Recovery trajectories following the reduction of urban nutrient inputs along the eutrophication gradient in French Mediterranean lagoons
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 (Viaroli et al., 2008; Zaldívar et al., 2008a,b; Souchu et al., 2010). In addition to this urban pressure, the transitional status of these waters makes these ecosystems particularly vulnerable to eutrophication (de Jonge and Elliott, 2001; Newton et al., 2014). Eutrophication of coastal ecosystems is a worldwide issue which has been described for decades (Nixon, 1995; Cloern, 2001). The increase in nutrient inputs, enhanced by urbanisation, agriculture or industry, leads to a complex cascade of both direct and indirect responses by ecosystems (Schramm, 1999; Viaroli et al., 2008). 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 (Cloern, 2001; Zaldívar et al., 2008a,b).
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 (Lie et al., 2011; Saeck et al., 2013; Staehr et al., 2017). While the degradation trajectories of coastal ecosystems subjected to eutrophication have been extensively described (Schramm, 1999; Cloern, 2001; de Jonge et al., 2002; Viaroli et al., 2008), recovery trajectories have only recently been studied (Duarte et al., 2009; Borja et al., 2010; McCrackin et al., 2017). 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 and to guide future management actions (McCrackin et al., 2017). Only a few studies have been conducted on the recovery process of French coastal lagoons, and they focused on selected water bodies (Collos et al., 2009; Leruste et al., 2016; Pasqualini et al., 2017). 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, >109 and >1010 cell.L−1, respectively, Souchu et al., 2010; Bec et al., 2011). Large-scale management actions were recently undertaken on waste water treatment systems to restore the quality and the functioning of the ecosystems (de Wit et al., 2015; Leruste et al., 2016). 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.
Section snippets
Study sites and sampling strategy
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 × 106 m3 (Souchu et al., 2010).
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,
Integrative parameters revealed a eutrophication gradient
The rare dataset we used to monitor coastal lagoons whose status ranged from oligotrophic to hypertrophic (Souchu et al., 2010; Bec et al., 2011), 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 (Meinesz et al., 2013; Gowen et al., 2015; Leruste et
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.
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 and to Elise Hatey for the analyses of chlorophyll a during all the years of the monitoring program; this article would not exist
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