Effects Of Exchange Fluxes Of Nutrients At The Sediment And Water Interface And The Huanghe Input On Nutrient Dynamics Of The Bohai Sea

With the rapid economic development around the Bohai Sea, the environmental quality of the Bohai Sea is not optimistic. Eutrophication and frequent redtide have become serious ecological problems. Bohai Sea is the only inland sea in our country and its ecological environment plays an important role in economic and social development. The nutrient concentrations and distribution in water and sediments of the Bohai Sea are discussed in this study; Exchange fluxes of nutrients at the sediment and water interface are evaluated based on diagenesis model and its contribution to nutrient loads of the Bohai Sea is also studied; The concentrations of nutrients in the Huanghe and its fluxes to the Bohai Sea are studied in order to understand the effect of Huanghe input on nutrient dynamics of the Bohai Sea.High concentrations of nutrients in the Bohai Sea are mainly shown near the estuaries and coastal areas. Compared with other seas, the proportion of ammonium in dissolved inorganic nitrogen is higher in the Bohai Sea due to the increase in chemical fertilizer application, domestic sewage and industrial waste over the catchments areas. Phosphorus is the potential limiting element for phytoplankton growth in the Bohai Sea. From 1982 to 2008, the concentrations of ammonium, nitrate, nitrite and dissolved inorganic nitrogen in the Bohai Sea increased, while the concentrations of phosphate and dissolved silicate decreased. The concentrations of nitrogen in the Bohai Sea increased with the increase in chemical fertilizer application, domestic sewage and industrial waste input; Phosphate is dominated by atmospheric deposition and riverine input in the Bohai Sea. Because very limited amounts of phosphorus canbe exported off the soil, the phosphate levels decreased in the Bohai Sea; The concentrations of silicate in the Bohai Sea decreased due to the decrease in fresh water discharge from the Huanghe.We studied distribution of organic carbon, nitrogen, phosphorus and biogenic silica in sediments of the southern and central Bohai Sea. The results indicated that the concentrations of various forms of nitrogen, phosphorus and organic carbon were higher in the shallow coastal sediments than those in the central Bohai Sea because of the effects of terrigenous inputs, and the distribution of biogenic silica is closely related to that of diatoms; Exchangeable nitrogen accounted for 3.7% of total nitrogen and fixed ammonium accounted for 38% affected by the content of organic carbon and clay mineral composition in sediments; The OC/BSi, ON/BSi and OP/BSi molar ratios are 2.3,0.19 and 0.03 respectively in surface sediments, lower than the Redfield ratio; The average molar ratio of OC/ON is 13.2 in surface sediments, which was higher in the northwest of the Bohai Sea and the Bohai Strait; OC and ON in surface sediments were mainly from terrigenous input; The molar ratios of OC/ON in sediment cores ranged from 6.0 to 14.0, which decreased slightly in the upper 5cm indicating that ON was enriched in the sediments.Exchange fluxes of nutrients at sediment and water interface in the Bohai Sea were evaluated based on diagenesis model:The fluxes of NH4+, DIN, PO43- and SiO32- are released from sediment to seawater, while NO3- shows a net exchange flux from water to sediment. The benthic nutrient fluxes are mainly affected by nitrification rate, denitrification, contents of organic nitrogen in sediments, authigenic mineral precipitation, silica dissolution, bioturbation and porosity in sediment. Nutient fluxes released from sediment can support primary production with 10% dissolved inorganic nitrogen,1.1% phosphate, and 10% silicate requirement for phytoplankton.Monthly fluxes of dissolved nutrients were observed from November in 2008 to December in 2009, which show a seasonaly change with high values in the flood seasons and low values in the dry seasons. During water and sediment regulation periods in the Huanghe in 2009, nutrients fluxes account for 40% of the annual fluxes, while the suspended particulate matter flux accounts for 79% of the annual fluxes; Due to the water and sediment regulation in Huanghe, the concentrations of NO2- and NH4+ decreased, the concentrations of NO3-, DIN, DON, TDN and TDP increased slightly, and the concentrations of PO43- and SiO32- increased significantly, while the concentrations of DOP fluctuated during investigation previous. The concentrations of NH4+, NO2-, NO3-, DIN, TDN, PO43-, TDP and SiO32- in the Huanghe Estuary increased due to the water and sediment regulation in Huanghe, and the concentrations of DON and DOP remained stable, while the molar ratio of N/P decreased and the molar ratio of Si/N increased. Compared with exchange fluxes of nutrients at the sediment and water interface, the flux of NO3- from Huanghe input accounts for greater contribution(27%) to the nutrient loads of the Bohai Sea, but the fluxes of NH4+, NO2 and PO43- account for smaller contribution. Over the last three decades, the concentrations of DIN increased significantly mainly affected by increase in population and application of nitrogen fertilizer, the concentrations of DON and PO43- fluctuated, while the concentrations of DOP and SiO32- decreased; From 2001 to 2009, the annual fluxes of DIN, TDN and SiO32- before 2004 increased, but decreased slightly in 2009 because of the decrease of water discharge, while the annual flux of DON fluctuated; The annual flux of PO43- increased from 2001 to 2009, the annual flux of DOP increased before 2003 and decreased after that; The annual flux of TDP was low in 2001 and 2002, and highest in 2003(11.95×106mol), but decreased after 2003.