A portable ammonium analyser was developed and used to measure in situ ammonium in the marine environment. The analyser incorporates an improved LED photodiode-based fluorescence detector (LPFD). This system is more sensitive and considerably smaller than previous systems and incorporates a pre-filtering subsystem enabling measurements in turbid, sediment-laden waters. Over the typical range for ammonium in marine waters (0–10 μM), the response is linear (r2 = 0.9930) with a limit of detection (S/N ratio > 3) of 10 nM. The working range for marine waters is 0.05–10 μM. Repeatability is 0.3% (n = 10) at an ammonium level of 2 μM. Results from automated operation in 15 min cycles over 16 days had good overall precision (RSD = 3%, n = 660). The system was field tested at three shallow South Florida sites. Diurnal cycles and possibly a tidal influence were expressed in the concentration variability observed.
