ENTRAINMENT OF NITRATE IN THE FRASER-RIVER ESTUARY AND ITS BIOLOGICALIMPLICATIONS .2. EFFECTS OF SPRING VS NEAP TIDES AND RIVER DISCHARGE

A 24-h time series of high-resolution vertical profiles of salinity, t emperature, NO3 and fluorescence were taken during spring and neap tid es at an anchored station (station 2) in the Strait of Georgia, 8 km s eaward of the mouth of the Fraser River in order to estimate entrainme nt of NO3 resulting from the outflowing riverine plume. The time serie s confirmed that more NO3 was entrained during the spring tide (24 mmo l m-2) than during the neap tide (17 mmol m-2). The contribution of th e entrained NO3 was 2.3 and 1.6 times that of the river-borne NO3 duri ng spring and neap tides, respectively. We hypothesize that spring tid es cause stronger bottom stirring which results in higher NO3 concentr ations in the deep seawater which is the source of NO3 that is entrain ed upward. The results from the times series taken during days of diff erent river discharge show that during higher river discharge (9000 m3 s-1) more NO3 (72 mmol m-2) was entrained than during lower (6720 m3 s-1) river discharge (32 mmol m-2). The entrained NO3 was 5.4 times th at of the river-borne NO3 during the higher river discharge and 3.3 ti mes during the lower discharge. The mechanism which explains the great er NO3 entrainment is that greater river discharge pushes the estuarin e plume seaward, further from the river mouth, and therefore there is a larger area of deep seawater (with higher NO3 concentrations) expose d directly to the riverine plume for entrainment.