Limited capacity of suspended particulate matter in the Yangtze River Estuary and Hangzhou Bay to carry phosphorus into coastal seas

Abstract

The Yangtze River carries high fluxes of suspended particulate matter (SPM) and associated particulate phosphorus into the Yangtze River Estuary (YRE), but primary production in coastal seas is phosphorus-limited. Understanding the kinetics of exchangeable inorganic phosphorus (Ex-iP) released from SPM in response to different salinities and SPM concentrations is the key to demonstrating their capacity to carry bioavailable phosphorus into coastal seas. We conducted field investigations and set up a series of incubation experiments with SPM collected from marine and freshwater settings under controlled conditions, with biotic activities inhibited.Our incubation experiments showed that initial rise in salinity substantially increases the efficiency and rates of Ex-iP release, leading to an early release of majority of Ex-iP from riverine SPM when encountering saline waters, but high salinity may result in desorption. Concentrations of released DIP are linearly related to SPM concentrations, but concentrations of DIP in micromoles per gram of SPM are negatively logarithmically related to SPM concentrations, which help prevent SPM from full release of Ex-iP at the river mouth. Release of Ex-iP is insufficient to buffer DIP in the YRE. The contribution of Ex-iP from SPM decline rapidly with the attenuation of SPM concentration, and becomes negligible offshore. Extremely high loads of SPM in Hangzhou Bay allows it to retain some capacity to carry Ex-iP offshore. High concentrations of SPM in a turbid estuary are more important in reducing the removal of DIP by phytoplankton uptake through restriction of light penetration, as the SPM possesses a very limited capacity to carry Ex-iP to coastal seas. However, such high concentrations might be a key contributor to the regeneration of remineralization of organic phosphorus in an estuary.