From Farm to Fork and from Land to Sea: using a biogeochemical model to understand the impact of agro-food scenarios on the quality of freshwaters and marine waters

The Seine River is one of the best examples of heavily populated rivers where the imprint of human activities on the biogeochemistry of the water can be tracked throughout many centuries. The downstream sector of the river opens out into a large macro tidal estuary, which, like the rest of the watershed, is subject to great human pressure: the estuarine basin hosts ca. 1M inhabitants and plays a fundamental role in the industrial and logistics sectors in France. This thriving activity has led to many morphological changes over the past century, and these deep physical transformations have impacted the role of the estuary as a biogeochemical buffer.

We here use a deterministic biogeochemical representation of the land-to-sea continuum that successively involves the GRAFS model of the agro-food system, the Riverstrahler model of the river network and an extended configuration of the ECO-MARS3D that allows assessing the role of the estuary in the transformation, storage, and elimination of nutrients, analysing what are the main biogeochemical processes and what are the estuarine sections where these occur.

We then use this unique modelling chain to project different future scenarios, placing particular emphasis on the changes in the agro-food system. The first scenario assumes the pursuit of the current trend of opening and specialisation of agriculture, as well as of the concentration of population within the Paris agglomeration. The second scenario assesses the generalisation of agroecological practices and a healthier human diet. A third hybrid scenario was elaborated assuming that agroecological practices were implemented only in some protected areas, making up about one-third of the total watershed area (in line with the EC Farm to Fork strategy). Results show that only the full agroecological scenario would be able to restore good water quality everywhere in the river network, as well as significantly decrease the risks of toxic algal blooms in the coastal zone. Intermediate situations, such as protecting specific areas, however attractive as a solution, are not enough to offset the impacts of intensive human activities unless the protected areas dedicated to compensating for damage are sufficiently large.