Towards sustainable nutrient management along the Land-Sea continuum, an integrated modeling perspective

The Land-Ocean Aquatic Continuum (LOAC) plays a pivotal role in the transfer and transformation of carbon and nutrients from terrestrial systems to coastal waters, critically influencing coastal eutrophication resulting from excessive nitrogen (N) and phosphorus (P) loads from rivers compared to Silica (Si). Indeed, both agricultural practices on land and biogeochemical processes in and near streams as well as within estuaries control the eventual export of carbon and nutrients into the coastal sea. To address the complex interplay of biogeochemical processes that govern these transfers, an integrated modeling approach combining agricultural practices (GRAFS, an agri-food system model), river network and wetland processes (pyNuts-Riverstrahler modelling framework), and estuarine dynamics (C-GEM model) was applied across metropolitan France over the 2014–2019 period. The estuarine dynamics were modelled only where relevant, on 40 macro-tidal estuaries along the French Atlantic coast. This comprehensive framework explicitly quantifies the cascading fluxes of Dissolved Organic Carbon (DOC), and different forms of N, P and Si from headwaters to estuarine outlets. In addition, three different scenarios of agricultural practices modulating N diffuse inputs were designed and applied ranging from ‘business as usual’ to a switch towards ‘agroecology’. The modeling chain described above was applied to all watersheds larger than 300 km² (n = 80) using reference conditions representative of the 2014-2019 period and validated by an extensive riverine database of 392,870 measurements from 929 stations.  

This integrated approach allows quantifying potential excess in nutrient export into the coastal seas compared to Redfield ratios between N, P and Si. Our simulations reveal that even under the most optimistic trajectories of nutrient reduction from agricultural practices, some coastal regions such as those flowing into the Celtic Sea will still experience nutrients exports above admissible values, despite in and near streams processes in rivers and estuaries typically removing 20-60% of the nutrient inputs from the land. Our results thus highlight the need for an integrated approach of nutrient management strategies encompassing terrestrial ecosystems, inland and coastal waters. Such an approach is needed to evaluate how these management strategies can help achieve sustainable water quality thresholds across the interconnected aquatic ecosystems of the LOAC.