Valérie Derolez
email: valerie.derolez@ifremer.fr

Beatrice Bec

Dominique Munaron

Annie Fiandrino

Romain Pete

Monique Simier

Philippe Souchu

Thierry Laugier

Catherine Aliaume

Nathalie Malet

Béatrice Bec

Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Mediterranean coastal lagoons are often surrounded by densely populated areas and are consequently subject to strong anthropogenic pressure, and particularly high levels of inputs of domestic sewage [START_REF] Viaroli | Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons : a brief overview[END_REF][START_REF] Zaldívar | Eutrophication in transitional waters : an overview[END_REF][START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. In addition to this urban pressure, the transitional status of these waters makes these ecosystems particularly vulnerable to eutrophication (de Jonge & Elliott 2001; [START_REF] Newton | An overview of ecological status, vulnerability and future perspectives of European large shallow, semi-enclosed coastal systems, lagoons and transitional waters[END_REF]. Eutrophication of coastal ecosystems is a worldwide issue which has been described for decades [START_REF] Nixon | Coastal marine eutrophication: A definition, social causes, and future concerns[END_REF][START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF]. The increase in nutrient inputs, enhanced by urbanisation, agriculture or industry, leads to a complex cascade of both direct and indirect responses by ecosystems [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EU-project EUMAC[END_REF][START_REF] Viaroli | Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons : a brief overview[END_REF]. Anthropogenic eutrophication causes ecological damage including anoxic crises, toxic algal blooms, even loss of species, and more widely, deterioration of ecosystem functions and the services they provide [START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF]Zaldívar et al. 2008).

In recent decades, several legal and managerial frameworks have been designed to reduce human pressure on aquatic ecosystems and to establish ambitious ecological quality targets, e.g., EC Nitrate Directive and Water Framework Directive (WFD) (EC 1991;EC 2000) or the US Clean Water Act (USEPA 2002). In particular, mitigation actions have targeted sewerage networks in estuarine watersheds, and have resulted in their recovery at varying speeds and with various patterns [START_REF] Lie | Changes in the nutrient ratios and phytoplankton community after declines in nutrient concentrations in a semi-enclosed bay in Hong Kong[END_REF][START_REF] Saeck | Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton[END_REF][START_REF] Staehr | Decadal changes in water quality and net productivity of a shallow Danish estuary following significant nutrient reductions[END_REF]. While the degradation trajectories of coastal ecosystems subjected to eutrophication have been extensively described [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EU-project EUMAC[END_REF][START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF][START_REF] De Jonge | Causes, historical development, effects and future challenges of a common environmental problem: eutrophication[END_REF][START_REF] Viaroli | Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons : a brief overview[END_REF], recovery trajectories have only recently been studied (Duarte et al. 2009;[START_REF] Borja | Medium-and long-term recovery of estuarine and coastal ecosystems: patterns, rates and restoration effectiveness[END_REF][START_REF] Mccrackin | Recovery of lakes and coastal marine ecosystems from eutrophication: A global meta-analysis[END_REF]. Moreover, the rate of improvement after mitigation works on coastal ecosystems is not well known, even though this information is indispensable to assess recovery effectiveness Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 2 and to guide future management actions [START_REF] Mccrackin | Recovery of lakes and coastal marine ecosystems from eutrophication: A global meta-analysis[END_REF]. Only a few studies have been conducted on the recovery process of French coastal lagoons, and they focused on selected water bodies [START_REF] Collos | Oligotrophication and emergence of picocyanobacteria and a toxic dinoflagellate in Thau lagoon, southern France[END_REF][START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF][START_REF] Pasqualini | Spatiotemporal dynamics of submerged macrophyte status and watershed exploitation in a Mediterranean coastal lagoon: understanding critical factors in ecosystem degradation and restoration[END_REF]. These studies emphasised shifts in communities of primary producers after mitigation actions. The originality of our study is that we used a large set of data collected from 2001 to 2014 from 16 French Mediterranean coastal lagoons, thereby covering the whole eutrophication gradient from oligotrophic to hypertrophic. Some of these lagoons have been receiving large quantities of urban nutrient inputs for decades, leading to extremely high levels of phytoplankton biomass, associated with high pico-and nano-phytoplankton abundances in the most degraded lagoons (annual median values >100 µg Chl a.L -1 , >10 9 and >10 10 cell.L -1 , respectively, [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. Large-scale management actions were recently undertaken on waste water treatment systems to restore the quality and the functioning of the ecosystems [START_REF] De Wit | Restoration ecology of coastal lagoons: new methods for the prediction of ecological trajectories and economic valuation[END_REF][START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF]. The unprecedented range of anthropogenic eutrophication and the remediation works carried out during the 14-year study period allowed us to analyse recovery trajectories. First we analysed changes in the eutrophication gradient over the study period and identified key parameters. We then focused on three contrasted ecosystems to test the following hypotheses: (i) reducing inputs of urban nutrients has a rapid effect on the nutrient levels and phytoplankton biomass in Mediterranean lagoons; and (ii) the pattern of the recovery trajectories depends on the degree of anthropogenic eutrophication before the remediation actions were undertaken. Analysis of available historical data provided useful information to improve our understanding of the patterns of change in the ecological functioning of coastal lagoons after remediation. Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 3 Data from 16 French Mediterranean coastal lagoons collected between 2001 and 2014, were analysed. Fourteen of the lagoons are located in the Gulf of Lions, from north of Perpignan to Montpellier, and two in Corsica (Fig. 1). The database includes polyhaline and euhaline microtidal lagoons (annual median salinity > 18) (see Supplementary Material), with surface areas ranging from 1.4 to 75 km², depths ranging from 0.7 to 9 m and volumes ranging from 0.7 to 260 x 10 6 m 3 [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. Details on the morphometric and hydrologic characteristics of the study sites are available in [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. The lagoons are all connected to the Mediterranean Sea, through direct inlets, or through indirect connections via a canal or another lagoon. The database covers the anthropogenic eutrophication gradient from oligotrophic to hypertrophic lagoons [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. In each of the 16 lagoons, water was sampled at one or two stations depending on the size of the lagoon, giving a total of 22 stations: Leucate (LES, LEN), La Palme (LAP), Bages-Sigean (BGS and BGN), Ayrolle (AYR), Gruissan (GRU), Thau (TES, TWS), Ingril (INN and INS), Vic (VIC), Pierre-Blanche (PBE), Arnel (ARN), Prévost (PRE), Méjean (MEW, MEE), Grec (GRC), Or (ORE and ORW), Diana (DIA) and Urbino (URB). We first analysed changes in the anthropogenic eutrophication gradient at the 22 stations over the 14-year study period.

