Long-term variations in dissolved silicate, nitrogen, and phosphorus flux from the Yangtze River into the East China Sea and impacts on estuarine ecosystem

Variations of dissolved silicate (DSi) flux in the Yangtze River have caused great concern among scientists. Analysis of spatial and temporal variations of DSi indicates that the distribution of DSi concentration (DSiC) is closely related to the occurrence of bedrocks in the river cathchment. On average, the upper Yangtze River and Dongting and Poyang Lake of the middle Yangtze basin serve as the major DSi sinks, retaining 3.39 × 104 t y−1, 5.59 × 104 and 2.51 × 104 t y−1. The middle and low Yangtze River remains DSi sources, emitting 2.85 × 104 and 2.48 × 104 t y−1, respectively. Geochemical data at Datong hydrological station recorded the flux of nutrients entering into the sea during the flood season, i.e. 74% for DSi, 73% for DIN (Dissolved Inorganic Nitrate) and 68% DIP (Dissolved Inorganic Phosphate). The yearly DSiC and flux show a sharp decrease since 1950s. The mean DSiC was 109.47, 91.09, and 77.56 μmol l−1 in the 1960s, 1970s and 1980s, respectively. The mean DSi fluxes for the same time period were 2.72, 2.23, and 2.13 × 106 t. A PP (primary productivity) model estimates 3.14 × 105 t of DSi (13.08% of the annual mean flux entering into the sea) was fixated within the 162 reservoirs in 2002. On the other hand, DIN and DIP concentration and flux have increased greatly since the 1950s. Fertilizer application peaked after the 1980s can interpret these high DIN and DIP. A sharply decreasing DSi flux and quickly increasing DIN and DIP fluxes into the sea have enhanced eutrophication and caused frequent harmful algal blooms in coastal waters. On average, red-tide frequency was from 0.04 during 1933–1979 to 7.0 during 2000–2002. The Skeletonema costatum (siliceous alga), the red-tide-predominant species that is in positive proportion to DSi flux, decreased from 33% during the 1980s to 24% during 2000–2002. The present data evidence the increase of Prorocentrum dentatum (non-siliceous alga) from 12.5% in the 1980s to 36% in 2000–2002, which tends to be dominant species of the red tides off the Yangtze estuary. The number of big dams in the Yangtze River basin will double in the next 30–50 years. This will significantly influence the variations of nutrient fluxes in the river basin and estuary, in relation to health management of river-coast ecosystem.