Tide drives nutrients variation and exchange flux in the semi-enclosed Shuidong Bay coastal water in winter, South China Sea

Nutrients play a key role in coastal ecosystems. Coastal tide is an important driver of the daily nutrient variability in semi-enclosed bay under the intensive human activities and climate change. However, the extent of tidal effects on the nutrients variation and exchange flux in the semi-enclosed bay in winter is still scarcely. In this study, seawater samples were collected by synchronous continuous observation during winter in 2021, to investigate tide drives nutrients variation and exchange flux between Shuidong Bay (SDB) and South China Sea (SCS). The results showed most nutrients were significantly impacted by the tidal variation in the SDB with the semi-diurnal cycle. Coastal flood water can increase seawater exchange capacity due to strong dilution influenced by the outer SCS water, resulting mitigate adverse effects of nutrient enrichment in the semi-enclosed bay. In addition, the concentrations of total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and silicate (DSi) were 146.75 & mu;mol/L, 1.06 & mu;mol/L, and 6.15 & mu;mol/L. Besides, there was a relative excess of nitrogen, phosphorus and silicate were limiting factors in the SDB, and nutrients significantly impacted the Chlorophyll-a in coastal water. Moreover, both TDP and DSi were transported from the SDB to the SCS with a net exchange flux of 0.23 t and 1.99 t, while TDN was transported from the SCS to the SDB with a net exchange flux of 0.14 t, resulting in nutrients ratio increase in the SDB coastal water. Statistical analysis identified the tide induced hydrological variations and terrestrial inputs dominated nutrient dynamics in the SDB coastal water. This study provided a new understanding of the variation and transport of nutrients driven by tide in the SDB, which is implications for understanding the biogeochemistry of nutrients in daily scale and water quality management in the semi-enclosed bay in future.