Seasonal Sources and Cycling of Nitrogen Revealed by Stable Isotopes in the Northeastern Beibu Gulf, China

Abstract

Isotope measurements were performed on dissolved nitrate (NO3−) and ammonium (NH4+) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ15N-NO3− and δ15N-NH4+ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO3- sources and the results showed that in the nearshore area, NO3− originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO3− isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ15N-NO3− and δ15N-NH4+ values in the offshore area in the fall, and the NO3− loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO3−) and positive dual nitrate isotopes but lower NO3− concentration in the offshore area during the spring. Overall, isotope analysis of NO3− and NH4+ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.

Sources of nitrate in a heavily nitrogen pollution bay in Beibu Gulf, as identified using stable isotopes

“…The distributions of isotopic values and nutrient levels are expected to fall on the mixing line between two endmembers. The deviation from the mixing line suggests external sources or the presence of transformation processes (Wankel et al, 2006;Yang et al, 2018;Lao et al, 2019a;Lao et al, 2021b;Chen et al, 2022).…”

“…The proportional contributions of potential nitrate sources can be quantified by a Bayesian mixing model, which was created and run in the SIAR (stable isotope analysis in R 4.1.2) package. This model has been widely used to quantify the potential nitrate sources in marine systems (Xue et al, 2009;Korth et al, 2014;Davis et al, 2015;Zhang et al, 2018: Lao et al, 2019aLao et al, 2021b andChen et al, 2022). The framework can be found in Xue et al (2009) and Moore and Semmens (2008) as follows:…”

“…These results suggest that decomposition/nitrification and/or external sources may occur in the bay. During the nitrification process, light N (d 14 N) in the NH 4 + pool is preferentially utilized by microorganisms, resulting in the enrichment of d 15 N-NH 4 + while adding depleted d 14 N to the residual nitrate pool (Sigman et al, 2005;Chen et al, 2009;Ye et al, 2016;Lao et al, 2021b). If nitrification is the dominant process in the bay, isotopic fractionation during the process can be estimated by opensystem Rayleigh fractionation (Altabet, 2006;Ye et al, 2016;Chen et al, 2022) as follows: + oxidation in seawater (Ye et al, 2016).…”

“…There are several rivers around the northern Beibu Gulf, and the influence of runoff input by the rivers and tidal influx from the outer gulf constitutes a well estuary-bay multi-ecosystem and has become an important mariculture area in China (Lao et al, 2020 andLiu et al, 2020;Xu et al, 2020: Xu et al, 2021Lao et al, 2021a;Lao et al, 2022a and2022b). However, with the rapid development of local urbanization and industrialization, the coastal bays in Beibu Gulf have been subjected to a high loading of contaminants from anthropogenic inputs, such as domestic, industrial, port and agricultural waste (Lao et al, 2019b;Xu et al, 2020: Lao et al, 2021aLao et al, 2021b;Lao et al, 2021c;Lao et al, 2021d: Xu et al, 2021and Lao et al, 2022a. The nutrients discharged into coastal bays from runoff input have increased over the past 20 years, and the increase in phosphorus input has decreased the N/P ratios in several coastal bays in the Beibu Gulf (Lao et al, 2020 andLao et al, 2021c).…”

“…Moreover, the eutrophication of some coastal bays in the Beibu Gulf has increased in recent decades due to the increase in terrestrial nutrient input, particularly the nitrogen input (Lai et al, 2014;Guo et al, 2020;Lao et al, 2021a andLao et al, 2021c;Pan et al, 2021). In addition, sewage is the dominant nitrate source in most bays/areas in the Beibu Gulf, which is identified by the dual nitrate isotopes (Pan et al, 2021;Lao et al, 2021b;Chen et al, 2022). This has resulted in an increase in the frequency of harmful algal blooms in the Beibu Gulf over the past decades, particularly in the past ten years (Xu et al, 2019).…”