Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part I: How to promote dinoflagellate dominance?

Salinity tolerance of the invasive blue crab Callinectes sapidus: From global to local, a new tool for implementing management strategy

The latest report from the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) raises concerns about the global proliferation of non-native species (NIS) driven by increasing temperatures. In 2023, the invasive blue crab Callinectes sapidus experienced a significant range expansion in the French Mediterranean lagoons, now present in over 20 lagoons and numerous estuaries with different invasion gradients. It has been established that this species is eurytherm (tolerates a wide range of temperatures; 0-40 °C), rendering temperature a non-limiting factor for its proliferation. On the other hand, salinity is a critical factor to consider, as C. sapidus requires brackish water for copulation and saline water for egg maturation. In this study, we present the salinity tolerance of C. sapidus using the metabolic performance to define the tolerance of the species to a large range of salinities (13 levels from 0 to 65 psu). Results showed that C. sapidus tolerates a large range of salinities with a minimum critical threshold (CTmin) at 0 psu, an optimum at 18.5 psu, and a CTmax at 62.4 psu. Using the performance curve equation, we applied the curve to real salinity data measured each month in 2023 in 20 lagoons invaded. Using this approach, we were able to map, on a fine spatial scale, the distribution of suitable habitats for C. sapidus in the year 2023. Additionally, we have developed a tool to prioritize the lagoons based on their suitability, thereby providing a decision-making framework for management measures in each lagoon. It emerged that 50 % of lagoons were priority areas, 10 % were intermediate and 40 % required monitoring. On the basis of this approach, we can respond to European regulations to help decision-makers implement action plans and/or early detection programs for invasive species management.

Socio-hydrogeological survey and assessment of organic pollutants to highlight and trace back pollution fluxes threatening a coastal groundwater-dependent ecosystem

Despite being a vector of pollution towards connected ecosystems, groundwater is often underestimated or not taken into account in management frameworks. To fill this gap, we propose to add socio-economic data to hydrogeological investigations to identify past and present pollution sources linked to human activities at watershed scale in order to forecast threats towards groundwater-dependent ecosystems (GDEs). The aim of this paper is to demonstrate, by a cross-disciplinary approach, the added value of socio-hydrogeological investigations to tackle anthropogenic pollution fluxes towards a GDE and to contribute to more sustainable management of groundwater resources. A survey combining chemical compounds analysis, data compilation, land use analysis and field investigations with a questionnaire was carried out on the Biguglia lagoon plain (France). Results show a pollution with a two-fold source, both agricultural and domestic, in all water bodies of the plain. The pesticide analysis reveals the presence of 10 molecules, including domestic compounds, with concentrations exceeding European groundwater quality standards for individual pesticides, as well as pesticides already banned for twenty years. On the basis of both the field survey and the questionnaire, agricultural pollution has been identified as very local highlighting the storage capacity of the aquifer, whereas domestic pollution is diffuse over the plain and attributed to sewage network effluents and septic tanks. Domestic compounds present shorter residence time within the aquifer highlighting continuous inputs, related to consumption habits of the population. Under the Water Framework Directive (WFD), member states are required to preserve the good ecological status, quality and quantity of water bodies. However, for GDEs it is difficult to achieve the 'good status' required without considering the groundwater's pollutant storage capacity and pollution legacy. To help resolve this issue, socio-hydrogeology has proved to be an efficient tool as well as for implementing effective protection measures for Mediterranean GDEs.

