Campagne de surveillance 2017 de l'état DCE des lagunes méditerranéennes oligo-et mésohalines françaises pour la physico-chimie, le phytoplancton et les macrophytes. Consolidation de l'indicateur macrophytes Bilan des résultats 2017

Ce rapport présente le bilan des résultats en 2017 des actions programmées dans le cadre du suivi des lagunes oligo- et méso-halines. Il concerne les diagnostics de l’eau, du phytoplancton et des macrophytes sur 7 masses d’eau : Campignol, Vendres, Bagnas, Marette, Charnier-Scamandre-Crey, La Palissade et Bolmon

Are mussels and oysters capable of reducing the abundances of Picochlorum sp., responsible for a massive green algae bloom in Thau lagoon, France?

After an exceptional hydroclimatic year, a “massive green algae bloom”, dominated by the autotrophic picoeukaryote Picochlorum sp, was observed in 2018–2019 in the Thau lagoon. Oyster farmers informed decision-makers and scientists about an alarming halt to growth, loss of flesh weight and abnormal mortality in commercial size oysters, whereas mussel farmers observed no adverse effects. Two hypotheses were tested. First, Mediterranean mussels are capable of filtering Picochlorum spp. whereas Pacific and flat oysters are not. Second, picophytoplankton filtration rates vary with the size of the oyster, spats having greater ability than juveniles and commercial size oysters. A series of experiments was conducted in February 2019 using a series of beakers containing (i) three different types of solutions (1 L): water from the Thau lagoon containing Picochlorum at a rate of 45.2 million cells.L−1 (P) or diluted to 20.7 million cells. L−1 (D) and a solution of Isochrysis galbana containing 9.4 million cells. L−1; (ii) in the absence or presence of five types of bivalves: Pacific oysters of three different sizes (spat, juvenile and commercial size), flat oysters, and Mediterranean mussels of commercial size. Four separate water samples were taken during a 40-min incubation period to measure fluctuations in picophytoplankton and nanophytoplankton abundances using flow cytometry. Retention efficiency (%) was compared according to species and size. In contrast to Pacific and flat oysters, mussels with an average weight of 21.4 g, were able to drastically reduce the abundances of Picochlorum and depleted nearly one million Picochlorum cells per minute at 11.4 °C. These results suggest that the 1228 t of mussels that died in 2018 before the Picochlorum bloom could have helped limit the Picochlorum bloom if they had survived the heat and anoxia.

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

Phytoplankton strategies to exploit nutrients in coastal lagoons with different eutrophication status during re-oligotrophication

International audienc

Evaluation of sequential filtration and centrifugation to capture environmental DNA and survey microbial eukaryotic communities in aquatic environments

Sequential membrane filtration of water samples is commonly used to monitor the diversity of aquatic microbial eukaryotes. This capture method is efficient to focus on specific taxonomic groups within a size fraction, but it is time‐consuming. Centrifugation, often used to collect microorganisms from pure culture, could be seen as an alternative to capture microbial eukaryotic communities from environmental samples. Here, we compared the two capture methods to assess diversity and ecological patterns of eukaryotic communities in the Thau lagoon, France. Water samples were taken twice a month over a full year and sequential filtration targeting the picoplankton (0.2–3 μm) and larger organisms (>3 μm) was used in parallel to centrifugation. The microbial eukaryotic community in the samples was described using an environmental DNA approach targeting the V4 region of the 18S rRNA gene. The most abundant divisions in the filtration fractions and the centrifugation pellet were Dinoflagellata, Metazoa, Ochrophyta, Cryptophyta. Chlorophyta were dominant in the centrifugation pellet and the picoplankton fraction but not in the larger fraction. Diversity indices and structuring patterns of the community in the two size fractions and the centrifugation pellet were comparable. Twenty amplicon sequence variants were significantly differentially abundant between the two size fractions and the centrifugation pellet, and their temporal patterns of abundance in the two fractions combined were similar to those obtained with centrifugation. Overall, centrifugation led to similar ecological conclusions as the two filtrated fractions combined, thus making it an attractive time‐efficient alternative to sequential filtration

Influence de la filtration de différentes espèces et tailles de bivalves sur les concentrations de Picochlorum sp., responsable du phénomène d'eaux vertes observées dans la lagune de Thau en 2018

