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

Abstract

Abstract. The evolution of eutrophication parameters (i.e., nutrients and phytoplankton biomass) during recent decades was examined in coastal waters of the Vilaine Bay (VB, France) in relation to changes in the Loire and Vilaine rivers. Dynamic linear models were used to study long-term trends and seasonality of dissolved inorganic nutrient and chlorophyll a concentrations (Chl a) in rivers and coastal waters. For the period 1997–2013, the reduction in dissolved riverine inorganic phosphorus (DIP) concentrations led to the decrease in their Chl a levels. However, while dissolved inorganic nitrogen (DIN) concentrations decreased only slightly in the Vilaine, they increased in the Loire, specifically in summer. Simultaneously, phytoplankton in the VB underwent profound changes with increase in biomass and change in the timing of the annual peak from spring to summer. The increase in phytoplankton biomass in the VB, manifested particularly by increased summer diatom abundances, was due to enhanced summer DIN loads from the Loire, sustained by internal regeneration of DIP and dissolved silicate (DSi) from sediments. The long-term trajectories of this case study evidence that significant reduction of P inputs without simultaneous N abatement was not yet sufficient to control eutrophication all along the Loire–Vilaine–VB continuum. Upstream rivers reveal indices of recoveries following the significant diminution of P, while eutrophication continues to increase downstream, especially when N is the limiting factor. More N input reduction, paying particular attention to diffuse N sources, is required to control eutrophication in receiving VB coastal waters. Internal benthic DIP and DSi recycling appears to have contributed to the worsening of summer VB water quality, augmenting the effects of anthropogenic DIN inputs. For this coastal ecosystem, nutrient management strategies should consider the role played by internal nutrient loads to tackle eutrophication processes.

Nutrient transport and transformation in macrotidal estuaries of the French Atlantic coast: a modelling approach using C-GEM

“…Phaeocystis blooms have been reported in the Somme Bight (Lamy et al, 2006;Lefebvre et al, 2011), while Dinophysis (dinoflagellates) and Pseudo-nitzschia (diatoms) are the dominant harmful algae in the Seine Bight (Cugier et al, 2005a;Garnier et al, 2019;Ménesguen et al, 2019). Whereas most of the rivers have shown a decrease in phosphorus in the last two decades (Romero et al, 2013), leading to a negative P-ICEP, as found here, algal biomass increased (diatoms and dinoflagellates) with a shift in the peak in the Vilaine Bight (Ratmaya et al, 2019), mostly due to N fluxes brought by the Loire River, which flows northwards with the currents (Ménesguen et al, 2018a(Ménesguen et al, , 2019 and to internal sources from sediments (Ratmaya et al, 2019). No strong eutrophication problems have been reported off the Loire estuary, in the North Biscay Bay, without major changes during the last two decades, in contrast to the North of France (Gohin et al, 2019).…”

“…These estuaries receive nutrient deliveries from the upper river basins with different land uses and population densities (Table 2): for example, the Seine, Vilaine, Loire, and Charente river basins are dominated by agricultural activities, sustaining a large proportion of the national livestock and/or production of crop agriculture (Billen and Garnier, 2007;Ménesguen et al, 2018a;Ratmaya et al, 2019). The percentages of different land use types for each basin are presented in Table 2.…”

“…The percentages of different land use types for each basin are presented in Table 2. The intensive agriculture and urbanization within the estuarine basins studied induced eutrophication, especially those of the north half of the French Atlantic coast, regarded as nutrient-enriched (Garnier et al, 2019;Ménesguen et al, 2019;Ratmaya et al, 2019).…”

“…The French estuaries along the northeastern Atlantic coast studied herein have been subjected to nutrient enrichment for many years (Garnier et al, 2019;Ménesguen et al, 2019;Ratmaya et al, 2019). Therefore, the objectives of this paper are to:…”

Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum

“…Consequently, P control may be effective in many lakes, but still allow N export to downstream ecosystems. Controlling P can exacerbate N export to N‐limited downstream systems because it reduces the uptake, sedimentation, denitrification and burial of N in upstream lakes and rivers (Elmgren & Larsson, ; Finlay, Small, & Sterner, ; Paerl, ; Ratmaya et al, ). Graneli, Wallstrom, Larsson, Graneli, and Elmgren () reached a similar conclusion regarding the movements of N and P through bays and into the open area of the Baltic Sea.…”