Influence des flux anthropiques de nutriments et des caractéristiques du territoire sur la qualité de l'eau : une perspective historique du bassin du Saint-Laurent

Nitrogen (N) and phosphorus (P) are at the heart of the challenges of reconciling food production and water quality. If N and P are essential nutrients for agricultural production, nutrient enrichment of aquatic ecosystems can lead to loss of oxygen in the water, disruption of food webs, proliferation of toxic algae, and loss of biodiversity. This eutrophication process therefore has important consequences for human health and local economies. The aim of this thesis is to improve our understanding of anthropogenic and natural factors governing N and P fluxes at the watershed scale, in order to foster a better nutrient management at the Food-Water Nexus. First, we examine the impacts of human activities on N and P fluxes in 76 watersheds of the St. Lawrence, from a historical perspective covering important social transitions of the last century. We show that since 1901, the N and P surpluses in the St. Lawrence basin have increased by 4.5 and 3.8 times respectively, with a peak in 1991 mainly due to atmospheric N deposition and the application of phosphate fertilizer. These net anthropogenic inputs to the landscape are strongly related to riverine loads where ~ 22% of N and ~ 17% of P surpluses are exported by rivers. By tracking the main sources of inputs to the landscape over time, this study emphasizes that strategies will need to be region-specific, although it also clearly shows the positive impact of large-scale legislation (Goyette et al. 2016). While the anthropogenic inputs of N and P to the landscape favor riverine exports, our understanding of other factors controlling the transfers remains limited. Also, beyond absolute nutrient loads to aquatic ecosystems, the relative concentrations of N and P influences water quality. We thus explore the role of different land and climate characteristics in controlling and decoupling N and P fluxes along the aquatic network. We show that precipitation and water retention capacity of the landscape act as the main controlling forces on these fluxes, and that lakes and reservoirs play a secondary but significant role. We also identify a legacy effect, especially during extreme precipitation events. We conclude that, in the face of climate change, increasing the water retention capacity of the landscape is essential to mitigating eutrophication (Goyette et al. in review a). Legacy P is known as a major water quality issue, yet time lags incurred by this effect remain uncertain. Moreover, the existence of an accumulation threshold beyond which a basin exceeds its optimal P retention capacity is unknown. We show that the accumulation threshold for optimal P retention by watersheds is particularly low and can be achieved in less than 10 years at current input rates in highly agricultural regions. Conversely, a return to initial conditions in terms of water quality could take several centuries. Our study presents for the first time a threshold of P buffering capacity at the watershed scale and highlights the need to exploit residual P stocks in soils (and sediments) in order to reconcile food production and water quality (Goyette et al. in review b).  Among various measures to mitigate nutrient pollution, increasing nutrient use efficiency in the agricultural sector is emerging as a key strategy. We therefore draw a portrait of temporal trends in N and P use efficiency in the agro-food sector of the St. Lawrence basin during the last century. We show that agriculture in this region was unsustainable between 1901 and 1960, gradually depleting soils of nutrients, and that through the use of fertilizers, the system rapidly tipped to an inefficient system where ~70% of N and ~90% and P resources are lost to the environment. We observe that an increase in efficiency within the agro-food system does not guarantee a decline in nutrient pollution and thus conclude that, in parallel with gains in efficiency, there is an urgent need to identify and constrain human activities to the carrying capacity of ecosystems (Goyette et al. in prep). By quantifying the different effects of N and P transport and retention by human activities and key climate and landscape features, this thesis provides relevant information to a better understanding of the dynamics of N and P within socio-ecological systems.

Keyphrases: climat, Nitrogen, Phosphorus, Eutrophication, Anthropogenic, Riverine loads, Water quality, Land use, Lakes, Dams, Climate, St. Lawrence basin, Historical