Abstract:High-density pond culture leads to water quality deterioration and wastewater discharge, which may result in pollution of pond and even coastal waters, and affect the sustainable development of pond culture. Domestic and foreign scholars generally believe that large-scale seaweeds cultivation is one of the effective ways to absorb and utilize nutrients and relieve the eutrophication.Sargassum thunbergii can rapidly absorb harmful substances in seawater, such as arsenic,zinc and cadmium, as well as NH+4-N,NO-3-N, P， etc.Therefore， it is suitable to cultivate this algae in cofferdam or artificial reef pond in co-culture with sea cucumber or abalone. S.thunbergi cultivation during spring and summer have been widely reported, however, culture during autumn and winter is still very limited.To investigate the growth characteristics of S.thunbergi in different stages and the impact of environmental factors, such as temperature, light intensity, salinity or pH, on both one year old (1-y algae) and two years old (2-y algae)seedlings, S.thunbergi were cultivated at different depths in pond from October 2010 to March 2011. Morphological observation was done and growth rate was determined on the seaweed.It was found that: 1)best growth was achieved at water-depths of 40~60cm, followed by that of 80~100cm,and the slowest growth was found at 0~20 cm;2) no receptacle was seen during the experiment, and 2-y algae set forth some 1~2 cm secondary branches, while 1-y algae did not; 1-y algae produced air sac in mid-November, while 2-y algae did the same at the beginning of the experiment; from mid-December, air sac began to decay and rot off, when water temperature was below 0℃ and pond was frozen in mid-January of the next year, most air sac fell off and the thalli began to rot with shortened length, thin frond, decreased fresh weight and black brown body;as water temperature gradually rose in late February, S. thunbergii began to grow, with increasing length and weight; some thalli produced air sac gradually in early March;3) during this experiment, 2-y algae grew better than 1-y algae, showing greater length and weight, It was found that factor S, which is the ratio of fresh weight to length of a single algae,was also greater in 2-y algae than 1-y algae.
