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Structural changes in phytoplankton communities have large influence on marine elemental cycling, food web dynamics and carbon export. Here we used data from two field expeditions, performed in spring and fall, over a coast-offshore gradient to investigate phytoplankton structure and dynamics in the Southwestern Tropical Atlantic (SWTA). Results revealed a predominant role of the thermohaline structure as the main driver of phytoplankton dynamics regardless the season. In fall, the thermocline and nutricline shallowing promoted a biomass increase, which was 3-fold higher around the oceanic islands. The structure of phytoplankton community mainly varied vertically, with Prochlorococcus pigments contributing greatly to the higher nutrient Deep Chlorophyll Maximum, whereas other Cyanophyceae predominated in nutrient poor surface layers during the two seasons. In addition, a clear coast-offshore variability in the new production (Fp) was observed, with the shelf region displaying higher values (up to 0.21), promoted by larger Bacillariophyceae pigments concentration, thus suggesting a coastal influence on shelf production. Although the phytoplankton biomass increased seasonally, our results highlighted a predominance of recycled production (Fp) and uncoupled dynamics between biomass and size of phytoplankton structure, with pico- and nanophytoplankton dominating the relative biomass, i.e., ca. 80% of the community in both seasons. We hypothesize that these patterns may result from a strong nitrogen limitation (N:P of around 3:1), which likely constrain a pronounced growth of the microphytoplankton.

WOS:000737215300006; International audience; Ecological baselines for the structure and functioning of ecosystems in the absence of human activity can provide essential information on their health status. The Glorieuses islands are located in the Western Indian Ocean (WIO) and can be considered as "pristine" ecosystems that have not been subjected to anthropogenic pressure. Their nutrient context and the microbial assemblages were assessed by determining the abundance of heterotrophic prokaryotes (archaea and bacteria), picocyanobacteria, picoeukaryotes, microphytoplankton and protozooplankton communities in five stations, during two contrasted periods (November 2015 and May 2016). Chlorophyll-a concentrations were always under 1 mu g/L and associated to very low levels in orthophosphates, nitrate and dissolved organic carbon, revealing an ultra-oligotrophic status for the Glorieuses waters. Picocyanobacteria confirmed the ultra-oligotrophic status with a predominance of Synechococcus. Zeaxanthin associated with the presence of picocyanobacteria represented the major pigment in both surveys. Three indices of diversity (species richness, Shannon and Pielou indexes) from microscopy observations highlighted the difference of diversity in microphytoplankton between the surveys. A focus on a 16S metabarcoding approach showed a high dominance of picocyanobacteria, Alpha- and Gammaproteobacteria, regardless of station or period. Multivariate analyses (co-inertia analyses) revealed a strong variability of ecological conditions between the two periods, with (i) high nutrient concentrations and heterotrophic nanoflagellate abundance in November 2015, and (ii) high heterotrophic prokaryote and picoeukaryote abundance in May 2016. The impact of a category 5 tropical cyclone (Fantala) on the regional zone in April 2016 is also advanced to explain these contrasted situations. Relative importance of top-down factors between bacterial and heterotrophic nanoflagellates was observed in November 2015 with an active microbial food web. All the results indicate that three microbial indexes potentially can be considered to assess the ecological change in Glorieuses marine waters.

International audience; Bacteria and phytoplankton are key players in aquatic ecosystem functioning. Their interactions mediate carbon transfer through the trophic web. Chemical contamination can alter the function and diversity of phytoplankton and bacterioplankton, with important consequences for ecosystem functioning. The aim of the present study was to assess the impact of chemical contamination on the interactions between both biological compartments. Two contrasting marine coastal ecosystems, offshore waters and lagoon waters, were exposed to chemical contamination (artificial or produced from resuspension of contaminated sediment) in microcosms in four seasons characterized by distinct phytoplankton communities. Offshore waters were characterized by a complex phytoplankton–bacterioplankton network with a predominance of positive interactions between both compartments, especially with Haptophyta, Cryptophyta, and dinoflagellates. In contrast, for lagoon waters, the phytoplankton–bacterioplankton network was simpler with a prevalence of negative interactions with Ochrophyta, Cryptophyta, and flagellates. Contamination with an artificial mix of pesticides and trace metal elements resulted in a decrease in the number of interactions between phytoplankton and bacterioplankton, especially for offshore waters. Resuspension of contaminated sediment also altered the interactions between both compartments. The release of nutrients stored in the sediment allowed the growth of nutrient limited phytoplankton species with marked consequences for the interactions with bacterioplankton, with a predominance of positive interactions, whereas in lagoon waters, negative interactions were mostly observed. Overall, this study showed that chemical contamination and sediment resuspension resulted in significant effects on phytoplankton–bacterioplankton interactions that can alter the functioning of anthropogenic coastal ecosystems

