How to avoid eutrophication in coastal seas? A new approach to derive river-specific combined nitrate and phosphate maximum concentrations

Abstract

Since 1950, increase in nitrogen (N) and phosphorus (P) river loadings in the North-East Atlantic (NEA) continental seas has induced a deep change in the marine coastal ecosystems, leading to eutrophication symptoms in some areas. In order to recover a Good Ecological Status (GES) in the NEA, as required by European Water Framework Directive (WFD) and Marine Strategy Framework Directive (MSFD), reductions in N- and P-river loadings are necessary but they need to be minimal due to their economic impact on the farming industry. In the frame of the "EMoSEM" European project, we used two marine 3D ecological models (ECO-MARS3D, MIRO&CO) covering the Bay of Biscay, the English Channel and the southern North Sea to estimate the contributions of various sources (riverine, oceanic and atmospheric) to the winter nitrate and phosphate marine concentrations. The various distributed descriptors provided by the simulations allowed also to find a log-linear relationship between the 90th percentile of satellite-derived chlorophyll concentrations and the "fully bioavailable" nutrients, i.e. simulated nutrient concentrations weighted by light and stoichiometric limitation factors. Any GES threshold on the 90th percentile of marine chlorophyll concentration can then be translated in maximum admissible 'fully bioavailable' DIN and DIP concentrations, from which an iterative linear optimization method can compute river-specific minimal abatements of N and P loadings. The method has been applied to four major river groups, assuming either a conservative (8μgChlL) or a more socially acceptable (15μgChlL) GES chlorophyll concentration threshold. In the conservative case, maximum admissible winter concentrations for nutrients correspond to marine background values, whereas in the lenient case, they are close to values recommended by the WFD/MSFD. Both models suggest that to reach chlorophyll GES, strong reductions of DIN and DIP are required in the Eastern French and Belgian-Dutch river groups.

Changes in chlorophyll concentration and phenology in the North Sea in relation to de‐eutrophication and sea surface warming

“…and Table ). The marine offshore sites did not exhibit trends in nutrient concentration as they are mainly under the influence of Atlantic waters (Dulière et al ; Lenhart and Große ; Ménesguen et al ). BE offshore is an exception as it shows a decreasing trend in winter DIP.…”

“…BE offshore is an exception as it shows a decreasing trend in winter DIP. Although that waterbody is dominated by Channel waters, it is also under the influence of the Scheldt and the remnant of the buoyant plumes from the Seine and Somme rivers (Ménesguen et al ). The dissolved silica (DSi) loads are lower than the N loads in most rivers, resulting in on average high N:Si ratios at the river outlets.…”

“…The Belgian continental shelf (BCS) is shallow (~ 20–50 m) with its coastal zone being permanently well‐mixed due to tidal energy and its offshore zone being subject to intermittent stratification in the northeastern part (Ruddick and Lacroix ; van Leeuwen et al ). The coastal area is under freshwater influence, mainly from the Scheldt but also from the Rhine/Meuse rivers (Dulière et al ), while the offshore waters are dominated by oceanic inputs including remnants of the buoyant plumes from the Seine and Somme rivers (Ménesguen et al ). In the BCS, the oceanic and freshwater end‐members are well‐mixed horizontally and vertically by tidal energy, resulting in a decreasing coastal‐offshore gradient in salinity, and nutrient and Chl concentrations (Desmit et al ).…”

Modeling the biogeochemical functioning of the Seine estuary and its coastal zone: Export, retention, and transformations

“…On the seaward boundary, bathymetric data and the tidal components were provided by the French Navy Oceanographic Department (Service Hydrographique et Océanographique de la Marine), and the biogeochemical variables came from the MANGA configuration of the ECO‐MARS3D model, a setup that encompasses the English Channel and the Bay of Biscay and uses a 16‐km‐resolution grid and a 4‐d time step (Ménesguen et al ).…”

Reduced phosphorus loads from the Loire and Vilaine Rivers were accompanied by increasing eutrophication in Vilaine Bay (South Brittany, France)

“…The Arzal 5 dam, 8 km fro m the mouth of the Vilaine, was constructed in 1970 to regulate freshwater d ischarge and prevent saltwater intrusion (Traini et al, 2015). The t wo studied rivers, especially the Lo ire, are the main nutrient sources in the northern Bay of Biscay, including VB (Guillaud et al, 2008;Ménesguen et al, 2018a).…”