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Document riverine inputs of organic carbon and nutrients into the marine estuary in relation to hypoxia and acidification

Context and project description

In the marine estuary of the St. Lawrence, the organic carbon and nutrient inputs (nitrates, phosphorous and nitrogen) associated with agricultural activities upstream may have damaging consequences on the ecosystem: eutrophication that leads to the proliferation of toxic or harmful algae and the development of acidified and hypoxic areas, meaning lacking in dissolved oxygen. These areas, already identified in the bottom waters of the estuary, raise concerns for aquatic life.

The project carried out by researchers as part of the St. Lawrence Action Plan studies the various aspects of the phenomenon. The effects to document occur in the estuary, but their source is in fresh water. As such, part of the work took place upstream in the St. Lawrence, namely taking measures at the Lake Ontario outlet (Wolfe Island) as well as at the mouth of the Ottawa (Carillon) and in Québec.


This project has helped to increase knowledge of the transport of nutrients in the St. Lawrence River to the estuary. Thanks to the creation of numerical tools, the project included an evaluation of the effect of nitrate inputs on hypoxia (lack of oxygen) and acidification of waters in the Lower Estuary of the St. Lawrence. The modelling work done in this project helped provide a better understanding of the influence of nitrate inputs and resulted in a scientific publication [1]. The project also led to the publication of a scientific article on the contribution of colloidal matter (mixture of suspended particles) to the organic carbon reservoir in the Lower Estuary [2]. An article quantifing the mean annual flux of Carbon, Nitrogen and Phosphorus to the St. Lawrence estuary, originating from a range of upriver sources (shoreline erosion, municipalities, tributaries and atmospheric deposition) was published as well [3]. Another article focusing on modelling of the effects of a reduction in nutrient inputs on hypoxia in the Lower Estuary of the St. Lawrence should be published soon. pollution of the St. Lawrence

[1] For more information, see the original article, “The Gulf of St. Lawrence biogeochemical model: a tool to study the evolution of hypoxic conditions and other habitat changes” (International Council for the Exploration of the Sea (ICES), CM 2015, R:14).

[2] For more information, see the original article, “Contribution of transparent exopolymeric particles (TEP) to estuarine particulate organic carbon pool” (Marine Ecology Progress Series, 2015, No. 529, pp. 17–34).

[3] For more information, see the original article, “Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Québec, Canada)” (Biogeochemistry, 2017, No 135, pp. 251-276).

Participating departments

Government of Canada

  • Fisheries and Oceans Canada
  • Environment and Climate Change Canada

Government of Quebec

  • Ministère du Développement durable, de l’Environnement et de la Lutte contre les changements climatiques