The fate of Phosphorus in the Yangtze (Changjiang) Estuary, China, under multi-stressors: hindsight and forecast

The present study provides evidence that large dams that trap riverine sediment, decrease significantly SPM from estuaries, making them less turbid and less of a particulate trap for phosphorus adsorption. The study gives science-based insights for a future integrated estuarine water management in the Yangtze (Changjiang) Estuary. The hindsight focuses on the evolution and fate of riverine dissolved inorganic phosphorus (DIP) in the Yangtze Estuary from 1999 to 2010. A significant correlation between phosphorus and suspended particulate matter (SPM) was established. The results show that, in the past decade, the estuary has shifted from being a source of DIP to being a sink, and from a heterotrophic system to an autotrophic system. The ecosystem shift may be explained by the combined impact of the construction of dams that retain SPM but not nutrients, and to increasing nutrient fluxes to the estuary due to increasing usage of fertilizer and sewage. The foresight study is based on likely future scenarios for 2050. These are estimated using historical data and the stratified and a muddy-LOICZ model, which takes stratification and adsorption-desorption of phosphorus into account. We forecast that in 2050 the effective DIP inflow into the Yangtze Estuary will increase by a factor of 1.5 if the SPM remains at the current annual average of 700 mg L^-1, and by a factor of 3.3 if the SPM decreases to 200 mg L^-1 as a result of the dams, which will further degrade the estuary.