Speaker: Dr. Cynthia Bluteau, Postdoctoral Fellow, University of Quebec at Rimouski

Title: “Nutrient enrichment and ventilation during winter in the St. Lawrence Estuary”

Date: April 23rd, 2:30-3:30 pm

Location: Rm. 226, Ellis Hall

 

Abstract

I will introduce a multi-disciplinary project during winter aboard a Canadian Coast Guard ice-breaker, the Amundsen, which is typically stationed in the Arctic during summer for scientific research. The Coast Guard approved for the first time in 2018 the presence of a science team during their normal de-icing operations and ship escorting operations in the St-Lawrence between Quebec City and the Cabot Straight.  Current monitoring by DFO is by helicopter, and so confined to conductivity-temperature vertical profiles and nutrients determined from water collected near the surface with Niskin bottles. 

Aboard the ice-breaker, sampling could be extended to include turbulence profiling, CTD, plankton nets, sediment and ice grabs. In this presentation, I will focus mainly on the turbulence dataset that was combined with oxygen and nutrients observations to estimate vertical fluxes of these two parameters in the St-Lawrence’s lower estuary and the Gulf. The goal with these estimates was to establish whether water in the lower estuary, downstream of the Saguenay Fjord, would be better oxygenated in winter because of convective mixing processes. The second objective was to assess nutrient transport pathways during winter.  

Preliminary results demonstrate that oxygenation is high at the head of the Laurentian channel near the Saguenay Fjord because of intense mixing. This mixing is attributed to a tidal upwelling event, which is common in summer but unlike summer,  this upwelling transports nutrient-poor water situated about 50-m beneath the surface. This nutrient-poor water then mixes with nutrient-rich water from upstream in the St-Lawrence before moving downstream where it progressively mixes with nutrient-poor surface waters in the Gulf.  This general picture is not new, but the relative importance of vertical mixing versus horizontal transport is currently debated given the lack of winter turbulence measurements.

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