International audience; The productivity of most marine ecosystems is limited by the availability of dissolved nitrogen (N) and phosphorus (P). Nutrient recycling is therefore a key process for ecosystem functioning. Fish recycle nutrients through the excretion of ammonia and phosphate and can influence the abundance and community structure of primary producers such as phytoplankton. However, the effect of fish on other plankton compartments, and whether all fish species have similar effects, is largely unknown. We used a tank experiment to test how 2 Mediterranean fish species, gilthead seabream Sparus aurata and golden mullet Chelon auratus, with distinctly different N and P excretion rates, can affect the abundance and community structure of 3 plankton compartments: phytoplankton, bacterioplankton, and microzooplankton. We found that the nutrients released by seabream (whose excreta had an N:P molar ratio greater than the Redfield ratio of 16:1) induced a substantial increase in the abundance of all plankton compartments. In addition, with seabream, the relative abundance of diatoms in the phytoplankton communities increased. However, no significant change was observed with mullet, which had a low excreta N:P molar ratio, suggesting that the growth of microbial plankton was limited by the availability of N. Our results demonstrate that nutrient excretion by fish affects the microbial food web through a species-specific bottom-up effect on the total abundance and community structure of the phytoplankton, bacterioplankton, and microzooplankton communities.

WOS:000572257800001; International audience; The current lack of knowledge on the trophic ecology of scyphozoans, particularly at the benthic stage, prevents a full understanding of the controls on many jellyfish blooms. The blooming scyphozoan (Aurelia coerulea) completes its entire life cycle in the Thau lagoon (southern France), where the annual population dynamics of both its benthic and pelagic stages have been described. This offered an exceptional framework to investigate the trophic processes regulating jellyfish populations over time. To this aim, stable isotopic signature analysis (δ13C and δ15N) was used to infer the diet of both A. coerulea scyphistomae and medusae over 1 year. These results were matched with medusae gut content analysis and with the monthly abundances of local plankton groups. Lastly, the isotopic signatures of A. coerulea scyphistomae and medusae were compared with those of the oysters (Crassostrea gigas) cultivated in the lagoon to evaluate the potential interspecific trophic competition. The results revealed two seasonal shifts in the trophic niche of A. coerulea and substantial overlap between the diets of its benthic and pelagic stages. Conversely, trophic niche overlaps with the oysters were restricted, suggesting a limited impact of the local jellyfish bloom on shellfish production. Phytoplankton, microzooplankton, mesozooplankton, and sedimentary organic matter were all important food sources during critical periods of A. coerulea life-cycle. However, microzooplankton abundance was found to be key for the production of buds by the scyphistomae and, therefore it is likely to control the benthic population size and, thereby, to modulate the intensity of its annual bloom in Thau.

International audience; Recently, CO2 recycling for the production of valuable microalgae has acquired substantial interest. Most studies investigating CO2 conversion efficiency in algal cultures were based on single species, although a stabilising effect of algal diversity on biomass production was recently highlighted. However, addition of CO2 into polyalgal cultures requires a careful control of pH; performance of CO2 conversion, growth and carbon biomass production are affected by pH differently, depending on the species of microalgae. This study investigates the efficiency of CO2 conversion by natural marine algal assemblage cultivated in open, land-based raceways (4.5 m3, 10 m2), working as high rate algal ponds (HRAP). Ponds were enriched with nitrogen and phosphate, pure CO2 was added and algal cultures were grown under three different fixed pH levels: pH 6, 7 and 8. The highest conversion of photosynthetically fixed CO2 into carbon biomass (40%) was reached at pH 7, an intermediate level, due to the partial CO2 asphyxiation of algal predators (copepods, ciliates), while being under the suboptimal conditions for the development of marine amoebae. Under this pH, the theoretical maximal biological conversion of available CO2 into carbon biomass was estimated to be 60% in naturally inoculated open ponds.

