Quantitative assessment of nitrate sources in Jiaozhou Bay and its terrestrial inflows using nitrate stable isotope techniques

Nitrate was the most abundant form of nitrogen in the biosphere after atmospheric nitrogen and was the source of nitrogen most widely used by living organisms such as higher plants, algae, fungi and bacteria (Pinar et al., 1997). Excessive inputs of nitrate can cause coastal eutrophication, leading to harmful algal blooms, low oxygen, ocean acidification and other hazards; thus, controlling nitrate inputs was the key to mitigating coastal eutrophication (Hinson et al., 2023; Maúre et al., 2021; Wang et al., 2018a).
The sources of coastal nitrate were complex, and the direct discharge of nitrate into the sea from rivers, atmospheric deposition and ground- water due to human activities constitute important sources of coastal nitrate, with different sea areas presenting unique characteristics. For example, the eutrophication of rivers in the southern Baltic Sea and Mediterranean Sea catchments was caused by land-based inputs, with contribution rates as high as 66.5 % and 41 %, respectively (De Giro- lamo et al., 2017; Korth et al., 2014), and the land runoff contributions of nitrate in the Bohai Sea region of China in summer and fall were 58.8-69.4 % and 79.7-86.4 %, respectively (Yu et al., 2024). The source of nitrate in Chesapeake Bay was dominated by atmospheric deposition, which can contribute 20-32 % of the total nitrate (Sheeder et al., 2002). The main contributor of nitrate to the waters adjacent to the islands of northeastern Mexico was submarine groundwater discharge (SGD), with an input rate three times greater than that of river-dominated fluxes (Adyasari et al., 2021; Park et al., 2024). Notably, in addition to direct physical inputs, biogeochemical processes, mainly nitrification, can convert other forms of nitrogen into bioavailable nitrate, which can be an important source of nitrate in some waters. For example, studies in the estuary of the Yangtze River and adjacent waters and in the shallow waters of the Arabian Gulf had revealed that nitrification-induced regeneration processes account for a significant proportion of nitrate sources (Sarkar et al., 2024; Wang et al., 2018b). In summary, the ac- curate quantification of nitrate sources forms the basis for the preven- tion and control of coastal eutrophication, but this was difficult owing to the complexity of nitrate sources.
Traditional nitrate identification technique mainly measures the concentration of nitrate, which has certain defects and is limited by factors such as the amount of water discharged, as well as factors such as the diversity of nitrate sources, which makes it impossible to qualita- tively and quantitatively analyse the sources of nitrate (Petitta et al., 2009; Andrade and Stigter, 2009; Lockhart et al., 2013; Niu et al., 2021). With the development of isotope analysis techniques, stable isotope techniques have been widely used to determine the source of nitrate in water bodies. Relevant studies have shown that the two-isotope signa- tures of nitrate can be utilized to determine the source of nitrate in water bodies and to elucidate the transformation mechanism of nitrate. However, there are overlapping ranges of stable isotope source signa- tures, while quantitative contribution analysis needs to be carried out using other means (Kumar et al., 2023; Chen et al., 2022; Shin et al., 2023). Different nitrate sources present different isotopic signatures that can be used to distinguish between nitrate sources, such as atmospheric deposition, sewage and other sources of nitrate, δ15N is − 13-13 %, 4–25 % between; at the same time, the presence of assimilation, nitrification and other biological effects, so that the isotope has a fractionation effect, changing the isotopic composition of nitrate (Nestler et al., 2011; Fenech et al., 2012; Niu et al., 2021). On this basis, source resolution studies combined with relevant models and appropriate modelling methods for onshore rivers and coastal bays can be used to more accurately identify sources of nitrate. In terms of overland rivers, Zhang and Wang (2020) calculated nitrate sources from alluvial fans in North China using the SIAR model and found that sewage and faeces in groundwater and river water were the main nitrate sources. Tian et al. (2022) carried out a study on the source of nitrate in the Bohai Sea using the LOICZ nitrate budget approach modelling tools and reported that land-based inputs and nitrification, as well as land-based inputs, are important sources of nitrate in the Bohai Sea, with very high contributions.
Jiaozhou Bay is located in the Yellow Sea, which is a marginal sea in the northwestern part of the northwestern Pacific Ocean. Due to the influence of human activities in neighbouring cities, the nutrient profile of Jiaozhou Bay had undergone obvious changes, and eutrophication of the bay had resulted in a significant decline in ecosystem health (Shen et al., 2006; Zhang et al., 2024; Shen, 2001). Researchers concluded that human activities, particularly aquaculture since the 1980s, have increased nutrient loads in Jiaozhou Bay’s environment; however, the rivers along the coast of Jiaozhou Bay, with the Dagu River and the Moshui River as typical rivers, had become channels for industrial sewage and municipal wastewater in Qingdao, which is an important cause of the eutrophication of the waters of Jiaozhou Bay (Wang et al., 2020; Liu et al., 2005). In recent years, in China, pollution prevention and control policies for rivers had been implemented, effectively reducing the input of nitrate into rivers through the control of chemical fertilizer use and sewage discharge, which had significantly decreased the TDN flux from land sources to Jiaozhou Bay (Li et al., 2018). Therefore, reassessing the nitrate input to Jiaozhou Bay and systemati- cally analysing the contributions of human activities and natural pro- cesses to the nitrogen pool of this typical bay were important for the prevention and control of eutrophication in the bay under these new policies.

This study focused on a typical eutrophic bay, Jiaozhou Bay, and its major land-based rivers, the Dagu River and the Moshui River. On the basis of nitrate concentration and nitrogen and oxygen stable isotope data, an SIAR model combined with corrected fractionation coefficients was used to quantitatively analyse the sources of nitrate in the Dagu River Basin and Moshui River Basin, and the nitrate fluxes in Jiaozhou Bay were quantitatively assessed using the LOICZ nitrate budget approach, which was intended to systematically elucidate the sources and sinks of nitrate along the land–sea continuum. The purpose of this study was to systematically elucidate the sources and sinks of nitrate in the land–sea continuum, to quantitatively assess the contributions of biogeochemical processes to the nitrate pool in the bay, and to provide data support for coastal environmental protection under the new conditions.