Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. III. Effects of Winds

Abstract

Two 24-h time series of vertical profiles of velocity, salinity, temperature, fluorescence and NO3were conducted during weak and strong winds (2·2 and 7·3m s−1, respectively) to demonstrate wind effects on entrainment of NO3near the mouth of the Fraser River. The results showed that wind-induced entrainment of NO3was mainly responsible for the increased NO3concentrations in the upper layer because shear velocities between the upper and lower layers were great enough to break down the pycnocline and allow diapycnal mixing to occur. Strong shear was indicated 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 NO3minimum in the water column was gradually eroded and disappeared at the end of the time series during the strong wind event. The amount of entrained NO3under windy conditions (44mmol m−2) was almost three times that (16mmol m−2) under weak winds. The high ratios of the amount of entrained NO3to river-borne NO3(12 under windy conditions and 5·6 under weak winds) indicates that wind-induced entrainment of NO3in summer is particularly important for new production. Because turbulent energy came from winds, mixing started at the surface and moved downward. Thus, phytoplankton cells remained in the surface mixed layer, and responded to the entrained nutrients and grew rapidly. Phytoplankton biomass and primary production in the water column increased at the end of the time series, compared to the beginning.