Spatial Distribution And Regulating Factors Of The Isotopic Composition Of Nitrogen Speciation In The Marine Environments:from Estuary To Open Ocean

Nitrogen(N)is a pivotal element in regulating marine primary productivity,and consequently its availability affects on the sequestration of anthropogenic carbon dioxide in coastal and marginal seas.Since the industrial revolution,anthropogenic N inputs have been increased rapidly,which have a similar amount of N as that of oceanic N2 fixation.Currently,the interference with the N cycle have already transgressed its boundaries,leding to an irreversible consequence.To better understand the feedbacks of marine N cycle on the antropogenic influences and global warming,interests in the marine N cycle has soared in the last decade.To understand the N cyling in the Jiulong River Estuary(JRE),the Changjiang river(CJ)plume and the Northwestern Pacific Ocean(NWPO),we measured the concentration and isotopic composition of N speciation(nitrate:NO3-,nitrite:NO2-,ammonium:NH4+ dissolved organic nitrogen:DON,and particulate nitrogen:PN).The main innovative results incuded:(1)In the JRE(Summer).A non-conservative pattern(N-shape)in ?15NNO2 versus 1/[NO2-]scatter plot suggested the involvement of four end-member rather than two end-member.Significant shift of N processes was found across the environmental gradient in the JRE.In the upper estuary,where tubidity was high,the highest ammonia oxidation rate(?20.43 ?mol L-1 d-1,62-fold higher than that of nitrite oxidation rate)contributed to the NH4+ removal and NO2-accumulation.However,in the middle-lower estuary,biogeochemical influences were insignificant due to the short residence time.According to the incubation experiments of N uptake(under continuous saturated illuminations),phytoplankton showed significant preference for NH4+ over NO3-and NO2-.Obviously,the suppression of NO3-uptake by NH4+ also contributed to the conservative behavior of NO3-.Basing on the significant NH4+ uptake(5.0-6.7 ?mol L-1 for 12-h-light:12-h-dark cycle),the water residence time was required to<1 d to maintain the conservative mixing of NH4+ in the middle-lower estuary.The isotope effect during NO3-assimilation by phytoplanktong were 15?NO3=8.4-9.6‰ and 18?NO3=9.9-11.2‰,respectively,which were consistent with the assimilation by diatom.However,the isotope effect during NO2-assimilation(15?NO2=1.9-3.2‰)decreased with salinity,indicating that it was sensitive to the surrounding environmental conditions(such as salinity and the concentration of substrate).(2)In the CJ plume(in Summer).Using observations of NO3-concentration and dual isotopic composition across the CJ plume within a three end-member mixing model,we found the deviations between the observed and expected values for mixing alone,revealing the non-conservative behavior of NO3-.Using cross correlations between concentrations and dual isotope deviations,we identified three N transformation zones,which correspond with separate portions of the plume.Nitrification and sedimentary denitrification occurred near the river mouth,nitrification prevailed further offshore under the plume,and finally phytoplankton assimilation in the outer surface plume(>100 km offshore),where it was fueled by NO3-that had already been strongly modified by microbial processes.Caution is required to derive biological isotope effects in a plume using the initial NO3-composition of the freshwater end-member.In the pool of total nitrogen,the propotion of NO3-was highest,closed followed by DON,and then PN.For ?15N,they were in an order of ?15NNO3>?15NPN>?15NDON.The minimum value of ?15NPN occurred in the outer plume where with high chlorophyll-a(>20 ?g L-1),associated with the isotope effect during NO3-assimilation.By adding ?13CPOC and C/N for consideration,?15NPN showed obvious marine source yet influenced by isotope effect.Significant removal of DON was observed in the middle-outer plume,yet,with less change in the ?15NDON(-2‰ to 5‰).Selective assimilation of low-molar-weight-DON by phytoplankton or microbial cells with little isotope effect can partly explain the less variable of ?15NDON.(3)In the NWPO(in Spring).?15NNO3 and ?18ONO3 showed a typical vertical distribution as the mesotrophic ocean,which with highest values in the euphotic zone.It was associated with the NO3-assimilation by phytoplankton,atmospheric N deposition and nitrification.The depths of nitricline and synchronous increase in NO3-dual isotope were uplifted in the stations influenced by cold eddies.?15NPN increased with depth in the K transect and B transect due to the isotope effect during remineralization.However,the lowest values of 815NPN in the surface were associated with the atmospheric N deposition.The highest PN values occurred in the Station A4,A6 and A8,located at the edge of warm eddies,since the upper nutricline was uplifted into the euphotic zone owing to the submesoscale upwelling.Note that,?15NPN in the 50-100 m were lower than that of<50 m,significant different from the patterns of?15NPN in the K transect and A transect.It was attributed to the lateral trasport.