BENTHIC FORAMINIFERAL ASSEMBLAGES FROM THE LAURENTIAN CHANNEL IN THE LOWER ESTUARY AND GULF OF ST. LAWRENCE, EASTERN CANADA: TRACERS OF BOTTOM-WATER HYPOXIA

Over the past century, an increase in temperatures and a decrease in dissolved oxygen (DO) concentrations have been observed in the bottom waters of the Laurentian Channel (LC), throughout the Lower St. Lawrence Estuary (LSLE) and the Gulf of St. Lawrence (GSL), eastern Canada. The oxygen depletion and the development of hypoxic waters (<62.5 µM DO) in the LSLE has been attributed to an increase of the organic matter and nutrient fluxes and eutrophication as well as to changes in large scale ocean circulation in the northwestern North Atlantic (i.e., decreased contribution of the Labrador Current to the GSL) resulting in increased bottom-water temperatures, enhanced respiration rates in the GSL and lower DO concentrations. To document the impact of these changes, we analyzed the benthic foraminiferal assemblages of four sediment cores taken in the LC in 2018. Radiometric measurements (²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs) indicate that the cores encompass the last 50 years of sedimentation in the LSLE and the last ∼160 years in the GSL. The sedimentary record shows a 60 to 65% decrease in benthic foraminiferal taxonomic diversity in the LC since the 1960s. An accelerated change in the foraminiferal assemblages is observed at approximately the same time at all studied sites, around the late 1990s and the early 2000s, towards populations dominated by the hypoxia-tolerant indicator taxa Brizalina subaenariensis, Eubuliminella exilis, and Globobulimina auriculata. This evolution of assemblages reflects incursions of the hypoxic zone into the western GSL over the last decades. Results of our multivariate analyses highlight the potential of benthic foraminiferal assemblages as a proxy of bottom-water hypoxia.

As a sudden drop of the bottom-water minimum DO concentrations was observed in the LSLE since 2019, from ~55-60 μM to ~35 μM in 2021, we analysed new sediment samples collected in 2021 at the head of the LSLE. Preliminary results of the assemblages reveal an important increase in the relative abundance of Globobulimina auriculata.