Phosphorus transport in the Three Gorges Reservoir over the past two decades

Phosphorus (P) is an essential nutrient in aquatic ecosystems and its dynamics in river systems are greatly affected by human activities. Based on the in situ measurements of water discharge (Q), suspended sediment concentrations (SSC), and concentrations (C) of total P (TP), particulate P (PP), and total dissolved P (TDP) from 1997 to 2017, P transport in the Three Gorges Reservoir (TGR) and the combined effects of cascade reservoirs operation and anthropogenic emissions have been explored. The whole period was divided into three sequential periods of January 1997 to May 2003 (period 1), June 2003 to September 2012 (period 2), and October 2012 to December 2017 (period 3), corresponding to the period prior to impoundment of the TGR, during the operation of the TGR, and after the impoundment of upstream cascade reservoirs, respectively. Results showed that the P concentrations were positively skewed and generally followed the three-parameter gamma and log-normal distributions. There was significant difference for SSC, TP, PP, and TDP between adjacent periods but not for Q. The SSC, TP, and PP presented downward trends during the periods affected by the impoundment of cascade reservoirs, while the TDP increased in period 2 especially after 2007 and decreased in period 3 due to the upstream reservoirs regulation and the pollution management in the tributaries. For the monthly variation, the SSC, TP, and PP became more evenly distributed throughout the year after the impoundment of cascade reservoirs, while the TDP tended to show a pattern of "higher in dry season and lower in flood season". The C-Q relationships indicated that the TP in the TGR was positively correlated with Q (transport limited), while negatively with Q for TDP (source limited). Generally, the TP, PP, and TDP were positively correlated with both SSC and anthropogenic P emissions, thus, the P concentrations declined with sediment retention in the reservoir but increased with time due to the increasing pollutant emissions. Correspondingly, the P dynamics were better depicted after considering the effects of sediment and pollutant emissions than using a simple C-Q relationship.