Abstract
A large amount of terrestrial nutrients are discharged into the ocean through rivers. However, the impact of seasonal hydrological variations on riverine nutrient concentrations and fluxes remains unclear, especially in the medium-sized subtropical rivers that are highly influenced by human activities. In this study, we investigated the monthly changes in nutrient concentrations (soluble reactive phosphorus, SRP; dissolved silicate, DSi; and dissolved inorganic nitrogen, DIN) in the North Stream (NS) and West Stream (WS) of the Jiulong River (JLR). The results show that the concentrations of SRP and DSi in the NS and the WS displayed a similar seasonal variability, which was different from the pattern of DIN. Hydrological conditions, chemical fertilizer loss and biogeochemical processes are responsible for the seasonal changes in the nutrients in the two streams, especially during extreme rain events. Nutrient concentrations in the NS exhibited a clockwise trajectory along with river discharge during rain events, while a reverse pattern in the WS was observed since it experienced a moderately long rain event. Different rainfall features between the two main tributaries resulted in the majority of nutrients being exported at the start of the rain event in the NS and the end of rain event in the WS. Indeed, the annual high flow (Q/Qm > 3) accounts for ~17.3% of the annual nutrient flux in the JLR even though this period spans only ~4.0% of a year, which suggests the importance of rain events on nutrient export in these subtropical rivers. Although the annual fluxes of DIN and SRP in the JLR were smaller than many rivers worldwide, higher areal yields of DIN and SRP were observed, indicating that the JLR is highly influenced by human activities. Our study systematically evaluated the response of nutrient concentrations to hydrological changes in two tributaries of the JLR, which is useful in better understanding the nutrient dynamics in medium-sized subtropical rivers.
Highlights
Due to human activity and climate change [6–8], riverine nutrient inputs in the ocean have increased several times since the preindustrial era [9,10], with the majority transported into the ocean during storm events [11–17] and accounting for up to 50% of the annual nutrient fluxes [18–20]
It is necessary to study the way in which nutrients respond to hydrological changes, such as storm events, in order to improve our understanding of nutrient dynamics and provide scientific information for model predictions and the management of aquatic ecosystems
The results show that the areal yields of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) in the Jiulong River (JLR) were high compared to other major world rivers [25,68], even though its catchment area is only 0.8% of the Yangtze River, 2.3% of the Pearl River and 1.9% of the Yellow River
Summary
Due to human activity and climate change [6–8], riverine nutrient inputs in the ocean have increased several times since the preindustrial era [9,10], with the majority transported into the ocean during storm events [11–17] and accounting for up to 50% of the annual nutrient fluxes [18–20]. In East Asia, increased riverine nutrient inputs can be transported into the estuaries and their adjacent coastal areas by the higher frequency of tropical cyclones and extreme storms [21]. It is necessary to study the way in which nutrients respond to hydrological changes, such as storm events, in order to improve our understanding of nutrient dynamics and provide scientific information for model predictions and the management of aquatic ecosystems
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