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.