Materials and methods

Study sites and sampling strategy

Next, three stations were selected for specific statistical analyses based on their degree of anthropogenic eutrophication according to the descriptions provided by [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]: Ayrolle (AYR station): oligotrophic, Bages-Sigean (BGN station): eutrophic, Palavasian complex (MEW station): hypertrophic (represented by asterisks in Fig. 1). At the beginning of monitoring in 2001, AYR station was characterised by transparent waters with dominance of marine phanerogams and associated macroalgae, whereas BGN station was characterised by phanerogams and proliferating macroalgae (stations AY and BN in Souchu et al. 2010). MEW station was almost entirely dominated by phytoplankton (station MW in [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. Details on Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 4 the main hydro-morphologic characteristics of these stations and the reductions in nutrient inputs are listed in Table 1. The three stations are located in shallow lagoons (mean depth < 1.5 m) [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF] with comparable ranges of salinities (median values: AYR=31.1, BGN=25.9, MEW=28.1; see Supplementary Material) and climatic conditions. MEW station is located on Méjean lagoon, which is part of the Palavasian complex, south of Montpellier (Fig. 1). Méjean lagoon has no direct connection to the sea and the water renewal is driven by the inputs of brackish waters coming from the « Rhône to Sète Canal » [START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF]. The Palavasian complex watershed is highly urbanised due to the presence of the city of Montpellier (250,000 inhabitants).

The construction of an upgraded wastewater treatment plant (WWTP) in 2005, followed by the diversion of its effluents via an outfall in the Mediterranean Sea located 11 km off-shore led to a sudden dramatic reduction in anthropogenic inputs of nitrogen and phosphorus into the lagoons. This included an estimated 83% reduction in total N and a 73% reduction in total P from the watershed [START_REF] Meinesz | Base de données "pressions sur les lagunes méditerranéennes" -Analyse des liens état -pression[END_REF]. Before remediation works, the contribution of WWTPs to total inputs to the Palavasian complex was lower for P than for N (respectively 78% and 89% of the total inputs), thanks to the physicochemical treatment of Montpellier's WWTP allowing better phosphorus abatement. As a consequence, the diversion of discharges from this WWTP in 2005 resulted in a larger decrease in N than in P total inputs. This difference in reduction rates between N and P is also explained by the increase in urban run-offs from the watershed, leading to an increase in P total inputs. Bages-Sigean lagoon is located south of the city of Narbonne (53,000 inhabitants). At BGN station, the water is mainly renewed by brackish waters coming from the southern part of the lagoon, which is directly connected to the sea [START_REF] Fiandrino | Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion[END_REF]. The construction of an upgraded WWTP in Narbonne in 2003 enabled a 55% reduction in total N and a 64% reduction in total P inputs from the watershed. Before remediation, the contribution of WWTPs to total inputs to Bages-Sigean lagoon was higher for P than for N (respectively 82% and 76% of the total inputs), this being explained by the lack of physicochemical treatment on Narbonne's WWTP before its refurbishment. The improvement of this WWTP in 2003, including additional treatment to ameliorate phosphorus binding, resulted in a larger decrease of P total inputs than in N total inputs. Oligotrophic Ayrolle lagoon has a permanent connection to the Mediterranean Sea and its watershed is not directly concerned by urban inputs [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]).

Laboratory analyses and data collection

Between 2001 and 2014, water was sampled each year at monthly intervals (June, July and August) in summer, when primary production is at maximum in Mediterranean lagoons [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. We avoided collecting samples during periods of sediment resuspension by not sampling for three days after any period in which wind speed exceeded 12.5 m s -1 . Temperature (Temp., °C), salinity (Sal.) and dissolved oxygen (O2, mg L -1 ) were recorded in-situ with field sensors. At each station, on each sampling occasion, one water sample was collected in a 1 L polypropylene bottle at the subsurface (in the middle of the water column for depths below 2 m and at -1 m from the surface at deeper stations) for laboratory analysis. The following parameters were analysed in the laboratory: dissolved inorganic nitrogen (DIN = NH4 + + NO3 -+ NO2, µM), dissolved phosphorus (DIP, µM), dissolved silicate (Si, µM), total nitrogen and phosphorus (TN and TP, µM) and turbidity. Nutrients were analysed using standard protocols [START_REF] Aminot | Dosage automatique des nutriments dans les eaux marines[END_REF], as described in [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. Turbidity (Turb., NTU) was measured with a HACH 2100N IS sensor following ISO 7027 protocols. Chlorophyll a concentrations (µg Chl a L -1 ) were measured by spectrofluorimetry [START_REF] Neveux | Spectrofluorometric assay of chlorophylls and phaeopigments using the least squares approximation technique[END_REF] with a Perkin-Elmer L650. Based on cytometric analyses, different size classes of phytoplankton were identified and counted with a FACSCalibur flow cytometer: autotrophic picoeukaryotes (≤ 3 µm) and Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 6 nanophytoplankton (> 3 µm) abundances (PICO and NANO, 10 6 cells L -1 ) (see [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF] for phytoplankton analyses).

Statistical analyses

Before statistical analyses, Chl a data were log10 transformed to homogenise the variances. We used non-parametric Spearman's rank correlation (ρ) analyses to identify potential relationships between the 12 environmental variables: Temp., Sal., O2, DIN, DIP, Si, TN, TP, Turb., Chl a, PICO and NANO.

