Iron And Phosphorus Limitation Of Nitrogen Fixation In The South China Sea And Its Response To Ocean Acidification

Biological N2 fixation is an important part of the marine nitrogen cycle as it provides a source of new nitrogen that can balance marine N-budget and regulate net sequestration of atmospheric carbon dioxide(CO2).The South China Sea(SCS),one of the world's largest marginal seas,is also a place where nitrogen fixation activities occur.So far,there are few studies on the nutrients limitation of N2 fixation in the SCS.In addition,the increase of atmospheric CO2 concentration caused by human activities directly changes the concentration of dissolved CO2 and the pH of seawater,indirectly affecting the surface temperature,the stratification of water bodies and the supply of nutrients.In turn,the growth,production and community structure of phytoplankton will be affected.Our study investigated the surface nitrogen fixation rate and the structure of diazotroph in the western of SCS and combined with hydrological environment parameter,revealed the upwelling and diluted water's effect on the distribution of the nitrogen fixation,and discussed the potential mechanism of regulating nitrogen fixation.In addition,the field culture experiment of nutrients added and acidizing experiment were conducted to define the type of phosphorus(P)and iron(Fe)nutrients limitation in the northern of SCS and discussed the effects of ocean acidification on nitrogen fixation and carbon fixation.The main conclusions were obtained as follow:(1)Nitrogen fixation rate for the surface water of the western of SCS varied from 0.04 to 1.42 nmol·N·L-1·d-1.High nitrogen fixation occurred in the southwest region where may be influenced by dilutes water of the Mekong river.Trichodesmium spp.was main diazotrophs in our study region(79%-92%).Under the influence of dilute water,the input of nutrients(such as P and Fe)and the strengthening of water stratification are beneficial to the occurrence of nitrogen fixation.However,in the northwest region of the survey,the upwelling caused by cold eddy increased the input of surface nutrients and stimulated the growth of non-nitrogen fixation organisms,but might cause a competitive disadvantage of nitrogen-fixing organisms.(2)The results of nutrients added experiment of P and Fe in the northern of SCS showed that the addition of P or P and Fe can increase the rate of nitrogen fixation,while the addition of Fe alone has no effect on the nitrogen fixation rate.Therefore,the surface nitrogen fixation of NL station in the northern of South China Sea may be limited by P.This may be due to the fact that the atmospheric subsidence is relatively adequate in the NL station,thus supporting the growth of phytoplankton,in turn leading to the P limitation.(3)The experimental results of acidification in the northern of SCS show that:acidification reduced the rate of nitrogen fixation of the +Fe and +P groups,while no effects on the treatment group with both P and Fe added.Our results show that acidification inhibits biological nitrogen fixation in the South China Sea,and the effects of acidification may be regulated by nutrients conditions.