Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. II. Effects of Springvs.Neap Tides and River Discharge

A 24-h time series of high-resolution vertical profiles of salinity, temperature, NO3and fluorescence were taken during spring and neap tides at an anchored station (station 2) in the Strait of Georgia, 8 km seaward of the mouth of the Fraser River in order to estimate entrainment of NO3resulting from the outflowing riverine plume. The time series confirmed that more NO3was entrained during the spring tide (24 mmol m−2) than during the neap tide (17 mmol m−2). The contribution of the entrained NO3was 2•3 and 1•6 times that of the river-borne NO3during spring and neap tides, respectively. We hypothesize that spring tides cause stronger bottom stirring which results in higher NO3concentrations in the deep seawater which is the source of NO3that is entrained upward. The results from the times series taken during days of different river discharge show that during higher river discharge (9000 m3s−1) more NO3(72 mmol m−2) was entrained than during lower (6720 m3s−1) river discharge (32 mmol m−2). The entrained NO3was 5•4 times that of the river-borne NO3during the higher river discharge and 3•3 times during the lower discharge. The mechanism which explains the greater NO3entrainment is that greater river discharge pushes the estuarine plume seaward, further from the river mouth, and therefore there is a larger area of deep seawater (with higher NO3concentrations) exposed directly to the riverine plume for entrainment.