Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

Abstract

Increased flux of carbon and nutrients from human activities in river basins were linked to acidification and deepwater hypoxia in estuaries and coastal areas worldwide. Annual loads (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) of suspended particulate matter (SPM), dissolved organic carbon (DOC), total nitrogen (TN) and total phosphorus (TP) were assessed at the Lake Ontario inlet of the St. Lawrence River (SLR) (7110 m 3 s -1 ) and its estuarine outlet at Québec City (12,090 m 3 s -1 ). Internal loads from the Ottawa River (1950 m 3 s -1 ), seventeen other tributaries, urban wastewaters, atmospheric deposition and erosion were also estimated. Erosion (65% of SPM, 29% of TP), inflow from Lake Ontario (42% of DOC, 47% of TN) and Ottawa River (28% of DOC) contributed important flux to the estuary. Loads from other tributaries (20 and 27% of TN and TP at Quebec City) largely exceeded municipal sources (6% of exported TN and TP) and require future remediation. Aquatic plants fixed 277,000 t of C, 49,000 t of N and 7000 t of P (May-Sept.), delaying the nutrient flux to the estuary and turning the SLR into a nutrient sink over summers of lowest discharge. Degradation of exported organic C could consume 5.4-7.1 million t O 2 year -1 in the estuary whereas SLR flux of N and P represent 31-47% and 7-14% of total annual estuarine flux, respectively. Carbon and Nitrogen flux from freshwaters partly explain the decline in pH and oxygen concentrations in deep estuarine waters thus highlighting the need to reduce diffuse sources of nutrients in the entire watershed.

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

“…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). The deep waters entering the Laurentian Channel from the North Atlantic are a combination of cold, oxygen‐rich shelf and offshore Labrador Current Waters (hereafter called LCW, see Section 2 for definition), and warm, oxygen‐poor North Atlantic Central Waters (NACW) that mix on the continental slope to form Cold Slope waters (Figure 1, Bugden, 1991; Gatien, 1976; Gilbert et al., 2005).…”

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

“…Assumptions and boundary conditions Precipitation (rainfall) and evaporation, as well as inputs from rivers other than the St. Lawrence and Saguenay Rivers are neglected, as their contributions to the nutrient budget are relatively small. Nitrogen uxes from the atmosphere and nitrogen xation are also not considered, a reasonable since atmospheric deposition in the LSLE (3 × 10 8 mol yr −1 , Prospero et al (1996)) accounts for only 1.5% of the nitrogen input to the system (Hudon et al, 2017). Diusion of nutrients out of the sediments is not explicitly resolved but is implicit to the model, since the only ux to sediments is permanent burial.…”

“…The St. Lawrence River drains highly populated areas -with associated discharge of waste waters to the river and its tributaries -and fertile lands that host intensive farming. These activities are the source of high nutrients and particulate organic matter (Hudon et al, 2017) whose export has increased substantially over the last decades (Clair et al, 2013;Marcogliese et al, 2015;Pocklington and Tan, 1987). A better understanding of the fate of these nutrients in the system is essential to assess the role of eutrophication on the observed bottom-water deoxygenation.…”

Seasonal Variation of the Physico-chemical Composition of Ottawa River Waters in the St. Lawrence River

“…Three 8-day sampling cruises were conducted in the St. Lawrence River (SLR) during spring (23-30 May), summer (9-15 August), and fall (11)(12)(13)(14)(15)(16)(17) 2006 aboard the RV "Lampsilis" from the Université du Québec à Trois-Rivières. We studied the SLR along a 450 km distance from its source at the outlet of the Great Lakes, until the interface with marine waters at the estuarine transition zone (ETZ), 50 km downstream from the marine intrusion ( Figure 1).…”

“…Many studies have analyzed the physicochemical and biological characteristics of these waters (e.g., [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]) and of related sediments [25][26][27][28], while other studies focused on optical characterization of these waters (e.g., [27][28][29][30][31][32]). Most of these studies analyzed the spatial variability of these characteristics in the St. Lawrence River, but very few looked at their seasonal and interannual variability.…”