Distribution features of nutrients and flux across the sediment-water interface in the Sanggou Bay

Four cruises are conducted in the Sanggou Bay in April, July, November 2006 and January 2007, nutrient distributions, structure features, primary control processes and exchange fluxes across the sediment-water interface are dscussed.The results indicate that nutrient concentrations show obvious seasonal changes. The concentrations of NO^-₃, NO^-₂, PO^3-₄, DOP, TDP and SiO^2-₃are the highest in autumn, while the concentrations of NH⁺₄-N, DON and TDN show the highest in summer. The concentrations of nutrients are the lowest in spring except DON. In spring, the water exchange between the Sanggou Bay and the Huanghai Sea and the uptake by large phytoplankton result in the concentrations of nutrients at lower level in the bay. In summer and autumn, the riverine input affects the nutrient levels. In winter, the nutrient levels are affected by coastal waters . The structure of P shows a more obvious change and DOP is the major component in summer which can be 81%. The structure of N transforms from DON to DIN domination. The atom ratio of Si to N changes slightly in the whole year, while the atom ratios of Si to N and N to P in spring and summer are higher those that in autumn and winter. Based on the analysis of nutrient limiting criterion and the lowest threshold values of the phytoplankton growth, P is the limiting factor for the phytoplankton growth in spring and summer in the Sanggou Bay. Si is also a potential limiting factor in spring with the concentration lower than the threshold value of the phytoplankton growth, which shows that the contributions of NH⁺₄, PO^3-₄ and SiO^2-₃fluxes released from the sediment to primary production is quite limited. Comparing to other shallow sea environments, the benthic nutrient fluxes in the Sanggou Bay are ranked at the lower level.