Nutrients And Primary Production Coupling In The Changjiang Estuary And Its Indication To The Trophic Status

Changjiang Estuary and adjacent East China Sea (ECS) was one of the most productive marginal sea in the world, nutrients was the basis of the primary production. It is of great importance to study nutrients support and limitation of primary production, as well as the influence of primary production process on nutrient concentrations and structure along the salinity gradient, trophic mode and status of the Changjiang Estuary pelagic ecosystem, which are the key issue to understand the estuarine nutrients and carbon balance, evaluate organic carbon production and export efficiency, forecast the tendency of ecological effects in the estuary.Nitrate (NO3-), phosphate (PO43-), silicate (SiO32-) behavior along the salinity gradient were discussed based on the four seasons data. Nutrients absolute limitation and potential relative limitation area were pictured with the nutrients concentration and the Redfield ratio. Phytoplankton growth and the impact on nutrients structure under fresh-saline water mixing and surface-bottom mixing were discussed according to the in situ simulation experiments. Based on the three end-member mixing model, aptake of nutrients by the primary producer and Net Ecosystem Production (NEP) were estimated in the euphotic layer. NEP at coast, plume and offshore were acquired using the variation of oxygen at the mooring stations along whole water column during 24 hours, which was corrected with air-surface oxygen flux. Finally, estuarine NEP in four seasons and 5 years along the 31°N transect were estimated with LOICZ water salinity balance model.The results showed:1. nutrients increased with salinity decreased, presenting generally conservative mixing in the four seasons, while deviation along salinity gradient occurred due to primary production. In summer at the plume, NO3-, PO43-, SiO32- were consumed to different extent.2. Generally coastal area presented phosphorus relatively potential limitation (P RPL), while Si and P RPL occurred at the plume in summer where diatom bloomed.3. The higher the freshwater percentage was, the faster the phytoplankton grew. Algae blooms could resulted in increasing DIN/P and decreasing Si/DIN, changing the nutrients structure of the seawater.4. Based on the nutrients deviation from the conservative mixing, NEP showed positive in the euphotic zone, which meaned autotrophic status. Whereas the whole pelagic ecosystem presented heterotrophic within the 31°N transect mooring stations in summer.5. The four seasons NEP in the area and 5-year NEP variation along the 31°N transect indicated heterotrophic status in Changjiang Estuary pelagic ecosystem, it was strongest in summer and weakest in winter. Moreover, estuarine heterotrophic status was accelerating based on the 31°N transect NEP variation.This study indicated nutrients support and limitated primary production, while primary producer uptake nutrients under different circumstances, hence resulting in nutrients structure change. The budget results of nutrients in the models indicated it was autotrophic in the euphotic zone in summer, but whole pelagic zone showed accelerating heterotrophic status, which was of great importance for studying Changjiang Estuary biological effects and evaluating its tendency.