Harmful algae and pathogens on plastics in three mediterranean coastal lagoons

Plastic is now a pervasive pollutant in all marine ecosystems. The microplastics and macroplastic debris were studied in three French Mediterranean coastal lagoons (Prevost, Biguglia and Diana lagoons), displaying different environmental characteristics. In addition, biofilm samples were analyzed over the seasons to quantify and identify microalgae communities colonizing macroplastics, and determine potentially harmful microorganisms. Results indicate low but highly variable concentrations of microplastics, in relation to the period and location of sampling. Micro-Raman spectroscopy analyses revealed that the majority of macroplastic debris corresponded to polyethylene (PE) and low-density polyethylene (LDPE), and to a far lesser extent to polypropylene (PP). The observations by Scanning Electron Microscopy of microalgae communities colonizing macroplastic debris demonstrated differences depending on the seasons, with higher amounts in spring and summer, but without any variation between lagoons and polymers. Among the Diatomophyceae, the most dominant genera were Amphora spp., Cocconeis spp., and Navicula spp.. Cyanobacteria and Dinophyceae such as Prorocentrum cordatum, a potentially toxic species, were also found sporadically. The use of Primer specific DNA amplification tools enabled us to detect potentially harmful microorganisms colonizing plastics, such as Alexandrium minutum or Vibrio spp. An additional in situ experiment performed over one year revealed an increase in the diversity of colonizing microalgae in relation to the duration of immersion for the three tested polymers PE, LDPE and polyethylene terephthalates (PET). Vibrio settled durably after two weeks of immersion, whatever the polymer. This study confirms that Mediterranean coastal lagoons are vulnerable to the presence of macroplastic debris that may passively host and transport various species, including some potentially harmful algal and bacterial microorganisms.

Biodiversity impacts by multiple anthropogenic stressors in Mediterranean coastal wetlands

Mediterranean coastal wetlands are considered biodiversity hot-spots and contain a high number of endemic species. The biodiversity of these ecosystems is endangered by several pressures resulting from agricultural and urban expansion, climate change, and the alteration of their hydrological cycle. In this study we assess the state-of-the-art regarding the impact of several stressor groups on the biodiversity of Mediterranean coastal wetlands (i.e., lagoons, marshes, estuaries). Particularly, we describe the impacts of eutrophication, chemical pollution, invasive species, salinization, and temperature rise, and analyze the existing literature regarding the impact of multiple stressors on these ecosystems. Our study denotes a clear asymmetry both in terms of study areas and stressors evaluated. The majority of studies focus on lagoons and estuaries of the north-west parts of the Mediterranean basin, while the African and the Asian coast have been less represented. Eutrophication and chemical pollution were the most studied stressors compared to others like temperature rise or species invasions. Most studies evaluating these stressors individually show direct or indirect effects on the biodiversity of primary producers and invertebrate communities, and changes in species dominance patterns that contribute to a decline of endemic populations. The few available studies addressing stressor interactions have shown non-additive responses, which are important to define appropriate ecosystem management and restoration measures. Finally, we propose research needs to advance our understanding on the impacts of anthropogenic stressors on Mediterranean coastal wetlands and to guide future interventions to protect biodiversity.

The challenge of assessing the proper functioning conditions of coastal lagoons to improve their future management

The sustainable management of coastal lagoon hydrosystems is a key issue for the socio-economic and environmental development of many coastal areas worldwide. Often maintained by direct or indirect groundwater supplies, they provide a large range of ecosystem services, from which human societies take advantage. Twenty years after its implementation, a large majority of the Mediterranean lagoons have still not reached the "good status" required by the WFD. Several questions then arise as to the representativeness of the WFD indicators or the relevance of the restoration objectives considering the complexity, evolutionary and unpredictable nature of lagoon hydrosystems. This study proposes an innovative, multidisciplinary, long-term approach to define the proper functioning conditions of a costal lagoon hydrosystems, i.e. all the factors that contribute to the functioning of coastal lagoon hydrosystems and the connectivity with other biodiversity reservoirs, be they ecological, hydrological, social or political. By considering the lagoons and all its hydrological, ecological and societal proper functioning conditions over almost 200 years, this approach makes it possible to assess the influence of past natural and anthropogenic disturbances and support the implementation of future relevant hydrosystem-based management plans which have to be coordinated and politically driven. Defining proper restoration and management objectives should ensure that ecological functions are maintained based on current and future ecosystem benefits and uses. Considering the highly unpredictable nature of coastal hydrosystems, the state of an ecosystem should not only be evaluated on ecological or chemical criteria but also take into account socio-economic and political indicators. The implementation of the 3rd river basin management plan of the WFD as soon as 2022 could be the appropriate occasion to reassess the restoration objectives towards more realistic goals and to give more significance to the definition of the "resilience capacity" of water bodies in place of inappropriate restricted restauration objectives.