Five environmental variables were selected for further analyses because of their key role in anthropogenic eutrophication processes in coastal lagoon waters [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]): DIN and DIP both cause eutrophication; Chl a represents phytoplankton biomass resulting from eutrophication; TP and TN correspond to causes and consequences and are used as integrated parameters for phytoplankton biomass and for organic matter. The five selected variables were standardised before between-and within-group principal component analyses (PCA), using time and space as instrumental variables to control multivariate analysis [START_REF] Blanc | Spatial and temporal co-structure analyses between ichthyofauna and environment: An example in the tropics[END_REF]. The withingroup PCA of the dataset was centred by group (same dimensions as the original dataset: 5 columns (environmental variables), 924 lines). The within-group analysis searched for information shared by the groups, while the between-group analysis searched for differences between groups. For example, in our study, between-station analysis was a PCA of the dataset containing of station means: 5 columns (environmental variables) and 22 lines (stations). In order to highlight the annual variability of each station with respect to its average position on the inertia axes, the initial data were used as supplementary individuals in the between-station PCA.

Between-group and within-group PCAs were performed to study station, year and month effects, using a reduced dataset with five selected parameters (Chl a, TN, TP, DIN and DIP). These PCAs made it possible to split the total inertia (i.e. the total information contained in the dataset) into between-and within-group inertia. Monte Carlo permutation tests were conducted on stations, sampling years and sampling months to identify the statistical significance of between-group analyses [START_REF] Manly | Randomization, bootstrap and Monte Carlo methods in biology[END_REF].

The distances of the trajectories recorded year to year (di, with i: period from year i to i+1) were calculated at MEW, BGN and AYR stations using the first two dimensions of the between-station PCA. The cumulative year-to-year distance on the first two dimensions was calculated (Δtotal=Ʃ|di|, from i=2001 to 2013). At MEW and BGN, annual average distances were also compared for the periods before (Δbefore) and after (Δafter) the works on the sewerage systems were completed.

A Mann-Kendall test was performed to assess monotonic temporal trends in environmental variables at MEW, BGN and AYR stations. Non-parametric Kruskal-Wallis one-way analysis of variance was also performed at these stations to identify significant differences between sampling years. At MEW and BGN stations, Wilcoxon rank sum tests enabled comparison of concentrations before (respectively, 2001-2005 and 2001-2003) and after (respectively, 2006-2014 and 2004-2014) works on the sewage systems.

Tests were considered significant at p < 0.05. All statistical analyses were performed with R software (R CoreTeam 2017).

Results

Key parameters highlighted a eutrophication gradient

The dataset revealed several significant correlations (Table 2) (see Supplementary Material for the ranges of the environmental parameters). Temperature, salinity, O2, Si, DIN and NANO were weakly correlated with other parameters (ρ < 0.5). Chl a concentrations were highly correlated with TP (ρ = 0.82, n = 900), TN (ρ = 0.70, n = 899), autotrophic picoeukaryote abundances (ρ = 0.67, n = 886) and turbidity (ρ = 0.66, n = 903). These five variables were significantly correlated with each other, demonstrating their link with the primary production process. Total phosphorus was significantly and much more strongly correlated with its inorganic forms DIP (ρ = 0.61, n = 902) than total nitrogen with DIN (ρ = 0.23, n = 901). The contribution of DIP to TP had a median value of 11.4%, while the contribution of DIN to TN was only 1.7%, highlighting the dominance of organic forms, especially of nitrogen. Ammonium (NH4 + ) was the main form of dissolved inorganic nitrogen (median 69.2%, n= 908).

Percentages of other inorganic forms of nitrogen were 18.2% for nitrates and 8.4% for nitrites. The weakest correlations with other environmental parameters were found for dissolved oxygen concentrations.

Permutation tests of between-group PCAs performed on the five selected parameters (Chl a, TN, TP, DIN and DIP) were all significant (Monte-Carlo's p < 0.001), but between-month inertia only accounted for a very low percentage of total inertia (0.7%), much lower than the percentages represented by the between-year and betweenstation inertias (respectively 8.5% and 41.5%). This led us to average the dataset per year and per station for the five water quality parameters. Then, as the spatial pattern was stronger than the temporal pattern (percentages of total inertia of 52.3% and 10.2%, respectively, Table 3), between-station PCA was selected to identify an anthropogenic eutrophication gradient among the 22 sampling stations and to reveal trajectories of change over the study period.

The first factorial axis of the between-station PCA (PC1, 81% of the between-station inertia) was highly correlated with Chl a, TN, TP, and to a lesser extent with DIP (Fig. 2a). This axis ordinated stations according to a clear anthropogenic eutrophication gradient (Fig. 2b). The second axis (PC2) accounted for only 11.5% of the betweenstation inertia and was correlated with DIN (Fig. 2). The following statistical analyses focused on three stations distributed along the anthropogenic eutrophication gradient identified by the between-station PCA: MEWhypertrophic station, BGN -eutrophic station -and AYR -oligotrophic station -(stations in colour in Fig. 2b). Figure 3 illustrates the trajectories of the three stations between 2001 and 2014 on the plane defined by the first two axes of the between-station PCA.

Recovery trajectories after remediation actions

During the study period, the variability of AYR station trajectory was low along the first axis and the first plane of the between-station PCA. The cumulative year-to-year distance on the first plane for AYR was the lowest (Δtotal=2.9). Conversely, BGN station shifted significantly along the first axis (PC1) towards oligotrophy after mitigation works were completed. The cumulative distance on the first plane was more than two times greater than that of AYR (Δtotal = 6.4) and the average annual distance decreased after mitigation works, indicating a reduction in ecosystem variability (Δbefore = 1.2 and Δafter = 0.4). From 2001 to 2014, the MEW trajectory also shifted significantly along PC1 towards oligotrophy. The cumulative year-to-year distance on the first plane for MEW was nearly eight times greater than that of AYR (Δtotal=21.5). The average annual distance of MEW station also decreased after mitigation works (Δbefore=2 and Δafter=1.5).

