Concentrations of phosphate in natural waters are often below the detection limits of conventional nutrient autoanalyzers, by either gas-segmented continuous-flow analysis orflow injection analysis. A liquid waveguide capillary flow cell has been used to extend the sensitivity of a conventional autoanalyzer for the automated analysis of nanomolar concentrations of phosphate in natural waters. Total reflection of light can be achieved within the liquid core of the flow cell because the refractive index of a cell wall coated with Teflon 1600 is lower than that of water. This property allows the manufacturers to construct long liquid waveguide capillary flow cells in a helical, rather than a linear shape, with compact dimensions. A small sample volume is required because the internal volume of a 2-m long capillary flow cell is only approximately 0.5 cm3. Adaptation of this long flow cell to autoanalyzers significantly enhances the sensitivity of automated colorimetric analysis of phosphate with a molybdenum blue method, allowing for the accurate and precise determination of nanomolar concentrations of phosphate in natural waters. The advantages of this technique are a low detection limit (0.5 nM), a small sample volume (2 mL), high precision (2% at 10 nM levels), and automation for the rapid analysis of a large number of samples.

Based on monthly monitoring data of unfiltered water, the nutrient discharges of the eight main rivers flowing into the coastal waters of China were calculated from 2006 to 2012. In 2012, the total load of NH3-N (calculated in nitrogen), total nitrogen (TN, calculated in nitrogen) and total phosphorus (TP, calculated in phosphorus) was 5.1 × 105, 3.1 × 106 and 2.8 × 105 tons, respectively, while in 2006, the nutrient load was 7.4 × 105, 2.2 × 106 and 1.6 × 105 tons, respectively. The nutrient loading from the eight major rivers into the coastal waters peaked in summer and autumn, probably due to the large water discharge in the wet season. The Yangtze River was the largest riverine nutrient source for the coastal waters, contributing 48% of the NH3-N discharges, 66% of the TN discharges and 84% of the TP discharges of the eight major rivers in 2012. The East China Sea received the majority of the nutrient discharges, i.e. 50% of NH3-N (2.7 × 105 tons), 70% of TN (2.2 × 106 tons) and 87% of TP (2.5 × 105 tons) in 2012. The riverine discharge of TN into the Yellow Sea and Bohai Sea was lower than that from the direct atmospheric deposition, while for the East China Sea, the riverine TN input was larger.

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