This study was aimed at determining the optimal depth and photon irradiance for growth of Sargassum fulvellum. Sampling and measurement of underwater irradiance were carried out at farms cultivating S. fulvellum at Wando, southwestern coast of Korea, from May 2004 to April 2005. Growth of thalli, underwater irradiance and photosynthetic quantum yield were measured over a range of depths for three culture stages. During their nursery cultivation stage (Stage I), length increase was greatest at 1.5 m depth (2.5±0.2 cm), where the average midday irradiance over 28 days was 488±58 μmol photons m −2 s −1 . During the pre-main cultivation stage (Stage II), the greatest length increase occurred at 1 m depth (10.9±0.1 cm) with an average irradiance of 845±169 μmol photons m −2 s −1 . For the main cultivation stage (Stage III) of the alga, thalli showed maximal length growth in March and early April at depths of 1-2 m and 3 m. These results suggest that growth at each cultivation stage of S. fulvellum could be controlled by depth of cultivation rope.

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