| Seven number of voyages investigation data obtained from 2005 to 2006. The environment quality of Hongsha Bay in Sanya was analyzed and evaluated based on three number of voyages investigation data which were the most representative data reflecting the environmental status in spring, summer and autumn of Hongsha Bay.The results indicated that the concentration of inorganic nitrogen of almost 100% stations in three seasons was inferior to the Gradeâ…¡of sea water quality standard, 47% of which stations was inferior to the Gradeâ…¢of sea water quality standard and 33.3% stations was poorer than Grade IV of sea water quality standard. Especially, the contamination of inorganic nitrogen was heavy in summer in the whole Bay and was attributed to the Gradeâ…£of sea water quality standard. However, the concentration of active phosphate of most of investigated stations was belonged to the Gradeâ…¡of sea water quality standard, while 13.3% of the stations did poorer than the Grade IV of sea water quality standard. The average nutritive indexes in each season was 3.66~4.33, presenting a heavy level of eutrophication. The majority of organic contamination was over medium, whereas that of 53.3% of the stations was heavy. The investigation also indicated that the discharge of industrial and agricultural waste water and the drainage of domestic sewage along the bay as well as the unorderly and excessive development of mariculture were the main factors resulting in eutrophication in the Hongsha Bay. It is necessary to control sewage draining and put more stress on protection of marine environment.During the investigation, two harmful algal blooms (HABs) were found; one occurred from April to May, another occurred in July. The process of Skeletonema costatum (S.costatum) bloom from 27 April to 4 May 2006 was monitored. We observed and analyzed the dynamic variety of environmental factors during this HAB, especially, the relationship between algae biomass density and environmental factors. The results indicated that the concentrations of dissolved inorganic nitrogen, inorganic phosphate and iron were high, which provided the necessary nutrient foundation for the outbreak of HAB. The persistent rain elevated water temperature and reduced the content of salinity before the HAB so that it conduced the rapid proliferation of S. costatum and resulted in the outbreak of this HAB. Then, the persistent high temperature and lack of wind after the rain promoted its development. Thereafter, the S. costatum bloom dissipated along with the exhaustion of phosphate and silicate. Three available statistical methods including correlation analysis, regression analysis and principal components analysis were used to determine the mechanism of HAB. The results showed that critical environmental factors, such as temperature,phosphate, silicate and iron levels, played important role in the process of the S. costatum HAB.In order to reduce the pollution and the HABs in Hongsha Bay, we adviced it is necessary to take measure on protecting the environment in the bay and doing the research of HABs prevention and cure. To sustainablely prevent and control occurrence of HABs effectively in tropical harbor,we suggested use ecology measures (such as planting mangrove, breeding seashell, fostering eucheuma cottonii and cultivating microbe which can improve marine sediment et al ) to prevent and cure the HABs in Hongsha Bay. Mangrove not only can cleanse sewerage but also have allelopathy to other plants. We studied the effect of mangrove powder, mangrove powder water steep, and mangrove root secretion on the growth of S. costatum. The results indicated that the growth of S.costatum was inhibited by mangrove powder, mangrove powder water steep and mangrove root secretion. We found that mangrove have allelopathy to the growth of S.costatum. Furthermore, there were not positive proportion between the effect of allelopathy and the content of total hydroxybenzene in mangrove plants, which implied that there was some other allelopathy substance in mangrove plants.
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