Exploring the Maximum Turbidity Zone in the St. Lawrence Estuary (Eastern Canada)

A highly turbid zone is observed in many estuaries, where the concentration of suspended particulate matter (SPM) is higher than both landward and seaward adjacent areas. Usually maintained by tides and river discharge, this maximum turbidity zone (MTZ) plays a crucial role in estuarine ecosystems, as high SPM impacts light and nutrient concentrations. The St. Lawrence Estuary (SLE), Eastern Canada, like other estuaries, is characterized by strong gradients of salinity, temperature and SPM concentration. SLE is subject to a particularly strong seasonal variability due to the formation of sea ice in winter, and snowmelt producing spring freshet.

In geological terms, the St. Lawrence is a young river in disequilibrium, and most of the suspended sediment originates from its bed and banks. However, sources, sinks and pathways of SPM yet remain unclearly defined, and current understanding of sediment budget is only partial. To quantitatively assess the sediment budget, we aim to implement a hydro-sedimentary model able to represent main processes (resuspension, transport, flocculation and deposition).

The development of this model requires observations to constrain the number of sediment classes and their properties (e.g. settling velocity). For this, we use observations obtained from a field campaign held in August 2019, when the sediment transport is assumed low. We measured vertical profiles of concentrations and grain-size of SPM at stations scattered across the entire area. We will present a three-dimensional snapshot of SPM properties across the SLE and discuss it in relation to ocean dynamics and historical data of seabed grain-size distribution. We will explore the MTZ contribution to the SLE sources and sinks of MPS and sediment budget.