A systematic approach to choosing an automated nutrient analyser for river monitoring

Automated high frequency nutrient analysers have recently become available for in-stream monitoring of freshwater ecosystems. These instruments permit observation of nutrients at the same temporal frequency as discharge measurements. In principle this development will overcome some of the limitations of current water quality sampling and enable a better understanding of coupled terrestrial and aquatic environmental systems. This paper presents a systematic approach to choosing such instruments for research applications and informing the design of prescribed water quality monitoring. The instruments considered are ion-selective electrodes, wet chemistry analysers and ultraviolet/visible light spectrophotometers. Before committing to a new technology, investigators should evaluate instrument related considerations and complementary, often project-specific factors, in a structured way. The instrument related considerations are the ability of the instrument to measure the required nutrient parameters, the temporal resolution, the detection limits and range of individual measurements, required accuracy and operating temperatures as well as the overall cost. The complementary factors to consider are the maintenance effort, operating conditions, major service expenses and special consideration associated with individual instruments. This evaluation is presented for a range of available instruments across the three instrument types. As supplementary material a tabular approach that combines these factors is proposed and illustrated with a case study where instruments were selected for researching nutrient movement in a catchment in northern Tasmania, Australia. Few of the instruments can provide all the essential requirements of the case study and significant compromise of maintenance costs and functionality was necessary. The approach is readily adaptable to choices of instruments for a wide range of investigations concerning aquatic water quality. Clearly the outcome of the choice process is likely to be different for different applications, locations and environments.