| Rushan, located in Shandong province, is famous for Pacific oyster （Crassostreagigas） culture. There are two different oyster culture ways performed in local area. Inone way oysters are cultured on suspended longlines throughout the year （AC）, whilein another way oysters farming on longlines only half a year in September-March（HC）. In this study, we selected two representative oyster farming areas for the twoculture ways, Xihuang （I） and Xujia （II）, and compared environmental parameters oftemperature, salinity, nutrient concentrations and abundances of phytoplanktonbetween the two areas over annual cycles.The averaged temperature, salinity and pH at the two sites are the same,14.2oC,30‰, and8.01. But the water transparency was quite different in the two areas, whichwas about108.2cm （I） and50.2cm （II）, respectively.Phosphorus content at the two sites was0.28±0.12μmol·L-1（I） and0.47±0.28μmol·L-1（II）, and the annual mean dissolved inorganic nitrogen （DIN） was12.9±6.6μmol·L-1（I） and11.9±9.4μmol·L-1（II）, respectively. Nitrate nitrogen accounted forthe vast majority of DIN, and the concentration was10.17±6.11μmol·L-1（I） and9.23±8.84μmol·L-1（II） in the two areas. The content of the nitrite and ammoniumnitrogen at Xihuang was0.75±0.73μmol·L-1and2.07±0.89μmol·L-1, while that atXujia was0.72±0.75μmol·L-1and2.04±1.53μmol·L-1, respectively. Owing to theconsistence of nitrogen and variation in ammonium between the two areas, it issuggested that the oyster farming might have significant effect on ammonium contentin seawaters through oyster excretion.The eutrophication status was assessed by eutrophication assessment indices （E）and nutritional quality indices （NQI）, respectively. Results showed that theconcentration of total nitrogen （TN）, total phosphorus （TP）, COD and Chl-a was atthe range of2.01-7.00μmol·L-1,11.38-75.45μmol·L-1,1.03-3.41mg·L-1and0.68-42.17μg·L-1, respectively. Basis on these data, E values of the two regions showed that spring and summer months both were below1, which indicating thatwater quality was in a benign condition. Nevertheless, almost year-round NQI valueswere higher than4, demonstrating that the annual water quality was in eutrophic state.The inconsistence might result from the effect of phosphorus-rich sediment particulatematters that resuspended to water column under the perturbations from tide and wind.The significantly different change in Chl-a showed clearly that biomass ofphytoplankton was inhibited by oyster cultivation. Despite no direct influence on thenitrogen and phosphorus concentration, the environmental factors could be changedby cultivated oysters after the accumulating sediments through their filtration wererecycled back into upper water column.A total of77species phytoplankton belonging to7phyla and42genera wereidentified in the studied waters. Diatoms were not only the majority in species, butalso the main phytoplankton group in biomass. The dinoflagellates were alsoimportant in the community, and could become the dominant species, especially inwarmer months. Though there were some differences in dominant group along withthe seasonal changes, the Chroomonas acuta, Nitzschia longissima, and Skeletonemacostatum were always found in the key species.Finally, the result of assessing carrying capacity at the oyster farming areas wasabout0.237million tons. The oyster production of Rushan was about0.174milliontons in2010. It was indicated that there was still some productive potential to beexploded. |
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