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

Abstract

<p><strong>Abstract.</strong> The evolution of eutrophication (i.e., phytoplankton biomass) during recent decades was examined in the coastal waters of Vilaine Bay (VB, France) in relation to those in their main external nutrient sources, the Loire and Vilaine Rivers. Dynamic Linear Models, corroborated by the Mann-Kendall test, were used to study long-term trends and seasonality of dissolved inorganic nutrient and chlorophyll <i>a</i> concentrations (Chl <i>a</i>) in rivers and coastal waters between 1980 and 2013. The reduction in dissolved riverine inorganic phosphorus concentrations (DIP) from the early 1990s led to the decrease in their Chl <i>a</i> levels. However, dissolved inorganic nitrogen concentrations (DIN) decreased only slightly in the Vilaine and actually increased in the Loire, especially during summer. Simultaneously, phytoplankton in the VB has undergone profound changes with: (1) increase in biomass, (2) change in the position of the annual peak from spring to summer, and (3) increase in diatom:dinoflagellate ratios, especially in summer. The increase in phytoplankton biomass in VB, particularly in summer, was probably due to increased DIN loads from the Loire, sustained by internal regeneration of DIP and dissolved silicate from sediments. This long-term ecosystem-scale analysis reports the consequence of nutrient management scenarios focused solely on P reduction. Freshwater ecosystems upstream reveal successful recoveries through the control of P alone, while eutrophication continues to increase downstream, especially during the period of N limitation. Therefore, nutrient management strategies, paying particular attention to diffuse N-sources, are required to control eutrophication in receiving coastal waters.</p>

Responses of Marine Diatom-Dinoflagellate Competition to Multiple Environmental Drivers: Abundance, Elemental, and Biochemical Aspects

“…On the one hand, interaction coefficient α showed a switch from a competitive superiority of the dinoflagellates to the diatoms as nitrate concentration increased at the low nutrient regime ( Figure 3 ), indicating that the dinoflagellates favor low nutrient condition, but the diatoms favor high nutrients. Our results are consistent with the succession trends in phytoplankton communities in the world ocean where high concentrations of nitrate are encountered, mainly in highly eutrophic bays and river inlets ( Aubry et al, 2004 ; Gettings et al, 2014 ; Mutshinda et al, 2016 ; Barcelos e Ramos et al, 2017 ; Ratmaya et al, 2019 ). For example, the Bay of Brest received high-nitrate-loaded freshwater (>700 μmol L –1 ) in winter in 1990s, which supported a diatom-dominated spring bloom ( Del Amo et al, 1997 ).…”

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.…”

Bi-decadal variability in physico-biogeochemical characteristics of temperate coastal ecosystems: from large-scale to local drivers

“…The consequences of such changes to coastal ecosystem functioning could be of various types. For example, changes in regional wind intensity and SLP modified water circulation dynamics (Somavilla et al 2016), inducing saltier waters to the midlatitudes in combination with decreasing precipitation and therefore a reduction in inputs from the continent: the flows of the main French rivers have decreased in the end of the twentieth century like in the Loire river (Ratmaya et al 2019). In the literature, similar processes have been reported in the Patos Lagoon, Brazil (Cloern et al 2016).…”