Nutrition Structure and Oligotrophy Analysis of Seawater in Liusha Bay (China)

The concentrations and distribution characteristics of nutrients in seawater at Liusha Bay of China were obtained based on the field observations in February (winter), May (spring), August (summer) and November (autumn), 2008. The results showed that the content of nutrients in seawater was rather low and the water body was oligotrolic in Liusha Bay. The annual average concentrations of NO₂-N, NO₃-N and NH₄-N were 0.77μmol L^-1, 3.44μmol L^-1 and 1.21μmol L^-1, respectively. And the annual concentrations of dissolved inorganic nitrogen (DIN), phosphate (DIP) and active silicate (DSi) were 6.17μmol L^-1, 0.30μmol L^-1 and 16.3μmol L^-1, respectively. As accounted for over 60% in DIN, NO₃ was the main form of DIN in seawater. The annual molar ratio of DSi/DIN/DIP was 54.0:20.4:1, which moved far away from the mean ratio of nutrients contained in marine diatoms. The marine productivity of Liusha Bay was restricted by DIN in summer and autumn and while by DIP in spring and winter. The nutrition level of seawater in Liusha Bay was assessed to be oligotrophic with the trophic state index. Excellent conditions of tidal current, existence of a large area of macroalgae, cultivated shellfish in a high density and few sources of nutrients input were considered as the main reasons of oligotrophy of seawater in Liusha Bay.