Fluxes of riverine nutrient to the Zhujiang River Estuary and its potential eutrophication effect

The Zhujiang River Estuary is becoming eutrophic due to the impact of anthropogenic activities in the past decades. To understand nutrient dynamics and fluxes to the Lingdingyang water via four outlets (Humen, Jiaomen, Hongqimen and Hengmen), we investigated the spatial distribution and seasonal variation of dissolved nutrients in the Zhujiang River Estuary, based on fourteen cruises conducted from March 2015 to October 2017, covering both wet (April to September) and dry (October to March next year) seasons. Our results showed that riverine fluxes of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) into the Lingdingyang water through four outlets varied seasonally due to the influence of river discharge, with the highest in spring and the lowest in winter. However, riverine flux of phosphate exhibited little significant seasonal variability. Riverine nutrients into the Lingdingyang water most resulted through Humen Outlet. The estuarine export fluxes of DIN out of the Zhujiang River Estuary derived from a box model were higher than fluxes of riverine nutrients in May, likely due to the influence of local sewage, while lower than riverine flux in August. The export fluxes of phosphate were higher than the fluxes of riverine phosphate in May and August. In contrast, large amounts of DSi were buried in the estuary in May and August. Although excess DIN was delivered into the Zhujiang River Estuary, eutrophication effect was not as severe as expected in the Zhujiang River Estuary, since the light limitation restricted the utilization of nutrients by phytoplankton.

Automated summary

The Zhujiang River Estuary is experiencing eutrophication due to anthropogenic activities. Nutrient dynamics and fluxes to the Lingdingyang water through four outlets were investigated. Riverine fluxes of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) varied seasonally, with the highest in spring and the lowest in winter. Riverine flux of phosphate showed little seasonal variability. Most riverine nutrients entered Lingdingyang water through Humen Outlet. Estuarine export fluxes of DIN were higher than riverine flux in May, potentially due to local sewage, while they were lower in August. Export fluxes of phosphate were higher than riverine flux in May and August. Large amounts of DSi were buried in the estuary in May and August. Despite excess DIN input, eutrophication effect was not severe due to light limitation.