Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. I. Effects of the Salt Wedge

A series of high-resolution vertical profiles of temperature, salinity, NO3and fluorescence were taken along a transect of the Fraser River estuary to investigate entrainment of NO3. In late spring and summer, the NO3-poor estuarine plume was found to invade the river with the advancing salt wedge on the flood tide and to form a middle layer between the river water and the NO3-rich deep seawater in the salt wedge, forming a three-layered system. Thus, during entrainment the upward flux of salt into the riverine plume does not necessarily result in an upward flux of NO3due to the entrainment of low NO3estuarine plume water. Therefore, the amount of the entrained NO3was determined by the amount of the entrained deep water. Entrainment was affected by the tides, with more entrainment occurring during spring than neap tides. At lower low water during a spring tide, the river outflow was stronger and it pushed the estuarine plume water seaward more effectively. Thus, the outflowing freshwater entrained more NO3-rich deep water during spring tides. There was a chlorophyll maximum located at the bottom of the interface between the estuarine plume and the deep water. The maximum was advected into the river with the invasion of the incoming salt wedge on flood tides and entrained into the outflowing riverine plume on tidal ebbs. The chlorophyll maximum under the riverine plume was often below the euphotic zone and little uptake of nutrients occurred in the dark in samples taken from it at the time of sampling. However, when the samples were incubated under different irradiances, uptake of nutrients increased with increasing irradiance. Thus the phytoplankton in the chlorophyll maximum could serve as a potential seed population for blooms in regions further seaward beyond the river mouth when they were entrained into a zone with improved irradiance and entrained nutrients.