However, from 2011 to 2014, the trajectory oscillated back and forth (Fig. 3), revealing instability of the ecosystem, especially from 2013 to 2014, when the trajectory displayed notable backtracking. throughout the study period and average Chl a concentrations measured before and after remediation works were divided by 15.7 (134 before and 8.6 µg L -1 after, p < 0.0001) (Fig. 4c). TP and TN followed the same trend, with a significant decrease (p = 0.009 and 0.002) and average concentrations divided respectively by 2.5 and 3.1 (20 before and 8.1 µM of TP after, p < 0.0001; 339 before and 109 µM of TP after, p < 0.0001) (Fig. 4b). As PICO abundances were highly correlated with Chl a (ρ = 0.65, p < 0.0001, n = 42), they also decreased significantly over the study period (p = 0.037), and their concentrations decreased by a factor of 2.4 after WWTP refurbishment (4.01

x 10 9 before and 1.65 x 10 9 cell L -1 after, p = 0.005). Abundances of nanophytoplankton underwent significant variations depending on the year, with a peak in 2006 (average 7.63 x 10 9 cell L -1 ). The decrease in phytoplankton biomass was accompanied by a decrease in turbidity and oxygen levels (p=0.0014 and 0.0017). Changes in DIN concentrations revealed no particular pattern over time.

The pattern of change at BGN station differed from that at MEW. From 2001 to 2003, i.e. before the upgraded WWTP began operating, the BGN station was eutrophic, according to [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. From 2003 to 2005, BGN started recovering, but less abruptly than MEW (Fig. 3). DIP concentrations dropped the year after the remediation works were completed (Fig. 4a) and a significant decrease was observed Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 11 over the study period (p = 0.007), with levels divided by 5.2 (from 1.6 to 0.3 µM, p = 0.004) after completion of the works. Changes in TP followed the same trend as DIP, with mean levels divided by 2.4 (from 3.3 to 1.4 µM, p = 0.01), but chlorophyll a concentrations only started decreasing after a lag time of two years. DIN increased after completion of the WWTP works and reached a maximum in 2005 (Fig. 4a).

After the delay in ecosystem response, the pattern of change suggested a gradual continuation of the recovery process from 2006 until 2014, with a decrease in Chl a and DIP. Chlorophyll a concentrations decreased significantly throughout the study period (p=0.001) and were divided by 1.6 after remediation works (from 4.5 to 2.8 µg L -1 , p=0.03). Chl a concentrations decreased after remediation works, with average levels divided by 1.2 (from 47.8 to 38.5 µM, p = 0.004). TN concentrations decreased rapidly from 2002 to 2006, but then increased again and stabilised below the level observed before remediation works. However, picoeukaryotes, nanophytoplankton and dissolved and organic forms of N did not decrease significantly and no significant between-year differences were observed.

At AYR station, despite a decrease in Chl a, DIN and DIP over the course of the study period (from 1.6 to 0.5 µg L -1 , p = 0.016; from 1.2 to 0.32 µM, p = 0.02; from 0.04 to 0.03 µM, p = 0.03, respectively), the orders of magnitude of the concentrations were much lower than those at BGN and MEW stations (mean levels respectively from 1.6 and 4.6 times lower (DIN) to 3.8 and 28.2 times lower (Chl a)) (Fig. 4). Phytoplankton abundances showed no significant trends over time.

The average TN:TP and DIN:DIP ratios at MEW station were already the lowest and decreased significantly after remediation works (from respectively 17.4 to 14.6, p=0.02; and from 4.2 to 2.2, p = 0.02). TN:TP ratios were below 20, suggesting N limitation according to the total nutrient based approach (Guilford & Hecky 2000 in [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. The average TN:TP ratio at BGN station (31.1) was farther from the Redfield ratio and did not change significantly after remediation works. The TN:TP ratio at BGN station highlighted a limitation by both P and N. Moreover, the average DIN:DIP ratio (9.5) characterised a limitation by N according to the dissolved nutrient based criteria (Justic et al. 1995[START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]. The average TN:TP and DIN:DIP ratios were the highest at AYR station (respectively 46.8 and 17.6) and highlighted a P limitation.

Discussion

Integrative parameters revealed a eutrophication gradient

The rare dataset we used to monitor coastal lagoons whose status ranged from oligotrophic to hypertrophic [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF], allowed us to analyse eutrophication and oligotrophication phenomena in these ecosystems. Some of these lagoons received large inputs of urban nutrients for several decades, but major management actions have recently been implemented on the waste water treatment systems in most of their watersheds [START_REF] Meinesz | Base de données "pressions sur les lagunes méditerranéennes" -Analyse des liens état -pression[END_REF][START_REF] Gowen | Response of diatom and dinoflagellate lifeforms to reduced phosphorus loading: A case study in the Thau lagoon, France[END_REF][START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF]. Fourteen years after the monitoring of French Mediterranean lagoons began [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]), an anthropogenic eutrophication gradient is still highlighted by the water quality parameters. The five parameters selected in our study to identify the anthropogenic eutrophication gradient are representative of nutrient cycling in the water column of coastal lagoons as they incorporate either the causes (DIN and DIP), or the consequences of the eutrophication process (Chl a), or both (TP and TN, which were closely correlated with each other and with Chl a). DIN and DIP represented very low percentages of total nutrients in summer (median 1.68% and 11.4%, respectively). Dissolved nutrient concentrations were very low because of continuous uptake by massive phytoplankton blooms, which occur mainly in summer [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]). These results are in agreement with observations made in shallow lakes [START_REF] Ibelings | Resilience of alternative stable states during the recovery of shallow lakes from eutrophication: Lake Veluwe as a case study[END_REF][START_REF] Jeppesen | Shallow lake restoration by nutrient loading reduction -Some recent findings and challenges ahead[END_REF], where the integrative parameters TN, TP and Chl a appear to Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 13 be good complements to the dissolved nutrients usually measured in transitional and coastal ecosystems [START_REF] Claussen | Assessment of the eutrophication status of transitional, coastal and marine waters within OSPAR[END_REF]Ferreira et al. 2011;[START_REF] Mclaughlin | A regional survey of the extent and magnitude of eutrophication in Mediterranean estuaries of southern California, USA[END_REF][START_REF] Lemley | Towards the classification of eutrophic condition in estuaries[END_REF]. These parameters appear to be suitable for monitoring programmes, such as WFD assessment programmes, whose aim is to evaluate the effectiveness of management measures.

