Water-authorities are prompt to consider coastal eutrophication in their management of water quality. Identifying admissible nutrient thresholds remains highly uncertain considering the complexity of biogeochemical processes involved in carbon and nutrient cycling that can either attenuate or exacerbate the imbalance in nutrients cascading from land to the coastal waters. A modelling chain including agricultural practices (GRAFS model), the transfer and transformations of carbon and nutrients along the hydrographic network (pyNuts-Riverstrahler model) and estuaries (C-GEM model) was designed and applied, for the first time at the scale of the entire metropolitan France, over the 2014-2019 period. Using a large database of riverine measurements (n=392,870 data from 929 stations), the results were validated for dissolved organic carbon (DOC), nitrate (NO3), dissolved phosphorus (PO4), total phosphorus (TP) and dissolved silica (DSi). This land-to-sea modelling chain allows identifying the origin of nutrient excess exported to coastal waters where it supports harmful algae blooms. Our work provides an accurate source apportionment of nutrients for 64 French sea outlets and demonstrates that efforts are still needed to reduce diffuse agricultural nutrients loads. Thanks to process-based and spatially explicit modelling tools, our approach identifies and quantifies the main processes governing the transfer of nutrients along the aquatic continuum and should ultimately sheds light on achievable levels of nutrients in marine and coastal areas.

Primary Presenter: Antoine Casquin, Sorbonne Université (antoine.casquin@sorbonne-universite.fr)

Authors:

Antoine Casquin, Sorbonne Université, UMR METIS 7619  (antoine.casquin@sorbonne-universite.fr)

Goulven Laruelle, Université Libre de Bruxelles  (goulven.gildas.laruelle@ulb.be)

Marie Silvestre, FIRE (Fédération Île-de-France de Recherche sur l'Environnement)  (marie.silvestre@sorbonne-universite.fr)

Josette Garnier, Sorbonne Université  (josette.garnier@upmc.fr)

Gilles Billen, Sorbonne Universite  (gilles.billen@sorbonne-universite.fr)

Vincent Thieu, Sorbonne Universite  (vincent.thieu@sorbonne-universite.fr)