The Loire is the longest river in France (1012 km) with a watershed of 117 000 km2, while the Vilaine watershed is only a tenth of the size, with an area of 10 800 km2 (Fig. 1). Their catchment areas are dominated by agricultural activity, sustaining two-thirds of the national livestock and half the cereal production (Bouraoui and Grizzetti, 2008; Aquilina et al., 2012). The Arzal dam, 8 km from the mouth of the Vilaine, was constructed in 1970 to regulate freshwater discharge and prevent saltwater intrusion (Traini et al., 2015). The two studied rivers, especially the Loire, are the main nutrient sources in the northern Bay of Biscay, which includes the VB (Guillaud et al., 2008; Ménesguen et al., 2018a).

The VB, with an average depth of 10 m, is located under direct influence of these two rivers (Fig. 1). The Loire river plume tends to spread northwestward with a dilution of 20- to 100-fold by the time it reaches the VB (Ménesguen and Dussauze, 2015; Ménesguen et al., 2018b). The model ECO-MARS3D, using numerical tracers (Ménesguen et al., 2018a), estimates that the Loire constitutes, on average, 30 % of VB dissolved inorganic nitrogen (DIN) concentrations during floods (min–max 2 %–49 %) and 18 % during periods of low water (min–max 6 %–29 %). The contributions of the Loire to VB dissolved inorganic phosphorus (DIP) concentrations can reach 36 % during flood periods (average 12 %) but remain <5 % during periods of low water. The Vilaine river plume tends to spread throughout the bay before moving westward (Chapelle et al., 1994). The Vilaine contributions to the DIN concentrations in the VB, estimated by the model, are slightly lower than those of the Loire whatever the period, on average 23 % and 17 % for flood and low-water periods, respectively. The Vilaine contributes, on average, to 5 % and 8 % of VB DIP concentrations during flood and low-water periods respectively.

The water residence time in the VB varies between 10 and 20 days depending on the season and tends to be longer during calm periods (Chapelle, 1991; Chapelle et al., 1994), with tidal ranges varying between 4 and 6 m (Merceron, 1985). The water circulation is characterized by low tidal and residual currents, driven mainly by tides, winds and river flows (Lazure and Salomon, 1991; Lazure and Jegou, 1998). During periods of prevailing winds, particularly from the southwest and west, the water column of the VB is subjected to vertical mixing, which can lead sometimes to sediment resuspension and high turbidity (Goubert et al., 2010). Except during winter and periods of high hydrodynamic activity, phytoplankton production in the VB is not limited by light (Guillaud et al., 2008).

The Loire Brittany River Basin Authority (http://osur.eau-loire-bretagne.fr/exportosur/Accueil, last access: 31 March 2017) furnished dissolved inorganic nutrients and phytoplankton biomass data (concentrations of DIP; DIN; dissolved silicate, DSi; and chlorophyll a, Chl a) in rivers, at pre-estuarine stations located closest to the river mouth upstream of the haline intrusion (Fig. 1). DIN was defined as the sum of nitrate, nitrite and ammonium, with nitrate as the major component (>90 %). Sainte-Luce-sur-Loire on the Loire and Rieux on the Vilaine provided DIP, DIN and Chl a, measured monthly since the 1980s. For Sainte-Luce-sur-Loire, the influence of tidal dynamics was avoided by discarding data collected during high tide. Monthly DSi data were available from 2002 at Montjean-sur-Loire on the Loire and at Férel on the Vilaine (Fig. 1).

In order to calculate riverine nutrient loads, gauging stations located close to the river mouth were selected. River discharge data were extracted from the French hydrologic “Banque Hydro” database (http://www.hydro.eaufrance.fr/, last access: 20 May 2016). For the Loire, river discharge measurements at Montjean-sur-Loire were used due to the absence of data at Sainte-Luce-sur-Loire. The difference in drainage areas between Montjean (109 930 km2) and Sainte-Luce (111 570 km2) is negligible (ratio of 1.01) and would not have significantly changed calculations of nutrient inputs. For the Vilaine, daily discharge data were available at Rieux from the 1980s. DIN and DIP loads from rivers were calculated using averaged monthly discharge and individual monthly nutrient concentrations (Romero et al., 2013).

Nutrient and Chl a concentrations, plus phytoplankton count data in the VB, provided by the French National Observation Network for Phytoplankton and Hydrology in coastal waters (REPHY, 2017), were collected from Ouest Loscolo station (Fig. 1). This station is representative of the VB coastal waters (Bizzozero et al., 2018; Ménesguen et al., 2019) and displayed the longest dataset (from 1983 for phytoplankton counts and 1997 for nutrient and Chl a concentrations).

