Abstract Cool, nitrate-enriched water is drawn into Cook's Passage by rapid inflowing tidal currents. The water is a mixture of surface Coral Sea water and subtropical lower water. At depths > 30 m, the range of the mean nitrate concentration for outflows, calculated from hourly measurements, was 0.25–0.8 μg 1 −1, and for inflows it was 1.0–1.5 μg 1 −1. The maximum values normally recorded were in the range 2.0–2.7 μg1 −1. The quantity of nitrate in the water column is not dependent solely on the tidal amplitude; undulations in the thermocline are believed to play a major role in determining how much enrichment takes place. In the 40 m water column examined, the nitrate concentration in outflowing waters appeared to be a function of the amount entrained on the previous incoming cycle, with a loss to the lagoon of approximately 16 mg under 1 m 2 of surface. The mean silicate concentrations in outflows and inflows were 32.0 ± 6.0 and 35.2 ± 4.6 μg 1 −1, respectively, with corresponding phosphate values of 4.5 ± 0.7 and 4.6 ± 0.9 μg 1 −1, respectively. Neither shows any distinctive relationship with tides.