A version of the orthophthaldialdehyde-fluorescence ammonium determination for flow injection analysis (FIA) is presented here, with a view to its use for in situ, low-power consumption systems, Thus, the reaction temperature was limited to 30 degreesC and FIA was used in stop-Bow mode (3 min stop). The calibration is linear up to 50 mu mol l(-1), but concentrations up to 100 mu mol l(-1) can be measured. Repeatability is around 1% in the range of 0.5-4 mu mol l(-1) and the detection limit is about 0.03 mu mol l(-1) Over the salinity range of 5-35 (seawater practical salinity scale) the salt effect is almost negligible (within +/- 2%); and below salinity of 5 it increases to a maximum of -9% in fresh water compared to seawater. Hydrogen carbonate, dissolved oxygen and turbidity (either suspended sediments or phytoplankton cells) have almost no adverse effect in a wide range of concentrations, covering most natural water conditions. Relative interference of primary amines is negligible and mercury (a common sample preservative) does not depress the signal up to 20 mg l(-1) Hg II. Sulfide, that may be present in areas with anoxic waters, depresses the signal only slightly and linearly (-9% at 100 mu mol l(-1) S2-). The method appears to be convenient for the determination of ammonium in most coastal, estuarine and fresh waters. Sample throughput is 9 h(-1). The performance of the method can be improved, either by increasing the reaction time (low throughput) or, if enough energy is available, by increasing the reaction temperature (non-stop-flow mode, high throughput). Combining FIA and fluorometry appears to be interesting for in situ determination (submersible devices) of dissolved compounds in environments with variable salinity and turbidity (especially coastal and estuarine waters). (C) 2001 Elsevier Science Ltd. All rights reserved.