Acquisition periods, sampling frequencies and methods of analysis are detailed in Table S1 in the Supplement. Briefly, nutrient concentrations were measured manually or automatically in flow analysis using standard colorimetric methods with fluorimetry or photometry detection. Chl a concentrations were measured with either spectrophotometry or fluorimetry. Microscopic quantitative micro-phytoplankton analyses in coastal waters were conducted on Lugol-fixed samples and counted according to Utermöhl (1958). Phytoplankton identification and counts were carried out for organisms whose size is >20 µm (i.e., micro-phytoplankton) and smaller species with chain structure. Further details about sampling and processing of phytoplankton species are available in Hernández-Fariñas et al. (2014) and Belin and Neaud-Masson (2017). In order to account for the role of DSi, of all the micro-phytoplankton classes, genera and species identified in the VB, only total counts of diatoms (Bacillariophyceae) and dinoflagellates (Dinophyceae) were used in this work. Other micro-phytoplankton classes (Dictyophyceae, Prasinophyceae, Cyanophyceae, Chrysophyceae and Raphidophyceae) together represented only 10 % to 15 % of the VB total counts (Belin and Soudant, 2018).

DIN:DIP, DIN:DSi and DSi:DIP molar ratios were calculated and compared with theoretical molar N:P:Si ratios of 16:1:16 (Redfield, 1958; Brzezinski, 1985) in order to assess the potential limitation of phytoplankton growth by nutrient in rivers and in the VB and to investigate for trends.

Spearman correlations were computed for annual median values of the common period in order to analyze relationships between variables and tested using STATGRAPHIC CENTURION software (Statgraphics Technologies Inc., Version XVII, Released 2014).

The daily discharge of the Loire varied between 111 and 4760 m3 s-1 for the period 1980–2013, with the DLM trend displaying oscillations with periodicities of 6–7 years (Fig. 2a). A significant negative trend was detected for the common period (1997–2013), with a decrease of 94 m3 s-1 (Table 1). The seasonality plot displayed no marked change, with maximum values always observed in winter (blue) and minimum in summer (orange/red; Fig. 2b) and no significant linear change whatever the season (Table 2). The Vilaine discharge (median of 32 m3 s-1 for the period 1980–2013) corresponded to 6 % of the Loire discharge and displayed a similar trend and seasonality to those of the Loire (Fig. S2, Tables 1, 2), as highlighted by the significant correlation between their annual medians (Table 3).

DIP in the Loire varied between 0.1 and 9.4 µmol L-1 for the period 1990–2013 (Fig. 3a). A significant decrease of 0.85 µmol L-1 was detected for the common period (Table 1). Also during this period, the seasonality plot indicated a noteworthy shift in timing of annual DIP minima from summer to spring, as indicated by its change in color from yellow/orange (summer) in 2000 to green (spring) from 2006 onwards (Fig. 3b). This change was accompanied by a significant negative trend for winter–spring seasonal components and a significant positive trend for summer–autumn ones (Table 2). DIP loads from the Loire ranged between <0.1 and 15 mol s-1 for the period 1990–2013, with the trend displaying oscillations reflecting the influence of river discharge (Fig. 3c). For the common period, the Loire DIP loads decreased significantly by 52 % (Table 1). The seasonality plot of DIP loads from the Loire reflected that of discharge, with annual minimum and maximum values always observed in summer and winter, respectively (Fig. 3d). Trends of DIP concentration and DIP loads for the Vilaine were similar to those for the Loire (Fig. S3, Tables 1, 2), as indicated by a significant correlation between annual medians of DIP in the two rivers (Table 3).

DIN in the Loire ranged between 11 and 489 µmol L-1 for the period 1980–2013, with the trend displaying a decrease between the 1980s and the early 1990s, followed by an increase (Fig. 4a). However, the increase was not significant for the common period (Table 1). The DLM Loire DIN seasonality plot indicated a decrease in the seasonal amplitude starting in 1990 (Fig. 4b). For the common period, this decreasing amplitude resulted from a significant decrease in winter DIN maxima on the one hand and significant increase in summer minima on the other hand (Table 2) by around 60 µmol L-1 (Fig. 4a). The DIN loads from the Loire varied from <1.0 to 1142 mol s-1 and displayed a similar trend and seasonality to those of DIN (Fig. 4c, d), with an increase in summer minima from around 5 to 50 mol s-1 for the common period (Fig. 4c, Table 2). The trend of DIN in the Vilaine displayed an oscillation (Fig. S4), with a slight significant decrease over the common period (Table 1) and no marked variation in the seasonality (Fig. S4b, Table 2). As for the Loire, the trend and seasonality of DIN loads from the Vilaine were similar to those of DIN (Fig. S4c, d, Tables 1, 2).

