The apparatus and methods used for the automatic, colorimetric determinations of dissolved nutrients (nitrate plus nitrite, nitrite, ammonium and orthophosphate) in natural waters are described. These techniques allow for the determination of nitrate plus nitrite for the concentration range 0.02 to 8 mg/L (milligrams per liter) as N (nitrogen); for nitrite, the range is 0.002 to 1.0 mg/L as N; for ammonium, the range is 0.006 to 2.0 mg/L as N; and for orthophosphate, the range is 0.002 to 1.0 mg/L as P (phosphorus). Data are presented that demonstrate the accuracy, precision and quality control of the methods. INTRODUCTION This report describes methods used to determine dissolved nutrients (nitrate plus nitrite, nitrite, orthophosphate and ammonium) in natural waters by air-segmented continuous-flow absorption spectrophotometry. Concentration ranges for the methods described in this report are tabulated below. Analyte Concentration ranges (mg/L as N or P) Nitrate + nitrite 0.02 2.00; 0.08 8.00 Nitrite 0.002 0.200; 0.01 1.00 Orthophosphate 0.002 0.200; 0.01 1.00 Ammonium 0.006 0.400; 0.02 2.00 Samples with dissolved nutrient concentrations that exceed these ranges must be diluted prior to analytical determinations. These working concentration ranges could be modified; 10to 500fold dilution of samples could be achieved using dilution loops or dialyzers, separately or in combination (Coverly, 1985; Patton and Wade, 1990); analytical sensitivity could be increased by increasing the gain of the photometer up to the limiting signal-tobackground ratio, increasing the path length of photometer flow cells, increasing the flow rate of the sample pump tube or decreasing the flow rate of the diluent pump tube. APPARATUS An air-segmented continuous-flow analyzer (Alpkem RFA-300) was used to implement the automated, colorimetric methods described in this report. System components included a 301 sampler, a 302 peristaltic pump, a 313 analytical cartridge base, a 314 power module, three 305A photometers, and a PC-based data acquisition and processing system. Alternative procedures for these methods could have been implemented using flow-injection analyzers or other types of air-segmented continuous-flow analyzers. Operation Principles The configuration of a generalized bench-top air-segmented continuous-flow analyzer is shown in figure 1. An automatic sampler alternately moves the sample withdrawal tube between a reservoir containing analyte-free wash solution and disposable cups containing samples. The sample withdrawal tube connects to one of the peristaltic pump channels, which in turn connects directly to the analytical cartridge (the part of the analyzer between the pump and the photometer in figure 1). Samples and the wash solution are pumped into the analytical cartridge as discrete plug flows that are separated by one or more air bubbles that form at the tip of the sample withdrawal tube each time it is exposed to the atmosphere. The introduction of several intersample air bubbles at the beginning of