Remediation works triggered a rapid and significant response by lagoon ecosystems

In our study, the hypothesis that the reduction in urban nutrient inputs would have a rapid effect on the water total nutrient levels and phytoplankton biomass was confirmed in hypertrophic and eutrophic lagoons. After a reduction of more than 50% in TP and of 80% in TN loadings from the watershed of these ecosystems, a recovery trajectory was rapidly identified using the integrative parameters Chl a, TN and TP. The concentrations of these parameters in the water column decreased significantly within one to three years. The response lag times and ranges observed after remediation actions in the eutrophic lagoon are in agreement with those reported in other coastal lagoons by [START_REF] Pasqualini | Spatiotemporal dynamics of submerged macrophyte status and watershed exploitation in a Mediterranean coastal lagoon: understanding critical factors in ecosystem degradation and restoration[END_REF] and in estuaries by [START_REF] Lie | Changes in the nutrient ratios and phytoplankton community after declines in nutrient concentrations in a semi-enclosed bay in Hong Kong[END_REF], Boynton et al. (2013[START_REF] Ní Longphuirt | Linking changes in nutrient source load to estuarine responses: an Irish perspective[END_REF]. Our results demonstrate that even hypertrophic coastal lagoons can recover following substantial reduction in nutrient loads and confirm the rare observations available on such degraded ecosystems, whose main nutrient sources are WWTPs [START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF], or on temperate lakes (Jeppesen et al. 2005;[START_REF] Ibelings | Resilience of alternative stable states during the recovery of shallow lakes from eutrophication: Lake Veluwe as a case study[END_REF][START_REF] Jeppesen | Shallow lake restoration by nutrient loading reduction -Some recent findings and challenges ahead[END_REF]. What is remarkable about our results is the speed (one to two years) and the extent of the recovery of the hypertrophic lagoon, the decreases in the concentrations of Chl a and TP being 10 fold greater than in the eutrophic lagoon. This difference may be due to the more drastic reduction in nutrient loading, which reached 83% for total N and a 73% for total P in the hypertrophic lagoon versus 55% and 64% in the eutrophic lagoon. We observed shorter recovery lag times than those observed by many authors in transitional water ecosystems, who reported 10 to 18 years [START_REF] García | Nutrient and oxygenation conditions in transitional and coastal waters: Proposing metrics for status assessment[END_REF][START_REF] Gee | Signatures of restoration and management changes in the water quality of a central California estuary[END_REF][START_REF] Saeck | Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton[END_REF]. We also observed stronger effects than those reported by other authors on water column quality after remediation works of comparable magnitude [START_REF] Kronvang | Nutrient pressures and ecological responses to nutrient loading reductions in Danish streams, lakes and coastal waters[END_REF][START_REF] Williams | Development and evaluation of a spatially-explicit index of Chesapeake Bay health[END_REF]).

The TN:TP ratios were the lowest for the hypertrophic lagoon and the closest to the Redfield ratio, intermediate for the eutrophic lagoon and the highest for the oligotrophic lagoon. Such increases in the TN:TP ratios from the most to the least eutrophic ecosystem may also reflect a reduction in the dominance of primary production by phytoplankton and also a limitation of phytoplankton growth by phosphorus, which is more frequently observed in undisturbed ecosystems [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Paerl | Evolving paradigms and challenges in estuarine and coastal eutrophication dynamics in a culturally and climatically stressed world[END_REF]. The increase in the DIN:DIP ratio from the hypertrophic to the oligotrophic lagoon confirms this shift from DIN limitation to DIP limitation. This result is in agreement with observations of coastal lagoons [START_REF] Zirino | Nitrogen to phosphorus ratio in the Venice (Italy) Lagoon (2001-2010) and its relation to macroalgae[END_REF]) and also of freshwater ecosystems [START_REF] Jeppesen | Lake responses to reduced nutrient loading -An analysis of contemporary long-term data from 35 case studies[END_REF][START_REF] Jeppesen | Shallow lake restoration by nutrient loading reduction -Some recent findings and challenges ahead[END_REF][START_REF] Yan | Phosphorus accumulates faster than nitrogen globally in freshwater ecosystems under anthropogenic impacts[END_REF]).

In the hypertrophic lagoon, the decrease in the TN:TP and DIN:DIP ratios corresponds to a decrease in urban nutrient inputs, which was greater for nitrogen (-83%) than for phosphorus (-73%). The decrease is also due to the higher DIP concentrations observed four years after remediation actions were completed. The increase in DIP concentrations could be caused by phosphorus released into the water column from highly enriched bottom sediments [START_REF] Gikas | Water quality trends in a coastal lagoon impacted by non-point source pollution after implementation of protective measures[END_REF], which is facilitated by the strong benthic-pelagic nutrient coupling in shallow coastal lagoons [START_REF] Kennish | Chemical Introductions to the Systems: Diffuse and Nonpoint Source Pollution from Chemicals (Nutrients: Eutrophication)[END_REF]. But this increase in DIP could also be the result of non-consumption by phytoplankton as a consequence of strong nitrogen limitation in summer [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF].

Recovery trajectories in coastal lagoons

The pathways of change in the three coastal lagoons in our study covered a wide range of anthropogenic eutrophication, from oligotrophic to hypertrophic ecosystems.

Environmental monitoring of hypertrophic and eutrophic lagoons for respectively nine and 11 years after significant reductions in urban nutrient loading made it possible to identify different recovery patterns. The conceptual models of changes to the status of an ecosystem with pressure proposed by Elliott et al. 2007 and[START_REF] Tett | Defining and detecting undesirable disturbance in the context of marine eutrophication[END_REF], were used to represent the trajectories of the oligotrophic, eutrophic and hypertrophic lagoons, at different phases of anthropogenic eutrophication and recovery processes (Fig. 5). The concentration of Chl a in the water in summer was chosen as the ecosystem status metric (for shared periods: before (years 2001 to 2003) and after remediation works (years 2012 to 2014), as this integrative parameter has been shown to be a relevant indicator of eutrophication status [START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF][START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF]Ferreira et al. 2011). Moreover, our results are in agreement with those of [START_REF] Bec | Distribution of picophytoplankton and nanophytoplankton along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF], showing that Chl a is closely correlated with autotrophic picoeukaryotes, which comprise the majority of the phytoplankton community in French Mediterranean lagoons. TN total inputs from the watersheds (before and after remediation works, Table 1) were chosen as the pressure metrics, as N is typically the limiting factor in eutrophic lagoons in summer [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF].

