Preliminary Study Of Nitrogen Fixation In The Northwestern Of South China Sea

Marine Biological nitrogen fixation is an important part of the marine nitrogen cycle,nitrogen anomalies in seawater and marine biological nitrogen fixation are closely linked.To use nitrate anomaly index N* and combined with the classic Redfeild theory,which are effective methods to judge the abnormal phenomenon of nitrogen in seawater,then through a mathematical model to calculate the budgets of “new” nitrogen from biological nitrogen fixation. Marginal sea are generally affected by a variety of physical processes, therefore the migration, transformation paths and budgets of “new” nitrogen from biological nitrogen fixation are with high spatial and temporal variability in marginal seas. The northwestern South China Sea(SCS) is an area with low latitude,high water temperature, adequate lighting, few nutrients, high atmospheric fallout, and a variety of physical processes, is an ideal area to carry out nitrogen fixation research in marginal sea.The winter of 2012 in the northwestern SCS open voyage, analyzed and researched of the typical stations collected samples.By analyzing the temperature, salinity and depth analysis, combined with the current observation data of history we could divide the northwestern South China Sea into Area A(the coastal waters of Western Guangdong),Area B(the eastern of Hainan Island) and Area C(the open waters).By comparison the development of nitrate abnormal or nitrogen abnormal index(N*)defining application in 3 areas,screened out an effective model to judge nitrogen anomaly in the northwestern of SCS.Further analysis of seawater nutrients concentrations of nitrogen,phosphorus and silicon and chlorophyll a concentration,the use of the classical Redfeild theory and N* definition, from two aspects to judge the nitrogen abnormal phenomenon in the northwest of SCS.Studying the sea voyage different regions nitrogen fixation found that there was a big difference in plane and the vertical distribution.Vertical distribution,from the surface to the bottom of N* values decreased in Area A,from the surface to the bottom of N* value was more consistent in Area B,from the euphotic N* values higher and more consistent in Area C,the N* value decreased rapidly from euphotic shallower.N* values in the plane distribution,asubstantially lower value of N*in the coastal area.Far shore regional distribution of N*value was higher. The minimum value of N* is-1.83 μmol/dm3 appeared in Area B of the S13 station 150 meters layer in area B,the maximum value of N* is 46.42 μmol/dm3 appears in area C of the Y105 station 50 meters layer in Area C,vertical distribution of oceanic biological nitrogen fixation was closer in Area C.In Area A and Area B,vertical distribution of biological nitrogen fixation may be affected that due to the SCS coastal current and vertical mixing physical processes,which was different from the model of open waters Area C.The voyage of the nutrient concentration distribution taken on the regional distribution:In Area A,higher concentration of nutrients mixed effects largely by the freshwater and coastal current;In Area B and Area C,The nutrient concentration was relatively low,where nutrients input from rivers are lack,But in Area C,a higher concentration of dissolved inorganic nitrogen,which may be related to nitrogen fixation of nitrogen to the euphotic zone.The correlation of nutrients showed that area B exist potential denitrification in winter of 2012;there may be a silicon deficiency phenomenon,especially in the euphotic layer(75 meters shallower).