Restoration, conservation and phytoplankton hysteresis

Phytoplankton growth depends not only upon external factors that are not strongly altered by the presence of phytoplankton, such as temperature, but also upon factors that are strongly influenced by activity of phytoplankton, including photosynthetically active radiation, and the availability of the macronutrients carbon, nitrogen, phosphorus and, for some, silicate. Since phytoplankton therefore modify, and to an extent create, their own habitats, established phytoplankton communities can show resistance and resilience to change, including managed changes in nutrient regimes. Phytoplankton blooms and community structures can be predicted from the overall biogeochemical setting and inputs, but restorations may be influenced by the physiological responses of established phytoplankton taxa to nutrient inputs, temperature, second-order changes in illumination and nutrient recycling. In this review we discuss the contributions of phytoplankton ecophysiology to biogeochemical hysteresis and possible effects on community composition in the face of management, conservation or remediation plans.

Development and application of a real-time PCR assay for the sensitive detection of diarrheic toxin producer Prorocentrum lima

Prorocentrum lima (P. lima) is a widely spread dinoflagellate in the Mediterranean Sea and it has become increasingly involved in harmful algal blooms. The purpose of this study is to develop a probe-based real-time polymerase chain reaction (PCR) targeting the ITS1-5.8S-ITS2 region for the detection and absolute quantification of P. lima based on linear and circular DNA standards. The results have shown that the quantitative PCR (q-PCR), using circular plasmid as a template, gave a threshold cycle number 1.79-5.6 greater than equimolar linear standards. When microalgae, commonly found in aquatic samples were tested, no cross-amplification was observed. The q-PCR brought about a good intra and inter-run reproducibility and a detection limit of 5 copies of linear plasmid per reaction. A quantitative relationship between the cell numbers and their corresponding plasmid copy numbers was attained. Afterwards, the effectiveness of the developed protocol was tested with 130 aquatic samples taken from 19 Tunisian sampling sites. The developed q-PCR had a detection sensitivity of up to 1 cell. All the positive samples were taken from three sampling sites of Medenine Governorate with cell abundances that ranged from 22 to 156,000 cells L-1 of seawater. The q-PCR assay revealed a high sensitivity in monitoring the aquatic samples in which the low concentrations of P. lima were not accurately detected by light microscopy. Indeed, this approach is at the same time rapid, specific and sensitive than the traditional microscopy techniques and it represents a great potential for the monitoring of P. lima blooms.

Combinations of geoenvironmental data underline coastal aquifer anthropogenic nitrate legacy through groundwater vulnerability mapping methods

Groundwater quality is strongly dependent on land use. Past and current anthropogenic activities can lead to the diffusion of contaminants in aquifers. This diffusion can threaten the resource exploitation for decades, thereby endangering the ecological health of groundwater dependent ecosystems. Thus, groundwater stakeholders need methods for long-term management which integrate groundwater vulnerability. This study was conducted on the shallow alluvial aquifer of the groundwater-dependent Biguglia lagoon on Corsica Island, France. The aquifer is exposed to anthropogenic contamination for many decades with nitrate contamination legacy linked to agricultural activities, uncontrolled urbanization and sewage leakages. In most cases, vulnerability mapping is done in the objective of comparing groundwater situation regarding an on-going contamination process. But the question is still pending for aquifers where contamination is inherited from past practices or contaminations and where anthropogenic influences have changed through time. To propose an effective and innovative method for territorial management in Mediterranean alluvial aquifers, four index-based groundwater vulnerability mapping methods were tested and compared: two intrinsic vulnerability mapping methods (DRASTIC and SINTACS) and two specific vulnerability mapping methods (Modified-DRASTIC and SI), the latter integrating land use in the accuracy of groundwater vulnerability. Novelty is coming from the comparison between vulnerability maps and their application and validation in a hydrosystem affected by nitrate legacy-type contamination. The specific vulnerability mapping methods are more likely to represent the current pressures to which groundwater are subject. Thus, specific vulnerability methods such as the SI one revealed here very relevant to assess groundwater quality and to react retrospectively. The comparison between groundwater nitrate legacy and intrinsic groundwater vulnerability methods appeared also useful to define priority protection areas in long-term territorial management planning (EU Water Framework Direction). In this sense, the SINTACS method seems to be the more appropriate in the Mediterranean and alluvial context of this study.

