Abstract: As a "Golden Coast" for foreign trade and tourism development, Qinzhou Bay has experienced increasingly prominent eutrophication issues in recent years alongside rapid economic and agricultural growth. To systematically investigate the characteristics and sources of nitrate (NO₃⁻) pollution in this region, this study conducted field monitoring in Qinzhou Bay in September, October, and December 2023. Based on the collected data, the spatiotemporal distribution patterns of nitrate were analyzed, and the MixSIAR model was applied to qualitatively and quantitatively identify its sources. The results revealed that the spatial distribution of NO₃⁻ generally exhibited a decreasing trend from north to south and from the inner bay to the bay mouth, with high-concentration areas primarily located near the estuaries of rivers such as the Qinjiang River in Maowei Sea. Temporally, the nitrate concentration in December(454.15±356.11) was significantly higher than that in September(144.22±81.51) and October(80.13±61.68), which may be attributed to seasonal biological activities and variations in terrestrial input. Further analysis indicated that nitrification was the dominant process of nitrate transformation in Qinzhou Bay, with no significant denitrification detected. Source apportionment results demonstrated that manure and sewage were the major sources of nitrate, accounting for 67% of the total contribution, followed by soil nitrogen (31%). In contrast, atmospheric deposition and chemical fertilizers made minor contributions, each constituting only 1%. These findings underscore that terrestrial inputs, particularly domestic wastewater and agricultural manure emissions, are the primary drivers of nitrate pollution in Qinzhou Bay.

Key words: Nitrate, MixSIAR model, pollution source identification