En 2018, dans un contexte climatique exceptionnel, la lagune de Thau a été impactée par une « malaïgue » causant des mortalités estivales massives puis par un phénomène d’eaux vertes, dominé par le picophytoplancton du genre Picochlorum durant l’automne 2018, entrainant un arrêt de croissance chez les huîtres. Inquiétés par le phénomène et les impacts engendrés sur la production, les acteurs locaux ont co-construit le projet de recherches « Overte » pour (i) mettre en place un suivi environnemental pour décrire ce phénomène et mieux le comprendre et (ii) mettre en évidence les interactions entre Picochlorum et les coquillages en élevage. Ce rapport aborde le second objectif. Ainsi une série d’expérimentations a été réalisée en février 2019 pour tester les capacités de différentes espèces et tailles de bivalves en élevage, à diminuer les abondances de Picochlorum par leur activité de filtration. Ces expériences ont été réalisées en laboratoire à l’aide d’une série de béchers contenant (i) 3 types de solutions (1L) : l’eau de la lagune de Thau contenant Picochlorum à 45,2 millions de cellules.L-1 (P) ou à 20,7 millions de cellules.L-1 (D) et une solution d’Isochrysis galbana ; (ii) en absence ou en présence de 5 types de bivalves : des huîtres creuses (Crassostrea gigas) de taille T6, T20 ou C3, des huîtres plates, Ostrea edulis, ou des moules méditerranéennes (Mytilus galloprovincialis). Une série de prélèvements d’eau a été réalisée au cours des 40 minutes d’incubation pour déterminer la variabilité temporelle des concentrations de picophytoplancton et de nanophytoplancton par cytométrie en flux. Cette étude a permis de démontrer qu’au contraire des huîtres creuses et des huîtres plates, les moules d’un poids moyen de 21,4 g sont capables de diminuer drastiquement les abondances de Picochlorum et ainsi de retenir près de 1 millions de cellules de Picochlorum par minute à 11,4°C. Ces résultats suggèrent que les 1228 tonnes de moules, mortes en 2018 avant l’efflorescence de Picochlorum, auraient pu participer au contrôle de l’efflorescence de Picochlorum si elles avaient survécu aux chaleurs et à l’anoxie. Au vu de ces résultats, il serait sans doute important d’assurer une gestion de stock des moules dans la lagune de Thau. Néanmoins il serait très important de récolter les moules en élevage avant les chaleurs estivales, pour éviter des mortalités massives telles que celles observées ces dernières années en lien avec le réchauffement climatique. Les perspectives de ce travail sont de tester les capacités de filtration des moules selon un gradient de température et de concentrations cellulaires pour déterminer le seuil de concentration à partir duquel Picochlorum ne serait plus ingéré et assimilé mais rejeté dans la colonne d’eau sous forme de pseudofécès

Impact of nutrient availability on the trophic strategies of the planktonic protist communities in a disturbed Mediterranean coastal lagoon

The impact of changes in nitrogen (N) and phosphorus (P) availability on the trophic strategies of planktonic protists was evaluated in a disturbed Mediterranean lagoon (Biguglia lagoon, France) using short-term bioassays. Natural communities were collected in three periods, i.e., autumn, spring and summer, to address the influence of the different environmental conditions. The responses of autotrophic plankton communities to experimentally induced N and/or P limitations were assessed as changes in chlorophyll a (Chl a) concentrations and in the abundances of potentially mixotrophic protists taxa. We observed blooms (> 105 cells l−1) of nanoflagellates in autumn, and of phycocyanin-rich picocyanobacteria in summer. Communities showed a co-limitation by N and P at the three sampling periods, despite high N:P ratios in autumn and spring. The high abundances of potentially mixotrophic dinoflagellates during these periods suggest the involvement of alternative trophic pathways for their maintenance in the lagoon. After bioassay incubations using different nutrient enrichment treatments, we often observed reduced abundances of mixotrophic protists containing Chl a with a concomitant increased abundance of protists without Chl a. This indicates a loss of chloroplasts and photoautotrophic abilities in protists cells, possibly reflecting a shift towards heterotrophy that could be sustained by phagotrophy

Fifty years of ecological changes: Regime shifts and drivers in a coastal Mediterranean lagoon during oligotrophication

Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series (1970–2018) of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 (1970–1992) was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993–2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993–2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003–2006), after which the water column became oligotrophic and seagrass began to recover (2007–2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted

Phytoplankton dynamics and bloom events in oligotrophic Mediterranean lagoons: seasonal patterns but hazardous trends

Detailed seasonal analyses of phytoplankton assemblages’ composition were performed on long-term datasets (20 years) of two oligotrophic Mediterranean lagoons (Diana and Urbino), in order to test if phytoplankton community and bloom events patterns rely on a seasonal basis. Our results highlight a similar phytoplankton composition between the lagoons, but different patterns in terms of phytoplankton abundances, of timing, magnitude and occurrence of Taxonomic Units, and of bloom events occurrence. Dominant diatoms group showed a seasonal repartition, with highest contribution of Skeletonema sp. during winter and spring, some other groups emerging in warmer seasons (e.g. Chaetoceros spp.), or not showing marked seasonality (e.g. Pseudo-nitzschia spp.). Dinoflagellates’ abundances were higher during autumn, besides exhibiting punctual proliferations over the year. Salinity was the strongest environmental parameter in controlling Diana lagoon phytoplankton community, while Urbino community was mostly controlled by turbidity. A worrying evolution of the toxic diatom Pseudo-nitzschia spp., linked to salinity and temperature changes, was detected in both lagoons. We further demonstrated changes in the phytoplankton community are ongoing in these lagoons, despite their contrasting human pressure contexts, indicating that serious impacts on phytoplankton dynamics and blooms triggering in Mediterranean lagoons shall be foreseen within the current and forecasted climate change scenarios