily compensation point.