A multidisciplinary study of hypoxia in the deep water of the Estuary and Gulf of St. Lawrence: Is this ecosystem on borrowed time?

Abstract

The dissolved oxygen concentration has progressively decreased in the bottom water of the Lower St. Lawrence Estuary (LSLE) during the last century and reached the severe hypoxic threshold ([O2] < 62.5 µmol L-1) in the 1980s where it has hovered ever since. This thesis investigates the causes and impacts of the large-scale persistent hypoxia in the bottom water of the LSLE, using multidisciplinary tools. The causes were identified, by examining the processes governing the oxygen distribution in the water column, using a two-dimensional advection-diffusion model representing the transport of oxygen in the bottom water along the Laurentian Channel (Gulf of St. Lawrence, Canada). The impacts of persistent hypoxia were highlighted on modifications of sediment chemistry, more specifically by the diagenetic response of three redox-sensitive elements, Mn, Fe, and As, as well as on the fluxes of nutrients and metabolites across the sediment-water interface. Results of numerical simulations revealed that the oxygen distribution in the water column is governed by the combination of physical and biogeochemical processes, but its vertical distribution is mostly controlled by the deep-water circulation. In other words, the vertical distribution is much more sensitive to variations in physical than biogeochemical processes, and oxygen conditions at the continental shelf edge, where Laurentian Channel bottom waters originate, are mostly responsible for the establishment of hypoxia in the Lower Estuary. Whereas the concentrations and vertical distributions of sedimentary Mn phases seem to adjust rapidly to the progressive depletion of oxygen in the overlying waters and remained at steady-state since the 1980s, the development and persistence of hypoxia strongly modified the chemistry of Fe and As in LSLE sediments. The lower overlying-water oxygenation increased the proportion of organic matter that is oxidized by anaerobic pathways in the sediments, which contributed to increase the proportion of dissolved and solid reactive phases of Fe and As. The greater availability of reactive Fe and As phases restricted the formation of pyrite, which, in turn, limited the sequestration of As with pyrite, increasing the availability of this potentially toxic trace element to benthic organisms. Despite the accumulation of Fe and As in sedimentary reactive phases over the past 25 years, it has not significantly modified their fluxes across the sediment-water interface, and their sequestration within the sediment is maintained.