Abstract:

As a “golden” coastal area for foreign trade and tourism development, Qinzhou Bay has experienced increasingly severe eutrophication in recent years, driven by the rapid expansion of economic and agricultural activities. This study investigated the spatiotemporal distribution of nitrate $(\text{NO}_{3}^{-})$in the bay using field monitoring data collected in September, October, and December 2023. Furthermore, a preliminary assessment of nitrate sources was conducted specifically for September and October, employing nitrogen and oxygen isotope analysis coupled with a Bayesian mixing model (MixSIAR). The results revealed that nitrate concentrations generally exhibited a decreasing trend from north to south and from the inner bay to the bay mouth, with high-concentration zones mainly distributed near Maowei Sea and the estuaries of rivers such as the Qinjiang River. Temporally, the nitrate concentration in December [(2.01 ± 1.58) mg·L^-1] was markedly higher than those in September [(0.64 ± 0.36) mg·L^-1] and October [(0.35 ± 0.28) mg·L^-1], which was attributed to seasonal variations in biological activity and terrestrial inputs. Further analysis indicated that nitrification was the dominant transformation process for nitrate during September and October, while denitrification was not significant. Source apportionment results showed that manure and domestic sewage were the primary nitrate sources, contributing approximately 66% of the total input, followed by soil nitrogen (~32%). In contrast, atmospheric deposition and chemical fertilizers made minimal contributions, each accounting for about 1%.

Key words: nitrate, MixSIAR model, pollution source identification, Qinzhou Bay