Evaluation of FluoroProbe® performance for the phytoplankton-based assessment of the ecological status of Mediterranean coastal lagoons

The European Water Framework Directive and several other legislations worldwide have selected phytoplankton for monitoring the ecological status of surface waters. This assessment is a complicated task in coastal lagoons due to their intrinsic variability, prompting moves to use real-time measurements. Here, we tested the ability of the submersible spectrofluorometer FluoroProbe® to accurately estimate the phytoplankton biomass and to efficiently discriminate spectral groups in Mediterranean coastal lagoons, by using sub-surface water samples (n = 107) collected at Biguglia lagoon (Corsica) in different environmental situations (salinity and trophic state) from March 2012 to December 2014. We compared the estimates of biomass and phytoplankton group composition obtained with the FluoroProbe® (in situ and lab measurements) with the spectrofluorimetrically measured biomass and HPLC-derived quantifications of pigment concentrations. FluoroProbe® provided good estimates of the total phytoplankton biomass (particularly, the lab measurements). The FluoroProbe® data were significantly correlated with the HPLC results, except for the in situ measurements of very weak concentrations of blue-green and red algae. Our findings indicate that factory-calibrated FluoroProbe® is an efficient and easy-to-use real-time phytoplankton monitoring tool in coastal lagoons, especially as an early warning system for the detection of potentially harmful algal blooms. Practical instructions dedicated to non-specialist field operators are provided. A simple and efficient method for discarding in situ measurement outliers is also proposed.

Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past

Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifers is important for the management of groundwater resources, and the ecological health of groundwater dependent ecosystems. This study investigates anthropogenic contamination in the shallow alluvial aquifer of the Marana-Casinca, hydraulically connected to the Biguglia lagoon (Corsica, France). A multi-tracer approach, combining geochemical and environmental isotopic data (δ¹⁸O-H₂O, δ²H-H₂O, ³H, δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹¹B), and groundwater residence-time tracers (³H and CFCs) was carried out in 2016, and integrated with a study of land use evolution in the catchment during the last century. Groundwater NO₃^- concentrations, ranged between 2 mg/L and up to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agricultural activities). Comparatively high δ¹⁵N-NO₃^- values (up to 19.7‰) in combination with δ¹¹B values that were significantly lower (between 23‰ and 26‰) than the seawater background are indicative of sewage contamination. The ongoing deterioration of groundwater quality can be attributed to the uncontrolled urbanization development all over the alluvial plain, with numerous sewage leakages from the sanitation network and private sewage systems. Integration of contaminant and water-residence time data revealed a progressive accumulation of pollutants with time in the groundwater, particularly in areas with major anthropogenic pressure and slow dynamic groundwater flow. Our approach provides time-dependent insight into nitrogen pollution in the studied aquifer over the past decades, revealing a systematic change in the dominant NO₃^- source, from agricultural to sewage contamination. Yet, today's low groundwater quality is to large parts due to legacy pollution from land-use practices several decades ago, underlining the poor self-remediating capacity of this hydrosystem. Our results can be taken as warning that groundwater pollution that happened in the recent past, or today, may have dire impacts on the quality of groundwater-dependent ecosystems in the future.

Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part II: Mixotrophy of dinoflagellates as an adaptive strategy?

Dinoflagellate proliferation is common in coastal waters, and trophic strategies are often advanced to explain the success of these organisms. The Biguglia lagoon is a Mediterranean brackish ecosystem where eutrophication has long been an issue, and where dominance of dinoflagellates has persisted for several years. Monthly monitoring of fluorescence-based properties of phytoplankton communities carried out in 2010 suggested that photosynthesis alone could not support the observed situation all year round. Contrasting food webs developed depending on the hydrological season, with a gradual shift from autotrophy to heterotrophy. Progressively, microphytoplankton assemblages became unequivocally dominated by a Prorocentrum minimum bloom, which exhibited very weak effective photosynthetic performance, whereas paradoxically its theoretical capacities remained fully operational. Different environmental hypotheses explaining this discrepancy were examined, but rejected. We conclude that P. minimum bloom persistence is sustained by mixotrophic strategies, with complex compromises between phototrophy and phagotrophy, as evidenced by fluorescence-based observations.