DIN:DIP ratios in both rivers ranged between 1.0 and 1000, with >80 % of values being higher than 30, and displayed an increasing trend between 1990 and 2013 (Fig. S5). A significant increase of 85 % and 303 %, respectively, for the Loire and the Vilaine, was detected for the common period (Table S3). DSi in rivers ranged between 46 and 261 µmol L-1 in the Loire and from 5.0 to 201 µmol L-1 in the Vilaine for the period of available data (2002–2013). More than 80 % of DIN:DSi ratios in rivers were higher than the theoretical molar N:Si ratio of 1 for the potential requirement of diatoms (data not shown).

Chl a in the Loire ranged between >200 µg L-1 during the 1980s and <1.0 µg L-1 in the 2010s. The Chl a trend remained stable between 1980 and 2000 before decreasing subsequently (Fig. 5a). For the common period, the Loire Chl a decreased by 93 % (54 µg L-1; Table 1). The DLM Loire Chl a seasonality plot displayed a shift in timing of the annual Chl a maximum, as indicated by its change in color from orange/red (summer) during 1980–1990 to green (spring) during 2005–2013 (Fig. 5b). For the common period, this change in timing was accompanied by a significant negative trend for autumn seasonal components and significant positive trend for winter and spring (Table 2). Results for Chl a in the Vilaine revealed a similar trend and seasonality to those in the Loire (Fig. S6, Tables 1, 2), as indicated by a significant correlation between Chl a annual medians in the two rivers (Table 3).

DIP in the VB varied between <0.1 and >1.0 µmol L-1 with no noticeable trend (Fig. 6a). A significant decrease of 0.05 µmol L-1 was detected over the common period (Table 1). The seasonality plot of the VB DIP revealed a change in timing of the minimum values, as indicated by its change in color from yellow/orange (summer) before 2006 to green (spring) afterwards (Fig. 6b). This shift was accompanied by a significant negative linear trend for spring seasonal components and a significant positive trend for summer (Table 2).

DIN in the VB varied between <1.0 and >200 µmol L-1, with the trend displaying an oscillation (Fig. 6c). A significant increase of 3.2 µmol L-1 was detected for the common period (Table 1). The DLM seasonality indicated that this increase was focused in winter (Fig. 6d, Table 2). Annual DIN medians in the VB were positively correlated with those of discharge from the two rivers (Table 3).

DSi in the VB varied between <1.0 and 100 µmol L-1 without a noticeable trend (Fig. 6e). For the common period, a significant increase of 3.6 µmol L-1 was detected, which was comparable to that of DIN (Table 1). The seasonality did not indicate any particular change (Fig. 6f, Table 2). Annual DSi medians in the VB were positively correlated with those of the Loire discharge and with the VB DIN (Table 3).

DIN:DIP and DIN:DSi ratios in the VB ranged between <1.0 and 650 and from <0.1 to 44, respectively (Fig. S7). Summer values of DIN:DIP and DIN:DSi ratios were often below theoretical values of 16 and 1, respectively, for potential requirements of diatoms (Fig. S7). DSi:DIP ratios in the VB ranged between <5.0 and >100, with >80 % of values being above the theoretical value of 16 (Fig. S7). The trends for dissolved inorganic nutrient ratios in the VB displayed a significant increase for the common period (Fig. S7, Table S3).

Chl a in the VB ranged between 0.1 and 116 µg L-1, with the trend displaying an increase (Fig. 7a). For the common period, the VB Chl a increased significantly by 126 % (2.1 µg L-1; Table 1). The seasonality plot of Chl a in the VB displayed a shift in the timing of the annual maximum, indicated by its change in color from green (spring) before 2006 to orange/red (late summer) afterwards (Fig. 7b). This change was accompanied by a significant negative linear trend for spring seasonal components (Table 2). Annual Chl a medians in the VB were negatively correlated with those of Chl a from both rivers and with DIP in the Vilaine (Table 3).

Diatom abundances varied between 200 and 1.3×107 cells L-1 for the period 1983–2013, with the DLM trend showing an increase (Fig. 7c). For the common period, diatom abundances increased significantly by 227 % (90×103 cells L-1; Table 1). Although diatom abundances continued to peak in spring (Fig. 7d), their seasonality plot indicated a significant increase in summer seasonal components over the common period (Table 2). Dinoflagellate abundances were about 10-fold less than those of diatoms, with values ranging between 40 and 3.4×106 cells L-1 over the period 1983–2013. Like diatoms, the DLM trend for dinoflagellate abundances in the VB displayed an increase (Fig. 7d). For the common period, dinoflagellates' abundances increased by 8×103 cells L-1 (108 %; Table 1). However, the DLM seasonality plot indicated that summer seasonal components of dinoflagellate abundances, corresponding to the dinoflagellate annual peak, displayed a significant decreasing trend over the common period (Fig. 7f, Table 2).