Like in many studies of the recovery of coastal ecosystems (except e.g. [START_REF] Kemp | Eutrophication of Chesapeake Bay : historical trends and ecological interactions[END_REF]or Ibelings et al. 2017), the states of the eutrophic and hypertrophic lagoons before degradation were not fully described. The observations on the oligotrophic lagoon revealed a high ecological status, with low Chl a concentrations in accordance with the reference conditions laid down by European WFD (MEDDE, 2015), and revealed slight inter-annual variations (Fig. 5-1). This pattern reflects the inherent variability of undisturbed coastal lagoons (Elliott and Quintino 2007), dominated by perennial benthic macrophytes [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EU-project EUMAC[END_REF]Viaroli et al. 2007).

The eutrophic lagoon, in which the presence of macrophytes had been demonstrated before remediation (Fig. 5-2), recovered to a mesotrophic state, with primary production less dominated by phytoplankton and more by phanerogams (Ifremer 2014).

If we hypothesise that the reference conditions attainable for the eutrophic lagoon were similar to those of the oligotrophic lagoon, the recovery was almost complete, indicating complete resilience, because the ecosystem status after remediation was close to the reference state (Elliott et al. 2007). Such shifts from pelagic-dominated to more benthic-dominated communities based on macroalgae and seagrasses during the oligotrophication process have already been reported in coastal and estuarine eutrophic ecosystems after remediation actions [START_REF] Lie | Changes in the nutrient ratios and phytoplankton community after declines in nutrient concentrations in a semi-enclosed bay in Hong Kong[END_REF][START_REF] Cebrian | Eutrophication-driven shifts in primary producers in shallow coastal systems: implications for system functional change[END_REF][START_REF] Saeck | Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton[END_REF]).

The hypertrophic lagoon, which was dominated by phytoplankton and where the bottom was bare sediments before remediation (Fig. 5-4), shifted to a eutrophic state (Fig. 5-3), with the appearance of fast-growing macroalgae after mitigation actions (Ifremer 2010). In contrast to the eutrophic lagoon, the recovery trajectory of the hypertrophic lagoon appears to be not yet complete, suggesting that all compartments of the ecosystem have not yet recovered, particularly the benthic compartments (Elliott et al. 2007;[START_REF] Tett | Defining and detecting undesirable disturbance in the context of marine eutrophication[END_REF]. Moreover, the trajectory of the hypertrophic lagoon displayed high variability and feedback responses, suggesting that the ecosystem regime had become unstable [START_REF] Scheffer | Catastrophic regime shifts in ecosystems: linking theory to observation[END_REF][START_REF] Ibelings | Resilience of alternative stable states during the recovery of shallow lakes from eutrophication: Lake Veluwe as a case study[END_REF][START_REF] Kennish | Chemical Introductions to the Systems: Diffuse and Nonpoint Source Pollution from Chemicals (Nutrients: Eutrophication)[END_REF]. Shortly after remediation actions, the hypertrophic lagoon displayed a renewed increase in DIP concentrations, mimicking the immediate decrease in Chl a, and later the appearance of macroalgae (observed between 2006(observed between and 2009(observed between (Ifremer 2010))). The increase in DIP could correspond to the release of phosphate accumulated in the sediments, which is favoured by summer conditions [START_REF] Kemp | Eutrophication of Chesapeake Bay : historical trends and ecological interactions[END_REF], and also to their incomplete consumption by phytoplankton in the water column. Another possible explanation is recycling of the organic matter originating from macroalgae degradation and leading to an increase in dissolved inorganic nutrients in the water column. In lakes, internal loading of phosphates from sediments has frequently been reported to delay recovery from anthropogenic eutrophication [START_REF] Gulati | Lakes in the Netherlands, their origin, eutrophication and restoration: State-of-the-art review[END_REF][START_REF] Jeppesen | Lake responses to reduced nutrient loading -An analysis of contemporary long-term data from 35 case studies[END_REF][START_REF] Jeppesen | Shallow lake restoration by nutrient loading reduction -Some recent findings and challenges ahead[END_REF]), more rarely in coastal or estuarine ecosystems [START_REF] Lillebø | Applying quality status criteria to a temperate estuary before and after the mitigation measures to reduce eutrophication symptoms[END_REF][START_REF] Ní Longphuirt | Linking changes in nutrient source load to estuarine responses: an Irish perspective[END_REF][START_REF] Riemann | Recovery of Danish coastal ecosystems after reductions in nutrient loading: a holistic ecosystem approach[END_REF]. The recent renewed increases in Chl a and DIP persisting in 2015 and 2016, confirmed the incomplete recovery of the hypertrophic ecosystem, which alternates blooms of macroalgae and phytoplankton depending on the monitoring year [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EU-project EUMAC[END_REF][START_REF] Viaroli | Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons : a brief overview[END_REF]. Although a significant decrease was observed, Chl a concentrations after remediation still correspond to a bad status according to the European WFD (MEDDE, 2015).

