N₂ Fixation In Typical Bays And The South China Sea

Modifications of 15N2 tracer assay were carried out in this study and the modified 15N2 tracer assays were used to field study.N2 fixation rate in the water of Jiaozhou Bay and Daya Bay were determined during 16 cruises.The nitrogenase gene were also determined to observe the composition of diazotrophs.The spatial and temporal variation and the controlling factors of N2 fixation in Jiaozhou Bay and Daya Bay were investigated.The modified 15N2 tracer assays were also used to determine the N2 fixation rate in the open areas in the South China Sea.The major results we obtained were as follows:(1)N2 fixation rate in Jiaozhou Bay and Daya Bay ranged from n.d.-1.53 and n.d.4.51 nmol N L-1 h-1 respectively.Such values were generally higher than those reported in the open waters of the Yellow Sea and the South China Sea,thereby suggesting that the changing coastal waters may be "hotspots" of N2 fixation,which have been ignored for a long time.(2)Diazotrophic proteobacteria were dominated in Jiaozhou Bay and Daya Bay,indicating that organic carbon may be an important controlling factor of N2 fixation.This may be ubiquitous in global oceans.(3)In Jiaozhou Bay and Daya Bay,the most likely autochthonous source of bioavailable organic carbon may be the produce of photosynthetic organic carbon by phytoplankton,which may stimulate N2 fixation.Phytoplankton may influence N2 fixation by controlling the quantity and quality of organic carbon in Jiaozhou Bay and Daya Bay.(4)N2 fixation rate in the 0-200 m column of the South China Sea ranged from n.d.-2.08 nmol L-1 d-1.N2 fixation rate in the northeastern South China Sea were higher than other areas.Diazotrophic proteobacteria and unicellular cyanobateria were major components of diazotrophs during the investigation in March 2017.According to the depth-integrated N2 fixation rate in this study,the flux of N2 fixation in the South China Sea is 3.6×1010 mol N yr-1.(5)Comparison of N2 fixation rate determined by 15N2-bubble method and 15Ndissolved seawater method were carried out in the Southwestern South China Sea and Daya Bay.N2 fixation rate determined by 15N-dissolved seawater method was about 0.8-18.5 times of that determined by 15N2-bubble method.The ratio of N2 fixation rate determined by the two methods varied obviously in different seasons.It is hard to determine whether the previously reported N2 fixation rate based on the existing 15N2bubble method were underestimated according to the comparison of the existing methods.A modified 15N2-bubble method was used to determine N2 fixation in Daya Bay and the northeastern South China Sea.The results indicated that N2 fixation rate measured by the existing 15N2-bubble method accounts for only 52%of the real rate.