Seaward Nutrient Transport in the Lower St. Lawrence Estuary

Abstract

A transect across the lower St. Lawrence estuary was occupied to estimate volume transport and nutrient distributions (total dissolved nitrogen and phosphorus, and silicate) during February and May through September. Seaward transport was restricted to the upper 50 m and varied from 17 × 103 m3/s (August) to 129 × 103 m3/s (May). Nutrient and volume transport data are combined to estimate seasonal variability in monthly surface nutrient transport towards the Gulf of St. Lawrence. The estimates vary as follows: 0.72–6.72 × 103 tons N/mo, 0.05–0.48 × 103 tons P/mo and 0.50–6.73 tons Si/mo. Ratios between monthly freshwater input into the estuary and the corresponding values for net seaward transport, which varied from 1.1 to 9.4, are combined with annual data (1960–73) on monthly freshwater input into the estuary to estimate potential annual variability in seaward transport for each of the above months. Observed nutrient data, with the above transport estimates, suggest the possible range in monthly seaward nutrient transport that would be associated with the annual variability in monthly freshwater input into the estuary. Seaward nitrogen transport from May through September, estimated at 2.3 × 104 tons, is considered in relation to published data on the primary production in the western part of the Gulf of St. Lawrence. The estuary supplied a smaller amount of nutrients to the Gulf than has previously been assumed.

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

“…waters of the LSLE. This amount of nitrate is comparable to the estimated seaward nutrient transport of 20 3 10 3 t of nitrate over the top 50 m, calculated by Sinclair et al [1976] for the same period at the Rimouski section, a transect across the LSLE passing through Station 23. These authors however pointed out that such a seaward flux was insufficient to sustain the primary production in the Gulf of St. Lawrence.…”

“…Note however, in a best case scenario, i.e., by considering the largest surface for the shoaling area (e.g., 230 km 2 ) and by increasing the upper bounds of the 95% confidence interval on F HLC and F 23 by a factor 1.5 and 1.6, respectively (to account for neap-to-spring modulation at HLC and boundary mixing effect in the LSLE), the maximum vertical fluxes would be 1300 mol s 21 , enough to account for Savenkoff et al [2001] results (685 mol s 21 ). Our results suggest however that the hypothesis from Sinclair et al [1976] is more likely, i.e., that the LSLE plays a relatively minor role in supplying nutrients to the GSL.…”

“…This supports the efficiency of the pumping mechanism described above. Sinclair et al [1976] suggested that nutrient cycling in the LSLE/GSL system was not as hypothesized by Steven [1971], whereby the LSLE was acting as a nutrient supplier to the GSL. They rather suggested that the surface estuarine transport of nutrients from the LSLE accounted for less than 1% of the nutrient supply to the GSL and that its effect was geographically limited to the western portion of the Gulf.…”

“…Since Figure 2a suggests that the nitrate- depleted layer roughly corresponds to the top 50 m of the water column, we calculated the mean flux between 25 and 50 m as a measure of replenishment of the surface layer. This depth range is above the nitracline (estimated to lie between 50 and 200 m in section 3.1), but encompasses the limit often considered between the surface and the cold intermediate layer [e.g., Sinclair et al, 1976;Therriault and Levasseur, 1985;Plourde and Runge, 1993;Savenkoff et al, 2001;Plourde and Therriault, 2004]. This interval has also been chosen to represent the flux at the base of the euphotic zone, typically found at a depth of 10-20 in the LSLE [Therriault and Levasseur, 1985;Vezina, 1994 Note that for fluxes calculations at Station 23, the diffusivity profiles used are from the station itself, i.e., far from the channel sloping boundaries.…”

First Observations of Benthos and Seston from a Submersible in the Lower St. Lawrence Estuary

“…The standing stock of zooplankton is also large (sometimes > 200 mgm-3 ) during the second half of the summer. Other investigators put less emphasis on the nutrient pump mechanism (SINCLAIR et al, 1976;GREISMAN and IN-GRAM, 1977) and indicate productivity values (~ 500 mg C-m-2 -day-1 ) smaller by an order of magnitude (SEVIGNY et al, 1979) for the Gaspé current, and even lower elsewhere.…”