Figure 5 suggests that there is a common restoration scheme in the studied lagoons following an equivalent decrease in anthropogenic pressure (nitrogen inputs). To what extent factors such as depth or salinity could modify this pattern remains to be determined. Following a significant reduction in nutrient inputs, hypertrophic or eutrophic coastal lagoons showed an almost immediate positive response in their environmental conditions. However, the oligotrophication trajectory and the likelihood of recovery, even partial, remain statistically difficult to predict (Duarte et al. 2009;[START_REF] Mccrackin | Recovery of lakes and coastal marine ecosystems from eutrophication: A global meta-analysis[END_REF]. Given the costs and difficulties in implementing management plans for nutrient reduction, managers and policy-makers should consider uncertainty and ecosystem specificity as their main guides when designing any remediation actions targeting oligotrophication of coastal lagoons. Indeed, as exemplified in this study, the level of recovery and the trajectory could be affected by: (i) external drivers, such as completeness of nutrient reduction, other stressors, connectivity, or climate change; (ii) internal factors such as physical traits (i.e. water renewal time; de [START_REF] De Jonge | Causes, historical development, effects and future challenges of a common environmental problem: eutrophication[END_REF][START_REF] Fiandrino | Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion[END_REF], lagoon history, sediment nutrient stocks (Duarte et al. 2009;[START_REF] Borja | Medium-and long-term recovery of estuarine and coastal ecosystems: patterns, rates and restoration effectiveness[END_REF] or the presence of residual seagrass patches or seed stocks [START_REF] Ibelings | Resilience of alternative stable states during the recovery of shallow lakes from eutrophication: Lake Veluwe as a case study[END_REF][START_REF] Le Fur | Submerged benthic macrophytes in Mediterranean lagoons: distribution patterns in relation to water chemistry and depth[END_REF]; and interactions between the two drivers. All of these may cause time lags, regime shifts and non-linearities [START_REF] Ibelings | Resilience of alternative stable states during the recovery of shallow lakes from eutrophication: Lake Veluwe as a case study[END_REF]Duarte et al. 2009;[START_REF] Riemann | Recovery of Danish coastal ecosystems after reductions in nutrient loading: a holistic ecosystem approach[END_REF] in ecosystem responses following remediation efforts. Specifically in the case of hypertrophic coastal lagoons, we showed that even 10 years after major reduction in nutrient loads, the environmental conditions necessary to restore the ecological functions and structure of the ecosystem, and the objectives laid down by the European WFD have not fully been achieved. It was not possible to estimate the recovery times needed to reach a good ecological status by all the lagoons studied. Further analyses of a larger number of lagoons will be necessary and should be completed by modelling to determine the effects of each factor and feedback interactions (positive or negative) with an impact on recovery trajectory and recovery time. At the time of writing, three strategies are feasible:

• "Wait and see": one could hypothesise that eutrophication pressure is now sufficiently low to allow the ecosystem to recover its ecological functions and structure of the reference conditions, after a period of hysteresis;

• "Go further" in reducing nutrient inputs: this is economically and technically hard to implement, and the likelihood of ecological recovery, even partial, is not guaranteed for the same reasons as discussed previously;

• "Give a helping hand": implementing active restoration programmes to enhance internal factors could facilitate ecological recovery [START_REF] Verdonschot | A comparative review of recovery processes in rivers, lakes, estuarine and coastal waters[END_REF].

Examples of possible active restoration actions are: restoration of seagrass meadows [START_REF] Orth | Eelgrass recovery induces state changes in a coastal bay system[END_REF]de Wit et al. 2017) to reduce nutrient sediment fluxes or limit turbidity; development of passive aquaculture for top-down phytoplankton control and/or nutrient export (Ferreira and Bricker 2016;[START_REF] Ahmed | Can greening of aquaculture sequester blue carbon?[END_REF][START_REF] Rcoreteam ; R Reitsma | Nitrogen extraction potential of wild and cultured bivalves harvested from nearshore waters of Cape Cod, USA[END_REF].

Two common results in studies of oligotrophication trajectories are the unpredictability of the degree of ecological recovery and its duration (at best 10 years) in the case of success (Duarte et al., 2009;[START_REF] Verdonschot | A comparative review of recovery processes in rivers, lakes, estuarine and coastal waters[END_REF][START_REF] Mccrackin | Recovery of lakes and coastal marine ecosystems from eutrophication: A global meta-analysis[END_REF]. Both call for continued long-term monitoring and research programmes (see list of priorities in [START_REF] Verdonschot | A comparative review of recovery processes in rivers, lakes, estuarine and coastal waters[END_REF] as well as adaptive recovery management plans, which emphasize institutional experimentation and learning by doing [START_REF] Jentoft | Limits of governability: institutional implications for fisheries and coastal governance[END_REF].

Table 1. Summary characteristics of the three sampling stations studied covering the eutrophication gradient. Lagoon and watershed areas, eutrophication status in 2001 (from [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF], residence time (Fiandrino et al. 2012) and year of the remediation works. Annual N and P inputs originating from WWTPs located in the watershed and annual N and P total inputs from whole watershed (including inputs from the WWTPs), before and after remediation works [START_REF] Meinesz | Base de données "pressions sur les lagunes méditerranéennes" -Analyse des liens état -pression[END_REF]. N and P inputs were standardized by lagoon volume (values in t.year -1 .10 6 m -3 ).

  Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 9

Figure 4

 4 Figure 4 shows the time series of DIP, DIN, TP, TN, Chl a, PICO and NANO for the three stations (AYR, BGN and MEW) used for a detailed examination of changes in trajectories. Between 2001 and 2004, i.e. before the remediation actions in the watershed containing the Palavasian complex, MEW station remained in its initial hypertrophic state, with average Chl a concentrations of 134 µg L -1 . The recovery process started in 2005 with no lag time, immediately after the construction of the

Figure captions : Fig. 1 .

 :1 Figure captions:

Fig. 2 .

 2 Fig. 2. Between-station PCA of 5 water column variables from the 22 stations and the 14 years of sampling. a) Loadings of the 5 active variables on the plane defined by PC1 and PC2. b) Location of stations on the PC1-PC2 plane. The colours show the degree of eutrophication of three of the stations: oligotrophic (AYR, blue), eutrophic (BGN, green) and hypertrophic (MEW, red). The d-values give the scale (i.e. the size of the grid is shown in grey).

Fig. 3 .

 3 Fig. 3. Trajectories between 2001 and 2014 of the AYR (oligotrophic, in blue), BGN (eutrophic, in green) and MEW (hypertrophic, in red) stations on the plane defined by PC1 and PC2 of the betweenstation PCA. The d-value gives the scale (i.e. the size of the grid is shown in grey).

Fig. 4 .

 4 Fig. 4. Monthly concentrations (diamonds) and means (curves) of summer concentrations (June-August) of: a) DIP and DIN (µM), b) TP and TN (µM), c) Chl a (µg L -1 , log10 scale) and d) picoeukaryote and nanoeukaryote abundances (10 6 cell L -1 ) from 2001 to 2014 at MEW (left column), BGN (middle column) and AYR (right column) stations. The shaded areas represent the period before the waste water treatment systems were improved.

