Winter observations alter the seasonal perspectives of the nutrient transport pathways into the lower St. Lawrence Estuary

Abstract

Abstract. The St. Lawrence Estuary connects the Great Lakes with the Atlantic Ocean. The accepted view, based on summer conditions, is that the estuary's surface layer receives its nutrient supply from vertical mixing processes. This mixing is caused by the estuarine circulation and tides interacting with the topography at the head of the Laurentian Channel. During winter when ice forms, historical process-based studies have been limited in scope. Winter monitoring has been typically confined to vertical profiles of salinity and temperature as well as near-surface water samples collected from a helicopter for nutrient analysis. In 2018, however, the Canadian Coast Guard approved a science team to sample in tandem with its ice-breaking and ship escorting operations. This opportunistic sampling provided the first winter turbulence observations, which covered the largest spatial extent ever measured during any season within the St. Lawrence Estuary and the Gulf of St. Lawrence. The nitrate enrichment from tidal mixing resulted in an upward nitrate flux of about 30 nmol m−2 s−1, comparable to summer values obtained at the same tidal phase. Further downstream, deep nutrient-rich water from the gulf was mixed into the subsurface nutrient-poor layer at a rate more than an order of magnitude smaller than at the head. These fluxes were compared to the nutrient load of the upstream St. Lawrence River. Contrary to previous assumptions, fluvial nitrate inputs are the most significant source of nitrate in the estuary. Nitrate loads from vertical mixing processes would only exceed those from fluvial sources at the end of summer when fluvial inputs reach their annual minimum.

Decadal Variability in Subsurface Nutrient Availability on the Scotian Shelf Reflects Changes in the Northwest Atlantic Ocean

“…The outflow from the GSL—occurring predominantly on the western side of Cabot Strait—reflects changes to the GSL system, which integrates changes in riverine inputs, inflow of predominantly Arctic‐origin waters from the Labrador Shelf (i.e., inner LC branch via the Strait of Belle Isle), as well as dense slope water contributions originating from both the LC and GS through Cabot Strait (Figure 1). The supply of nutrients to the surface of the Lower St. Lawrence Estuary is mainly driven by vertical upwelling and mixing of nutrient‐rich deep waters at the head of the Laurentian Channel (Cyr et al., 2015; Ingram, 1983; Jutras, Mucci, et al., 2020; Lavoie et al., 2021), augmented by the horizontal advection of high‐nutrient riverine inputs from the upper estuary (Bluteau et al., 2021; Hudon et al., 2017; Lavoie et al., 2021). The relative nutrient contribution of the SLR compared to the total supply at the head of the Laurentian Channel varies between 25% and 50%, depending on the year and season (Lavoie et al., 2021, and references therein).…”

Dissolved Methane in the World's Largest Semi‐Enclosed Estuarine System: The Estuary and Gulf of St. Lawrence (Canada)

“…Strong mixing and high turbidity limit primary production in the USLE (Therriault et al., 1990). In the LSLE and the GSL, the more stable water column, in conjunction with nutrients delivered from the St. Lawrence River and upwelled near the head of the LC (Bluteau et al., 2021), leads to higher primary productivity, with peak values occurring in summer in the LSLE (Mei et al., 2010; Therriault & Levasseur, 1986) and in spring in the GSL (de Lafontaine et al., 1991; Mei et al., 2010).…”