Titel: Nitrate budget and cycling in the east China seas revealed by stable nitrogen isotopes
Sprache: Englisch
Autor*in: Tian, Shichao
Schlagwörter: East China seas; Nutrients and stable isotopes; Internal cycle of nitrogen; Nitrification; Nitrate loss; Nitrate budget
Erscheinungsdatum: 2022
Tag der mündlichen Prüfung: 2022-04-13
The east China seas comprise one of the broadest continental shelves around the world and have experienced increasing input of anthropogenic reactive nitrogen (Nr) in recent decades. The supply of nutrients, markedly that of Nr, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. The excess Nr created numerous ecological problems, such as increasing incidences of macroalgae blooms, harmful algae blooms, and more frequent occurrence of low oxygen regions near the estuaries. The cycling of Nr in the east China seas therefore is a potentially important ecosystem function mitigating effects of anthropogenic nitrogen inputs. Therefore, it is crucial to quantify the Nr input to this region, and to determine the quantities of nitrogen eliminated in and exported from it.
Samples of water, suspended matter and sediments were taken in the Bohai Sea (BHS) and Yellow Sea (YS) in spring (March and April) and summer (July and August) 2018. The Yellow River (YR) was sampled in May, July to November and Daliao River, Hai River, Luan River and Xiaoqing River were sampled in November 2018. In addition to nutrients, particulate organic carbon, the dual isotopes of nitrate (δ15N and δ18O), nitrogen contents and δ15N of suspended matter and sediments were determined in order to identify nitrogen sources and better constrain nitrogen budgets.
First, based on the available mass fluxes and isotope data, we propose an updated nitrogen budget of the BHS. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrate sources are rivers contributing 19.2–25.6 % and the combined terrestrial runoff (including submarine fresh groundwater discharge of nitrate) accounting for 27.8–37.1 % of the nitrate input to the BHS while atmospheric input contributes 6.9–22.2 % to total nitrate. An unusually active interior nitrogen cycling contributes 40.7–65.3 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (70.4–77.8 %) and only very little is exported to the YS (only 1.8–2.4 %). At present denitrification is active in the sediments and removes 20.4–27.2 % of nitrate from the pool.
Second, we compared the different seasonal dissolved inorganic nitrogen (DIN) variation between the north and south Yellow Sea Cold Water Masses (YSCWM). The nitrate concentration decreased from 2.41 ± 1.58 μmol L-1 in spring to 0.81 ± 0.61 μmol L-1 in summer in the northern part of the YSCWM. In contrast, nitrate increased from 5.49 ± 2.24 μmol L-1 to 7.58 ± 2.20 μmol L-1 in the southern part, accompanied by a decrease in δ15N-NO3- and δ18O-NO3- from values near 10.0 ‰ in the southern deep-water mass to values of about 5.0 ‰. The increasing nitrate concentration and decreasing δ15N-NO3- and δ18O-NO3- values of nitrate are indicative of nitrification that was a significant source of recycled nitrate in the southern part of YSCWM. To quantify this regenerated nitrate, we use a mixing model with end members of preformed nitrate in spring and regenerated nitrate in summer both, with their distinct dual isotope values. The values of δ15N and δ18O for regenerated nitrate were first constrained as 3.3 ‰ and 0.9 ‰, respectively. The results of the combined mass and isotope balance model suggest that in the southern YSCWM, in summer 65 % of the nitrate was regenerated and only 35 % was a residual of nitrate preformed in spring.
Our findings indicate that the northern part of YSCWM is a sink of Nr in the sub-thermocline water column, mainly because of denitrification in sediments. In contrast, the BHS and southern pool of YSCWM are a growing reservoir of regenerated terrestrial reactive nitrogen, the addition of which compensates for the removal by co-occurring sediment denitrification. The excess terrestrial nitrogen, delivered mainly by Changjiang River to the south YS and YR to the BHS, has already exhausted the attenuation capacity for Nr of those regions. In consequence, the BHS and southern YS are facing a higher ecological risk than the northern YS, when excess Nr discharge from Changjiang River and YR continues at present levels or even increases. A further eutrophication of the BHS and southern YS could induce water column hypoxia, as is increasingly observed in other marginal seas and seasonally off river mouths.
URL: https://ediss.sub.uni-hamburg.de/handle/ediss/9589
URN: urn:nbn:de:gbv:18-ediss-100266
Dokumenttyp: Dissertation
Betreuer*in: Gaye, Birgit
Emeis, Kay-Christian
Enthalten in den Sammlungen:Elektronische Dissertationen und Habilitationen

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