Fig. 5 .

 5 Fig. 5. Changes in chlorophyll a in the water at stations BGN and MEW measured before (years 2001-2002-2003) and after mitigation actions (years 2012-2013-2014) versus TN total inputs from the watersheds (dots: average; crosses: outliers). Boxplot of Chl a at AYR station represents data measured in 2001-2002-2003 and in 2012-2013-2014. Data are represented in the conceptual model of changes in the ecosystem status with pressure (adapted from Elliott et al. 2007 and Tett et al. 2007). Illustrations of ecosystem status: oligotrophic (1) mesotrophic (2), eutrophic (3) and hypertrophic (4) (© Réseau de Suivi Lagunaire).

Table 2 .

 2 Spearman's rank correlations (ρ) between environmental variables measured monthly at the 22

	Lagoon	Lagoon	Lagoon	Watershed	Residence time	Year of the	N and P inputs from the WWTPs, in	N and P total inputs from the
	(sampling	area	volume	area (km²)	(d)		remediation	t.year -1 .10 6 m -3			watershed, in t.year -1 .10 6 m -3
	station -eutrophication	(km²)	(10 6 m 3 )		(volume/residence time, 10 6 m 3 d -1 )	works	N before	N after works	P before	P after works	N before	N after works	P before	P after works
	status in 2001)							works		works		works		works
	Ayrolle (AYR -	13.4	9.9	119.8	ND		-	0	0	0	0	0.05	0.05	0.006	0.006
	oligotrophic)
	Bages (BGN -	24.4	47	468.6	210 (0.23)		2003	6.2	0.9	0.9	0.09	8.2	3.7	1.1	0.4
	eutrophic)
	Palavasian	7.2	9.1	662	32 (0.28)		2005	157.7	3.9	13.5	0.6	177.2	30.2	17.3	4.6
	complex (MEW
	-hypertrophic)
		stations from 2001 to 2014 (***p < 0.001; **p < 0.01; *p < 0.05; ns: not significant; strong correlations
		(|ρ|≥0.5) in bold and underlined). Temp.: temperature, Sal.: salinity, O2: dissolved oxygen, Turb.: turbidity,
		Si: dissolved silicate, DIN: dissolved inorganic nitrogen, TN: total nitrogen, TP: total phosphorus, DIP:
			dissolved inorganic phosphorus, Chl a: chlorophyll a, NANO: nanophytoplankton, PICO: autotrophic
							picoeukaryotes
			Temp.	Sal.	O2	Turb.	Si	DIN	TN	TP		DIP	Chl a	NANO
		Sal.	0.07*
		O2	-0.10**	-0.24***
		Turb.	ns	-0.32***	ns
		Si	0.14**	-0.24***	-0.09	0.31***
		DIN	ns	-0.19***	0.14***	0.24***	ns
		TN	0.13***	-0.36***	ns	0.73***	0.46***	0.23***
		TP	0.22***	-0.27***	ns	0.74***	0.36***	0.20***	0.81***
		DIP	0.16***	-0.10**	ns	0.34***	0.34***	0.09**	0.38***	0.61***
		Chl a	0.29***	-0.19***	ns	0.66***	0.38***	0.12***	0.70***	0.82***	0.45***
		NANO	0.10**	-0.11***	ns	0.35***	ns	0.10**	0.31***	0.36***	0.13***	0.41***
		PICO	0.12***	-0.22***	ns	0.57***	0.28***	0.13***	0.56***	0.65***	0.28***	0.67***	0.38***

Table 3 .

 3 Inertia and first eigenvalues (PC1) associated with global, between-and within-group Principal Component Analyses (PCA) of the environmental dataset (DIN, DIP, Chl a, TN, TP)

	PCA	Global	Within-year	Between-year	Within-station	Between-station
	Inertia	5	4.49 (89.7%)	0.51 (10.2%)	2.38 (47.7%)	2.62 (52.3%)
	PC1	3.12	2.75	0.41	1.07	2.12

Acknowledgements

This study was based on data collected during the projects "Réseau de Suivi Lagunaire" and Water Framework Directive, which received financial support from Ifremer, Agence de l'Eau Rhône Méditerranée Corse, Région Languedoc-Roussillon/Occitanie and Cépralmar. The authors are extremely grateful to Martine Fortuné and Grégory Messiaen for the analyses of nutrients andto Elise Hatey for the analyses of chlorophyll a during all the years of the monitoring program; this article would not exists without their rigorous work in the laboratory. We thank our colleagues from Ifremer LERLR, LERPAC and LERPAC-Corse, Laurent Benau (Narbonne Regional Park), lagoon management structures for their help during field investigations.

The authors would like to thank Maela Kloareg and Claire Bissery/Kuzulia for their help Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4 20 with data analyses. Thanks to Grégory Messiaen, Cédric Duvail and Pierre Lopez for the design and layout of the figures.

Ecosystems 10: 4-16, https://doi.org/10.1007/s10021-006-9009-4. Ifremer, 2010. Réseau de Suivi Lagunaire du Languedoc-Roussillon. Bilan des résultats 2009 (report in French). http://archimer.ifremer.fr/doc/00118/22919/ (accessed 14 May 2018). Ifremer, 2014. Réseau de Suivi Lagunaire du Languedoc-Roussillon. Bilan des résultats 2013 (report in French). http://archimer.ifremer.fr/doc/00210/32154/ (accessed 14 May 2018). Foundation for Statistical Computing. Vienna, Austria. Derolez et al. 2018. Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons. OCMA_2018_443_R4

Author contributions

All authors contributed to the conception and design of the study, to the selection, analyses and interpretation of the data and relevant literature. VD led the writing of the manuscript. BB, DM, AF, RP, MS, PS, TL, CA and NM critically revised the manuscript.

All authors approve the manuscript in its current form and agree to be accountable for all aspects of the works. Duarte, C.M., Conley, D.J., Carstensen, J., Sánchez-Camacho, M., 2009