Nitrogen And Oxygen Isotopic Composition Of Nitrate In Polar Oceans And Nitrification In The Antarctic Peninsula

Nitrogen and oxygen isotopic compositions of nitrate and nitrification rate were measured in the Bering Sea during the 6th Chinese Arctic Research Expedition and in the Antarctic Peninsula during the 32nd Chinese Antarctic Research Expedition.Spatial variations of nitrate ?15N and ?18O value and their controlling factors were discussed.The fractionation factors of nitrogen and oxygen isotope during phytoplankton assimilating,as well as 815N value of nitrate from remineralization of particulate organic matter in the Bering Basin were estimated based on the isotopic model.The variation of nitrification rate and its controlling factor in the Antarctic Peninsula was discussed.The ?15N and ?18O values of nitrate in surface water were higher than those in the deep water in the Bering Basin.The ?15N and ?18O values of nitrate decreased with the increasing depth in the euphotic layer,and correlated with the concentration of NO3-,indicating that the variation of nitrate ?15N and ?18O values was mainly regulated by nitrate assimilation by phytoplankton.Based on the Rayleigh model,the fractionation factors of nitrogen and oxygen isotope during assimilation were estimated as 5.1‰ and 4.9‰ respectively.In 100-500 m layer in the Bering Basin,the remineralization of particulate organic matter was the major process controlling the variation of nitrate and oxygen isotopic compositions of nitrate.The ?15N values of the newly nitrified nitrate were estimated as 4.2-4.9‰ based on the isotopic mass balance.The ?15N and ?18O values of nitrate were almost constant as?6.0‰ and 1.5-4.0‰o respectively,in deep water below 500 m in the Bering Basin.The ?15N and ?18O values of nitrate in the Bering shelf were in the range of 6.9-17.6‰ and 3.0-10.7‰,respectively.Their variations were mainly controlled by phytoplankton assimilation.Nitrification also has an effect on the variation of the?15N and ?18O value in nitrate,but to a less extent.The ?15N and ?18O values of nitrate in the bottom water were affected by denitrification in addition to assimilation and nitrification.The fractionation factors of nitrogen and oxygen isotopes during nitrate assimilation were estimated as 5.9‰ and 5.6‰ in the Bering Strait via the steady state model,and 4.2‰ and 3.1‰ in the Bering Shelf via a the Rayleigh model,respectively.The spatial variation of fractionation factors of nitrogen and oxygen isotopes was influenced by the growth rate and species composition of phytoplankton.The ?15N and ?18O values of nitrate were 5.2-13.7‰ and 1.8-9.6‰ in the Antarctic Peninsula,respectively.The high values of ?15N and ?18O were observed in surface water,and decreased with the increasing depth.The averaged ?15N value of nitrate in the winter residual water was 7.5 ±0.4‰.The variations of nitrogen and oxygen isotopes in nitrate were mainly controlled by nitrate assimilation by phjytoplankton in the Antarctic Peninsula.A positive correlation between nitrate 815N and ?18O value and the fraction of sea-ice melted water was found,further indicating the role of phytoplankton assimilation on isotopic composition of nitrate.The melted waters of sea-ice or glacier may strengthen the stratification of water column,and stimulate the growth of phytoplankton,and thus regulate the nitrogen and oxygen isotope composition of nitrate via assimilation.The ratio of ?18O to 515N in the temperature minimum layer with the residual of the winter water was slightly larger than 1:1,which may reflect the effect of nitrification,but this effect was low compared to that of phytoplankton assimilation.The fractionation factors of nitrogen and oxygen isotopes during nitrate assimilation were estimated as 4.1‰ and 4.3 ‰ in the Antarctic Peninsula based on the Rayleigh model.Nitrification rates in the Antarctic Peninsula were determined as 0.61-5.95 nmol/dm3/d in the upper 100 m with an average of 1.59-4.13 nmol/dm3/d.The highest rates of nitrification occurred at depths of 25 m and 50 m with the low rates observed in surface water and 100 m layer.A positively linear correlation between nitrification rate and NH4+ concentration was observed,indicated that ammonium plays an important role in nitrification process.Nitrification in the surface water was suppressed by light intensity and the competition for ammonium with phytoplankton.