ENTRAINMENT OF NITRATE IN THE FRASER-RIVER ESTUARY AND ITS BIOLOGICALIMPLICATIONS .3. EFFECTS OF WINDS

Two 24-h time series of vertical profiles of velocity, salinity, tempe rature, fluorescence and NO3 were conducted during weak and strong win ds (2.2 and 7.3 m s-1, respectively) to demonstrate wind effects on en trainment of NO3 near the mouth of the Fraser River. The results showe d that wind-induced entrainment of NO3 was mainly responsible for the increased NO3 concentrations in the upper layer because shear velociti es between the upper and lower layers were great enough to break down the pycnocline and allow diapycnal mixing to occur. Strong shear was i ndicated by Richardson numbers less than 0.25 near the depth at which flows in the upper and lower layers moved opposite to each other. As a result of wind-induced entrainment and mixing, the NO3 minimum in the water column was gradually eroded and disappeared at the end of the t ime series during the strong wind event. The amount of entrained NO3 u nder windy conditions (44 mmol m-2) was almost three times that (16 mm ol m-2) under weak winds. The high ratios of the amount of entrained N O3 to river-borne NO3 (12 under windy conditions and 5.6 under weak wi nds) indicates that wind-induced entrainment of NO3 in summer is parti cularly important for new production. Because turbulent energy came fr om winds, mixing started at the surface and moved downward. Thus, phyt oplankton cells remained in the surface mixed layer, and responded to the entrained nutrients and grew rapidly. Phytoplankton biomass and pr imary production in the water column increased at the end of the time series, compared to the beginning.