An ecological demographic study was conducted from January to December 2009 at Changshan Island on the northern side of Shandong Peninsula, China. The study investigated stem regeneration and the resource trade-off between sexual reproduction and vegetative regeneration of the brown alga Sargassum thunbergii at the individual thallus level. Stem regeneration from the holdfast occurred year round but with significant seasonality, peaking in the early summer. Both percentage of fertile stems and reproductive effort were negatively correlated with stem regeneration in the middle and later periods of sexual reproduction. Furthermore, the correlation coefficients increased significantly with time. These observations suggested that an increase in resource allocation to sexual reproduction would lead to a decrease in allocation to vegetative regeneration during the reproductive period. Reproductive effort was lower and stem regeneration was higher in the wave-exposed location compared with the sheltered location, indicating that allocation of resources between sexual reproduction and vegetative regeneration varied with location. When reproductive tissues were removed, the number of new stems produced from holdfasts increased, and when new stems were removed, an increase in reproductive effort occurred. These results suggest that trade-off between sexual reproduction and vegetative regeneration occurs in S. thunbergii.

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Abstract:The responses of Sargassum thunbergii germlings to high temperature, low salinity, desiccation, combined thermal and osmotic stress (35 ºC combined with 12 psu), anthracene, and eutrophication were examined. Probit regression analysis results showed that the median lethal time (LT50) values of high temperature decreased with the increase in temperature. The 24 h median lethal temperature was 36.9 ºC. For salinity treatment, the LT50 value of fresh water was 47.6 h. Survival rates of germlings were over 60% when germlings were exposed to salinities ranging from 27 psu to 7 psu at a time interval of 108 h post-treatment. The LT50 values of desiccation and combined thermal and osmotic stress (35 ºC combined with 12 psu) were 7.0 h and 9.8 h, respectively. Anovas showed that germlings were inhibited by high concentrations of anthracene (5 mg L -1 and 10 mg L -1 ) with low survival rates of below 50% and low relative growth rates of below 1% after 25 days of culture; however, low concentrations (0.01-1 mg L -1 ) had no significant effects. In addition, neither severe eutrophication nor disproportionality of N/P showed any significant effect on the survival and growth of germlings. Of the environmental stresses tested, possible occurrence of high temperature of 40 ºC and combined thermal and osmotic stress directly impacted the survival of germlings, suggesting that the deterioration of S. thunbergii bed may be related more to increasing extreme climatic events.