Riverine fluxes of different species of phosphorus in the Pearl River estuary

Phosphorus is the most limiting nutrient in coastal waters of China, particularly in the Pearl River (PR) estuary. Rivers have different P forms including dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP), and particulate organic phosphorus (POP). Their input to coastal seas has been overlooked. We hypothesize that DIP is a small fraction of total phosphorus (TP). We investigated these P forms and estimated their fluxes in PR eight outlets during 2015–2019. DIP on average is only a 30.90 % fraction of TP with PIP, POP and DOP accounting for 22.43, 31.56 and 15.37 %, respectively. The average annual fluxes of TP, DIP, DOP, PIP and POP were 12.58×, 3.34×, 1.68×, 3.19× and 4.26 × 106 mol/month, respectively, which are regulated by runoff and suspended particulate matter (SPM). The finding reveals the importance of other P forms for phytoplankton in the Pearl River estuary and their bio-availability deserves further study. •There were significant differences in SPM and P among 8 outlets of the PRE.•A lot of PIP and POP were input to the PRE along with SPM.•Large input of particulate phosphorus can be an important P source for phytoplankton.