| Sargassum thunbergii (Mert.) O'Kunte, a temperate macroalgae of high economic value which is quite common in western north Pacific, has a wide distribution along the coastal line of China from Liaotung Peninsula in the north to Leizhou Peninsula in the south. It has a broad application in fields of pharmacy and foodstaff with a long history in China. Sargassum thunbergii not only plays an important role in the marine ecosystem but also has great potential utilization in the exploiture of pharmacy, health care, aquaculture and chemical industry.Through both field studies and indoor experiments, the growth under natural environment, morphology of sexual reproduction, reproduction ecology of S. thunbergii were studied in Luyang Bay, Yantai,Shandong Peninsula (37°80′N, 120°75′E). The purpose of this paper was to probe into the ecological process of S. thunbergii respond to transformation of environmental factors to provide some help for artificial development of S. thunbergii and marine ecology protection.1 Studies on growth of S. thunbergii under natural environment Ecological investigation methods were applied to the study of growth and seasonal morphology diversification of S. thunbergii from Feburary 2005 to January 2006. S. thunbergii attained peak biomass (250.16±9.37 g dry weight m-2) and peak mean thallus length (587.43±156.75 mm) in July 2005, at that period, the sea water temperature was 22.5 oC which had not attained its peak yet. Another highest value recorded in December (46.27±2.74 g dry weight m-2, 132.58±21.79 mm). S. thunbergii recorded lowest biomass and mean thallus (17.83±3.03 g dry weight m-2, 38.92±16.35 mm) in September 2005. The seasonal varies of mean thallus length and the biomass of S. thunbergii showed the same trend. Length classe distribution relative frequency monitored from February 2005 to January 2006 showed S. thunbergii in this area belonged to smaller size. Significantly positive correlation was found between thallus and biomass(r = 0.968, p < 0.01). Significantly positive correlation was also found between biomass and air vesicles (r = 0.990, p < 0.01) which was firstly observed in early May 2005. Research of the environmental factors including temperature, wave action, tidal level and their associated effect on biomass was monitored from June 9 to August 1, 2007 in which 4 samples were collected. Results showed that temperature, tidal level, wave action had significant influence respectively on the growth of S. thunbergii at the initial stage (Jun.9 to Jun. 23) and later stage (Jul.20 to Aug.1) of sexual reproduction. Single influence of temperature, tidal level, wave action went down while the interaction between temperature and tidal level, temperature and wave action got strengthened at middle stage (Jun.23-Jul.20). Obviously, influence of these environmental factors on the growth of S. thunbergii showed a dynamic process.2 Studies on reproduction of S. thunbergiiS. thunbergii at Luyang Bay is available on the lower intertidal coast throughout the year. Thallus differed morphologically and physiologically with different color. In summer the thallus was filemot and beared lateral braches, air vesicles and receptacles while it was heavy dark from autumn to late spring next year beared no organs most of the period mentioned above. Eggs adhered to the receptacle when first released from their conceptacles. The zygote disengaged from receptacle 20 h approximately after fertilization. After taking transverse division twice firstly, one terminal cell developed into rhizoid and the two others developed into the germling. The juvenile started growing new branch when nearly reached 2mm in length.Receptacle which is the sexual reproduction organs of S. thunbergii was first observed in early June 2007 when the sea water temperature was 15.5℃. The sexual reproduction period was mainly from June to August. S. thunbergii reached peak sexual reproduction in middle July, with the biomass reproductive allocation 76.7±7.2% observed on July 15. The sexual reproduction came to an end in early August. Study showed that there is a significantly positive correlation between BRA and average length of lateral branches (p < 0.001, r = 0.869). The research of environmental factors on the fertilization of S. thunbergii was also carried out and the effect of these environmental factors also showed a dynamic process as it did on biomass. Four samples were collected from June 10 to July 22, 2007. ANOVA analysis showed that at the early stage (6.10-6.23) the influence of temperature, wave action, interaction between temperature and wave action, tidal level and wave action were significant. At the middle stage (6.23-7.10) temperature and wave action have significant influence on the sexual reproduction of S. thunbergii respectively. Then at the later stage (7.10-7.22) the only influence was from the temperature. Most fertile thallus had a length of over 10cm. Vegetative reproduction occurred in every month except from December 2006 to early March 2007 due to low temperature. But it has two concentrated growing periods which was respectively from April to July 2007 named"spring period"and from August to November 2006 named"autumn period". The latter produced more young thalli than the former. Vegetative reproduction is a principal strategy for the maintenance of S. thunbergii population.S. thunbergii life can be divided into 4 significant periods: inactivity period, growth period, reproductive period and senescent period. Before late March 2005, S. thunbergii was at inactivity period for low temperature. In this period, nearly no new thallus generated from holdfast, thallus grew slowly and both thallus length and biomass were at a low-level. From early April to mid June 2006 was the growth period in which the temperature increased, the thallus length and the biomass went up swiftly. Then came to the sexual reproduction period from mid June to late July in which the receptacle appeared and became fertile gradually. Both thallus length and biomass attained peak in this period. The last was senescent period which began from late July to September. In this period sea water temperature reached its peak, both main and lateral branches decayed, thallus length and biomass attained the lowest of the whole year at this time. Another growth period heeled the senescent before S. thunbergii went into the inactive period. There were two growth period in the one-year life circle which named spring-summer growth period and autumn-winter growth period respectively.
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