Nutrient cycling in the Lower St. Lawrence Estuary: Response to environmental perturbations

Abstract

We present a simple linear three-box model of nutrient cycling in the Lower St. Lawrence Estuary (LSLE).A present-day nutrient budget is obtained for xed-nitrogen, phosphorus, and silica, from which the model's parameters are derived. The model is used to (i) test the sensitivity of each layer's nutrient concentration to perturbations in nutrient and water volume inputs, (ii) obtain the response time of the system to a new steady state following a perturbation, and (iii) estimate bottom-water oxygen consumption. We nd that most of the dissolved nutrients (70% of xed-nitrogen, 90% of phosphorus) that reach the surface waters in the Lower Estuary originate from the deep waters, implying that the anthropogenic eutrophication potential of the St.Lawrence River is moderate. Our nutrient budget suggests that the Lower St. Lawrence Estuary acts as a nutrient pump for the Gulf of St. Lawrence. Nitrate appears as the limiting nutrient to surface productivity in the LSLE. This model can be used to test the impact of natural or anthropogenic perturbations on nutrient and oxygen concentrations in the LSLE.

Temporal Changes in the Causes of the Observed Oxygen Decline in the St. Lawrence Estuary

“…Between 1970 and 1995, nearly half of the oxygen decline observed at the head of the Laurentian Channel is explained by an increase in biological oxygen demand during the transit of deep waters along the Laurentian Channel ( + = 24 μmol kg −1 ). The increase in eutrophication is substantiated by an increase in nutrient inputs to the St. Lawrence watershed over the same period (Figure 7d, reproduced from Goyette et al., 2016), We focus on the nitrogen input because nitrate is the limiting nutrient in the Estuary and Gulf (Jutras et al., 2020). The increased oxygen utilization derived from the eOMP analysis is consistent with previous reports of increased organic matter flux and eutrophication (Thibodeau et al., 2006) as well as enhanced respiration rates due to increased bottom‐water temperatures (Genovesi et al., 2011) in the St. Lawrence Estuary.…”

“…13 of 20 O O does not directly yield the oxygen utilization during the transit, since vertical mixing brings oxygen from the oxygen-saturated surface waters to the deep waters of the Laurentian Channel. To calculate the 1970 estimate, we assume that the turbulent fluxes were invariant over the 4-7-years transit time from the Atlantic to the head of the Laurentian Channel, in which case the difference in  Lawrence watershed over the same period (Figure 7d, reproduced from Goyette et al, 2016), We focus on the nitrogen input because nitrate is the limiting nutrient in the Estuary and Gulf (Jutras et al, 2020). The increased oxygen utilization derived from the eOMP analysis is consistent with previous reports of increased organic matter flux and eutrophication (Thibodeau et al, 2006) as well as enhanced respiration rates due to increased bottom-water temperatures (Genovesi et al, 2011) in the St. Lawrence Estuary.…”

“…As a result, waters at the head of the Laurentian Channel are depleted in oxygen compared to the North Atlantic waters (Figure 2a). In recent decades, observations have revealed increased respiration rates (Genovesi et al., 2011) under higher deep‐water temperatures (Gilbert et al., 2005) and eutrophication (Jutras et al., 2020; Thibodeau et al., 2018). The latter is fostered by an increase in organic matter and nutrient exports to the estuary's main tributary, the St. Lawrence River (Environment and Climate Change Canada, 2018; Goyette et al., 2016), that drains highly populated and industrialized areas as well as intensively farmed lands (Hudon et al., 2017).…”

Nutrient transport pathways in the Lower St. Lawrence Estuary: seasonal perspectives from winter observations

“…CC BY 4.0 License. Biogeochemical box-models have been used to evaluate the importance of vertical mixing processes in supplying nutrients into the LSLE's surface (e.g., Savenkoff et al, 2001;Jutras et al, 2020). These box-models consider the fluvial nutrients advected into the LSLE and assume that the estuarine circulation of the LSLE can be idealized using a few homogeneous layers in a steady-state.…”

“…Vertical mixing processes brought 685 mol s −1 near the surface of the LSLE -more than 5 times than fluvial waters (Figures 6 and 10 of Savenkoff et al, 2001). Jutras et al (2020) revisited the nutrient loads with a three-layer model representative of the LSLE's summer conditions. Their vertical flux of dissolved nitrogen in the surface (1050 mol s −1 ) was three times larger than the contributions from fluvial sources.…”