A nitrate budget of the Bohai Sea based on an isotope mass balance model

[摘要] The Bohai Sea (BHS) is a semi-closed marginal sea impacted by oneof the most populated areas of China. The supply of nutrients, markedly thatof reactive nitrogen, via fluvial and atmospheric transport has stronglyincreased in parallel with the growing population. Therefore, it is crucialto quantify the reactive nitrogen input to the BHS and understand theprocesses and determine the quantities of nitrogen eliminated in andexported from the BHS. The nitrogen budget and in particular the internalsources and sinks of nitrate were constrained by using a mass-based and dualstable isotope approach based on δ 15 N and δ 18 O ofnitrate (NO 3 - ). Samples of water,suspended matter, and sediments were taken in the BHS in spring (March andApril) and summer (July and August) 2018. The Yellow River (YR) was sampledin May and July to November, and Daliao River, Hai River, Luan River, andXiaoqing River were sampled in November of 2018. In addition to nutrient,particulate organic carbon, and nitrogen concentrations, the dual isotopes ofnitrate ( δ 15 N and δ 18 O), δ 15 N ofsuspended matters, and sediments were determined. Based on the available massfluxes and isotope data an updated nitrogen budget is proposed. Compared toprevious estimates, it is more complete and includes the impact of interiorcycling (nitrification) on the nitrate pool. The main nitrate sources arerivers contributing 19.2 %–25.6 % and the combined terrestrial runoff(including submarine fresh groundwater discharge of nitrate) accounting for27.8 %–37.1 % of the nitrate input to the BHS, while atmospheric inputcontributes 6.9 %–22.2 % to total nitrate. An unusually active interiornitrogen cycling contributes 40.7 %–65.3 % to total nitrate vianitrification. Nitrogen is mainly trapped in the BHS and mainly removed bysedimentation (70.4 %–77.8 %), and only very little is exported to theYellow Sea (YS) (only 1.8 %–2.4 %). At present denitrification is activein the sediments and removes 20.4 %–27.2 % of nitrate from the pool.However, a further eutrophication of the BHS could induce water columnhypoxia and denitrification, as is increasingly observed in other marginalseas and seasonally off river mouths.

[学科分类] 大气科学