Distribution, fluxes and decadal changes of nutrients in the Jiulong River Estuary, Southwest Taiwan Strait

The Jiulong River Estuary (JRE) is a typical subtropical macro-tide estuary on the southwest coast of the Taiwan Strait (TWS), which has been greatly impacted by human activities over the past 30 years. To understand nutrient dynamics and fluxes under such a heavy background of anthropogenic perturbation, eight cruises were conducted from April 2008 to April 2011, covering both wet (May to September) and dry (October to April next year) seasons. Nutrient concentrations were very high for the freshwater end-member in the upper reach of the JRE (nitrate (NO3-N): 120–230 μmol L−1; nitrite (NO2−N): 5–15 μmol L−1; ammonium (NH4−N): 15–170 μmol L−1; soluble reactive phosphorus (SRP): 1.2–3.5 μmol L−1; dissolved silicate (DSi): 200–340 μmol L−1). In dry seasons, concentrations of these nutrients were higher than in wet seasons. Nitrate was the dominant chemical species of dissolved inorganic nitrogen (DIN), with percentages of 67%–96% in wet seasons and 55%–72% in dry seasons. Distributions of NO3-N and DSi against salinity were nearly constant during all cruises, and showed generally conservative mixing behaviors in the estuary (1<Salinity<32). The concentrations of SRP varied within a narrow range of 1.0–2.0 μmol L−1 in low/middle salinity areas, and they were quickly diluted by relatively oligotrophic near-shore seawater in the high salinity region. Based on a temporally high-resolution water discharge dataset, riverine fluxes of DIN, SRP and DSi into the JRE were calculated at 34.3×103 t N a−1, 0.63×103 t P a−1 and 72.7×103 t Si a−1, respectively. In comparison, estuarine export fluxes of DIN, SRP and DSi from the JRE to the TWS were estimated at 34.8×103 t N a−1, 0.82×103 t P a−1 and 71.6×103 t Si a−1. The estuarine addition flux of SRP was independently estimated at 0.16×103 t P a−1. In comparison with major world rivers, the Jiulong River shows a very high areal yield rate of NO3-N. In comparison with historical datasets from 1980s–1990s, concentrations of NO3−N and SRP increased 2–3 times in upper/middle areas of the JRE, while DSi remained at the same level. The latter is much different from decadal nutrient changes in the Mississippi River and the Yangtze River/Estuary. Such nutrient changes may fundamentally contribute to recent red tide events in the JRE and adjacent Xiamen Bay.