Study On Nitrate Biogeochemical Processes In The Chinese Continental Sea Based On The Stable Nitrogen Isotope Method

As an advanced and efficient pretreatment for nitrogen isotopic analysis of nitrate, the denitrifier method was established and completed in this thesis based on related references. The accuracy and precision were tested via measuring international standard substances. According to the results, correlation coefficients are better than 0.997, and the mean standard deviation is 0.26‰. The results were verified repeatedly. The analytical accuracy and precision were excellent and were satisfied to the demand of seawater analysis. Based on this technique, investigations were conducted in the southern Yellow Sea(SYS), the Changjiang River estuary and adjacent waters(CREAW) and the East China Sea(ECS) during May to June(spring) and Octorber to November(autumn). Samples were collected for analyzing the nitrate nitrogen isotopes(δ15NNO3), nitrogen and phosphorus nutrients, dissolved oxygen(DO) and chlorophyll-a(chl-a). Physical parameters, such as salinity, temperature and density were measured instantly via the CTD recorder. The isotope dilution in situ experiments were handled in significant regions of the Yellow Sea Cold Water Mass(YSCWM) and the CREAW. These experiments were applied for measuring the key biogeochemical processes of nitrate. This thesis aims to illustrate the sources and biogeochemical processes of nitrate systemically and comprehensively. The specific outcomes are as follows.In situ investigations were conducted along 4 transects in the SYS. Samples were collected from varying layers of 28 sites during spring and of 27 sites during autumn. The samples were measured and the spatial and temporal distributions were analyzed. Results show that terrigenous input was the main source of NO3 in the SYS during investigated period. NO3 were assimilated by phytoplankton in the surface water, which was significant during spring and weak in autumn. The YSCWM, which located in the offshore deep waters, was larger in spring than in autumn. The mean value of δ15NNO3 in the YSCWM was 7.57 ‰ in spring and was 5.27 ‰ in autumn. Combined with the lower NO3 concentrations in spring than that in autumn, it indicated that nitrification process probably exsited in the YSCWM. The isotope dilution experiments were conducted in the sites of YSCWM for verification. The results demonstrated that nitrification was the dominant process in spring, which had an actual rate as 0.27 μmol N·L-1·h-1. However, NO3 was consumed during autumn, and the actual rate of uptake was 0.52 μmol N·L-1·h-1.In the investigations of the CREAW, we collected water samples from different depths along three transects and analyzed the vertical distribution and seasonal variations. Results show that the Changjiang River diluted water(CDW) was the main factor affecting the shallow waters(above 10 m) of the CREAW, and CDW tended to influence the northern areas in June and the southern areas in November. δ15NNO3 values in CDW ranged from 3.21–3.55‰. In contrast, the deep waters(below 30 m) were affected by the subsurface water of the Kuroshio Current, which intruded into the waters near 31°N in June. The δ15NNO3 values of these waters were 6.03–7.6‰, slightly higher than the values of the Kuroshio Current. Nitrate assimilation by phytoplankton in the shallow waters of the study area varied seasonally. Because of the favorable temperature and nutrient conditions in June, abundant phytoplankton growth resulted in harmful algae blooms(HABs). Therefore, nitrate assimilation was strong in June and weak in November. The δ15NNO3 fractionations caused by assimilation of phytoplankton were 4.57‰ and 4.41‰ in the shallow waters in June and November, respectively. These results are consistent with previous laboratory cultures and in situ investigations. Nitrification processes were observed in some deep waters of the study area, and they were more apparent in November than in June. The fractionation values of nitrification ranged from 24–25‰, which agrees with results for Nitrosospira tenuis reported by previous studies. The isotope dilution method measured the rates of nitrate assimilation in the surface waters near the Changjiang River estuary,which were 0.26–1.17 μmol N·L-1·h-1 during spring and 0.16–0.17 μmol N·L-1·h-1 during autumn. The rates of nitrification were stronger than past investigations.In the southern ECS and east of Taiwan(only in spring), seawater samples were collected from 36 stations of 6 transects and 29 stations of 6 transects in spring and autumn, respectively. Spatial and temporal distributions were analyzed after key parameters were determined. Results show that the Kuroshio subsurface water intrudes into the ECS and separates into two branches on the continental shelf: the nearshore Kuroshio branch current(NKBC) and the offshore Kuroshio branch current(OKBC). The NKBC extended to coastal area of Zhejiang Province in spring while it reached to 27.5°N, 122°E in autumn. In spring, the nitrate concentration in the Kuroshio subsurface water was relatively stable east of Taiwan, supplied by upwelling currents northeast of Taiwan. Continuous nitrification occurred in the NKBC after intrusion into the ECS, revealed by the gradual consumption of DO in the bottom water of the NKBC. This process may significantly contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that NKBC assimilation is likely where nitrification occurs because the isotope fractionation ratio of oxygen and nitrogen(18?:15?) in NO3 was 1.18 in spring. We calculated the flux and NO3 budget in the NKBC in spring via numerical simulations and demonstrated that nitrification has occurred at a rate of at least ~0.52 kmol NO3·s-1 since the DH9 transect and that ~0.11 kmol NO3·s-1 was consumed in the coastal DH4 transect. According to the Rayleigh model, primary production was supported by the intrusion of the Kuroshio subsurface water into the southern ECS, causing 5.5‰ isotope fractionation. Moreover, at partial nearshore stations north of the region investigated, the assimilated nitrate originated from the mixing of NKBC and coastal currents from the Changjiang diluted water(CDW).