Nitrogen And Oxygen Isotopic Characteristics Of Nitrate In Zhanjiang Bay Water

Nitrogen（N）is an essential element for the growth of marine biota.Its concentration and distribution are related to the productivity of marine organisms.There are various types of N existing in the ocean,and nitrate is a main type available to marine life.However,an obstacle occurs in resolving the occurrence of N and its biogeochemical process in the water due to the complex migration.NO₃^--δ¹⁵N and NO3--δ¹⁸O technology is an important means studying the nitrogen cycle in the water at present.Zhanjiang Bay is located on the northeastern side of the Leizhou Peninsula,western Guangdong Province.The study of nitrogen biogeochemical process in the waters of Zhanjiang Bay basically remains in traditional methods,leaving an unknown in the source of nitrogen and biogeochemical process in the Zhanjiang Bay waters.Curious about the above questions,three field surveys were carried out in January,May and September in 2016 separately,combing with dual nitrate isotopic technology.The results are as follows:（1）The concentration of nitrate in 2016 in Zhanjiang Bay ranged from 10.57 μmol/L to 32.05 μmol/L in January,from 11.29 μmol/L to 26.13 μmol/L in May,and from 10.69 μmol/L to 24.77 μmol/L in September.In 2016,NO₃^--δ¹⁵N varied from 5.40‰¹0.80‰,（January）,6.28‰⁹.44‰（May）,4.60‰¹0.90‰（September）,with a mean value 8.43‰,7.63‰,7.88‰,separately.NO3--δ¹⁸O varied from-0.30‰⁸.00‰,（January）,2.84‰¹0.72‰（May）,-0.40‰⁹.50‰（September）,with a mean value 3.89‰,6.87‰,3.56‰,separately.NO₃^--δ¹⁵N of three months are close,and NO3--δ¹⁸O of May is the highest.In the horizontal distribution,the concentration of nitrate and dual nitrate isotopic showed an increase from the estuary to the Bay area.（2）The study investigated the nitrogen and oxygen isotopes of nitrate in the Zhanjiang Bay.Thus,the contribution of the three potential major sources of the Zhanjiang Bay was estimated using the end-member method.The results are as follows: The contribution rate of manure and sewage to NO3-in the estuary area of the Zhanjiang Bay was 44.4%,and fertilizer was 55.6%.Because agriculture is the dominated industrial structure in Zhanjiang.A large amount of fertilizer is used and nitrogen was discharged into the estuary area of the Zhanjiang Bay.Simultaneously,the coast of the estuary area is a densely populated area where a large amount of manure and sewage was discharged into nitrogen.（3）NO₃^--δ¹⁵N and δ¹⁸O was mainly falled on the the straight line of 1:1 in May,which should be the rusult of strong assimilation.In May,the Zhanjiang Bay is early summer and temperature is 26.89-30.92°C,which is suitable for the growth of phytoplankton.NO₃^--δ¹⁵N and δ¹⁸O were mainly distributed in the area below the straight line of 1:1,which might be related with the combined effect of assimilation-nitrification.The longer water residence times in January and September has a positive effect on the mineralization of particulates and the nitrification of ammonium.（4）In January,May and September,the value of △（15,18）of the Zhanjiang Bay showed an increasing trend from the Estuary area to the Bay area,indicating that the different biogeochemical processes should be happen in Zhanjiang Bay.There may be stronger nitrification in the estuary area and strong assimilation in the bay area.