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Effects of salinity, light intensity and sediment on growth, pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 µmol photons m−2 s−1) and sediment (0, 0.67 and 2.28 mg L−1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 µmol photons m−2 s−1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 ± 3.0 %DW) was at 25 psu, 150–400 µmol photons m−2 s−1 and no sediment. The highest pigment contents were phycoerythrin (0.8 ± 0.5 mg g−1FW) and phycocyanin (0.34 ± 0.05 mg g−1 FW) produced in low light conditions, at 150 µmol photons m−2 s−1. The highest photosynthesis rate was 161.3 ± 32.7 mg O2  g−1 DW h−1 in 25 psu, 400 µmol photons m−2 s−1 without sediment in the short period of cultivation, (3 days) and 60.3 ± 6.7 mg O2 g−1 DW h−1 in 25 psu, 700 µmol photons m−2 s−1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid‐January to the end of April (dry season), and from mid‐July to the end of September (first rainy season) when provided sediment was restricted.