The decrease in Chl a in pre-estuarine stations on the Loire and Vilaine rivers over the past decades reflects the global diminution in eutrophication in North American and European rivers (Glibert et al., 2011; Romero et al., 2013; Dupas et al., 2018). This decrease in Chl a was also observed in the Upper and Middle Loire (Larroudé et al., 2013; Minaudo et al., 2015). However, the Loire did not retrieve its oligotrophic state of the 1930s (Crouzet, 1983). At the studied stations, the annual Chl a peak decreased and shifted from late summer to spring (Fig. 8a, b). The parallel decrease of DIP and Chl a in the Loire and Vilaine rivers underlines the role of decreasing P in reducing phytoplankton biomass (Descy et al., 2012; Minaudo et al., 2015), as also found in other river systems, such as the Danube (Istvánovics and Honti, 2012), the Seine (Romero et al., 2013) and some Scandinavian rivers (Grimvall et al., 2014). This decreasing trend of DIP is a result of improved sewage treatment, decreased use of P fertilizers and the removal of P from detergents (Bouraoui and Grizzetti, 2011). However, the decline of Chl a in both studied rivers began several years after that of DIP when the latter reached limiting concentrations for phytoplankton, as deduced at Montjean on the Loire by Garnier et al. (2018). The change in timing of the annual DIP minima from summer to spring in the Loire and Vilaine rivers during the last decades of the studied period, concomitant with that of the annual peak of Chl a, can be explained by the increasingly early depletion of DIP by phytoplankton (see Floury et al., 2012, for the Loire).

The trend of DIN in studied rivers reveals the general trends observed in other large European rivers, showing a slight decrease, a steady trend or even an increase, depending on the degree of fertilizer application in catchment areas (Bouraoui and Grizzetti, 2011; Romero et al., 2013). The increase in summer Loire DIN since the early 1990s was offset by the decrease in winter values, which is related to the reduction in N point source emissions and N fertilizer application (Poisvert et al., 2016; data from French Ministry of Agriculture, Sylvie Lesaint, personal communication, 2017). An increase in summer DIN of several tens of micromole per liter was also reported in the Middle Loire (Minaudo et al., 2015). This increase in summer DIN is the result of a delayed response due to the long transit time of DIN through soils and aquifers in the Loire catchment (up to 14 years; Bouraoui and Grizzetti, 2011). The decreasing DIN uptake by phytoplankton in the Loire may have also contributed to the increase in summer DIN (Lair, 2001; Floury et al., 2012). Concerning the Vilaine, the slight decrease in DIN from the early 1990s reflects the decrease in N fertilizer application in the Vilaine catchment (Bouraoui and Grizzetti, 2011; Aquilina et al., 2012), which is facilitated by a relatively short transit time of DIN in the Vilaine watershed (∼5–6 years; Molenat and Gascuel-Odoux, 2002; Aquilina et al., 2012).

DSi data series in both rivers were too short to investigate long-term trends and seasonality but provided values in order to examine nutrient stoichiometry. Larroudé et al. (2013) observed no significant trend in DSi between 1985 and 2008 in the Middle Loire, as also confirmed at Montjean station by Garnier et al. (2018). The decrease in DIP led to the increasing trend of DIN:DIP ratios, and probably DSi:DIP, in both rivers, as was observed in numerous rivers (Beusen et al., 2016). Based on these trends, the DIP limitation has thus been reinforced in studied rivers during the last decades, and potentially in receiving coastal waters, regardless of the season.

In contrast to what happened in rivers, eutrophication in the downstream VB coastal waters has worsened during recent decades, as indicated by a significant increase in Chl a, also confirmed by the significant augmentation of both diatom and dinoflagellate abundances. The increase in Chl a in the VB was accompanied by a shift in its annual peak from spring to summer (Fig. 8c, d). This modification in the seasonal course of phytoplankton biomass coincides with the increase in diatom abundances, occurring mainly in summer. The dynamics of phytoplankton in the VB during the last decade of the studied period thus underwent important changes: (1) an increase in biomass, (2) a change in timing of the annual peak from spring to summer and (3) a modification in the seasonal course of diatoms and dinoflagellates.

In theory, several external nutrient sources could have contributed to nutrient availability in the VB: atmospheric, oceanic and riverine inputs. DIN inputs from rainwater estimated by Collos et al. (1989) represent only 1 % of river inputs, while levels of nutrients and Chl a in the Bay of Biscay always remained low during the studied period (Table S2). The proximity of the VB to the Loire and Vilaine rivers designates riverine inputs as main external nutrient sources in these coastal waters (Ménesguen et al., 2018a, b).