Water renewals in the Saguenay Fjord

Abstract

Water renewals and renewal times of the Saguenay Fjord are investigated and classified according to their intrusion depth. Renewal dynamics are controlled by a shallow sill (∼20 m) at the fjord mouth, by large tides that are a distinguishing feature of the Saguenay Fjord and by large vertical mixing inside the inner basin ( K∼10−4 normalm2 normals−1). A mooring was deployed in the inner basin of the fjord to provide a clearer quantitative understanding of the complexity and seasonality of water renewals in this seasonally ice‐covered fjord. The mooring provided information on currents over nearly the entire water column, along with temperature‐salinity at a few discrete depths. Hydrographic temperature and salinity transects spanning multiple seasons and years as well as turbulence profiles were also collected. The observations show that the fjord dynamics are more complex than previously hypothesized, with large changes in renewal event depths leading to three different renewal regimes. Part of this renewal depth variability may be explained by the seasonality of the St. Lawrence estuarine circulation. Because of the large turbulence within the inner basin bottom layer, the density decreases over time such that new deep renewals can occur every year. The mechanisms behind the large vertical mixing cannot yet be clearly identified but a statistically significant correlation ( K∝N−1.3) suggests that internal wave breaking may be a significant contributor to deep turbulence mixing in the inner basin. The renewal time of the inner basin waters is estimated to be between 1 and 6 months.

The Gulf of St. Lawrence Biogeochemical Model: A Modelling Tool for Fisheries and Ocean Management

“…In the LSLE, PON sinking speed is enhanced by flocculation processes and thus more organic matter is remineralized at depth. The greater DO consumption combined with the reduced ventilation of the water below 175 m (Lavoie et al, 2000;Belzile et al, 2016) leads to low DO concentrations (50-100 mmol m −3 ) and saturation values (as low as 20%) in the deeper layers of the LSLE (Figures 6C, 11). The deeper parts of the GSL (below 325 m in the eastern part of the Laurentian Channel) contain water entering from the North Atlantic which are characterized by higher DO concentrations (160-180 mmol m −3 , Figure 6C).…”

“…The nitrate flux from the St. Lawrence River is maximal in winter and spring, due to the combination of high runoff and high nitrate concentration (Figure 3B). The depth from which water is upwelled at the head of the Laurentian Channel is also maximal in winter (Saucier et al, 2009), which combined with a shoaling of the upper limit of Atlantic water (Galbraith, 2006), leads to upwelling of water with higher salinity and temperature (Belzile et al, 2016), higher nitrate concentration, and lower DO and pH than in summer. These two processes lead to the delivery of a large amount of nutrients to the LSLE when primary production is at a minimum.…”