International audience; Several studies focused on wastewater treatment in High Rate Algal Ponds (HRAP) suggest that highly variable climatic conditions cause large variations of microalgal biomass productivity. In the present study, we show that similar carbon, nitrogen and phosphorus removal efficiencies were reached in different HRAPs treating urban wastewaters located in two different temperate (Mediterranean and oceanic) climates. Furthermore, similar ecological successions were observed in these HRAPs. During the start-up phase, the consumption of organic matter by detritivores, already present in the wastewater, appears to be necessary for the microalgae to grow within two weeks in spring. The growth of the rapid-growing species, Chlorella sp., followed by the grazing-resilient species, Scenedesmus sp., combined with nitrifying and denitrifying bacterial activity, removed most the ammonia. The resulting exhaustion of ammonia would limit the complete removal of dissolved COD by bacteria and phosphate by microalgae in the HRAPs. This study shows that similar biological and environmental constraints were applied on the HRAPs, making the process efficiency highly reproducible under different temperate latitudes.

Lake Dziani Dzaha (Mayotte Island, Indian Ocean) is a tropical thalassohaline lake which geochemical and biological conditions make it a unique aquatic ecosystem considered as a modern analogue of Precambrian environments. In the present study, we focused on the diversity of phytoplanktonic communities, which produce very high and stable biomass (mean2014–2015 = 652 ± 179 μg chlorophyll a L−1). As predicted by classical community ecology paradigms, and as observed in similar environments, a single species is expected to dominate the phytoplanktonic communities. To test this hypothesis, we sampled water column in the deepest part of the lake (18 m) during rainy and dry seasons for two consecutive years. Phytoplanktonic communities were characterized using a combination of metagenomic, microscopy-based and flow cytometry approaches, and we used statistical modeling to identify the environmental factors determining the abundance of dominant organisms. As hypothesized, the overall diversity of the phytoplanktonic communities was very low (15 OTUs), but we observed a co-dominance of two, and not only one, OTUs, viz., Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta). We observed a decrease in the abundance of these co-dominant taxa along the depth profile and identified the adverse environmental factors driving this decline. The functional traits measured on isolated strains of these two taxa (i.e., size, pigment composition, and concentration) are then compared and discussed to explain their capacity to cope with the extreme environmental conditions encountered in the aphotic, anoxic, and sulfidic layers of the water column of Lake Dziani Dzaha. Electronic supplementary material The online version of this article (10.1007/s00248-019-01332-8) contains supplementary material, which is available to authorized users.

Phytoplankton and bacterioplankton are the key components of the organic matter cycle in aquatic ecosystems, and their interactions can impact the transfer of carbon and ecosystem functioning. The aim of this work was to assess the consequences of chemical contamination on the coupling between phytoplankton and bacterioplankton in two contrasting marine coastal ecosystems: lagoon waters and offshore waters. Bacterial carbon demand was sustained by primary carbon production in the offshore situation, suggesting a tight coupling between both compartments. In contrast, in lagoon waters, due to a higher nutrient and organic matter availability, bacteria could rely on allochthonous carbon sources to sustain their carbon requirements, decreasing so the coupling between both compartments. Exposure to chemical contaminants, pesticides and metal trace elements, resulted in a significant inhibition of the metabolic activities (primary production and bacterial carbon demand) involved in the carbon cycle, especially in offshore waters during spring and fall, inducing a significant decrease of the coupling between primary producers and heterotrophs. This coupling loss was even more evident upon sediment resuspension for both ecosystems due to the important release of nutrients and organic matter. Resulting enrichment alleviated the toxic effects of contaminants as indicated by the stimulation of phytoplankton biomass and carbon production, and modified the composition of the phytoplankton community, impacting so the interactions between phytoplankton and bacterioplankton.

International audience; This work describes a method to model the optical properties over the (400-750 nm) spectral range of a dense microalgal culture using the chemical and physical properties of the algal cells. The method was based on a specific program called AlgaSim coupled with the adding-doubling method: at the individual cell scale, AlgaSim simulates the spectral properties of one model, three-layer spherical algal cell from its size and chemical composition. As a second step, the adding-doubling method makes it possible to retrieve the total transmittance of the algal medium from the optical properties of the individual algal cells. The method was tested by comparing the simulated total transmittance spectra for dense marine microalgal cultures of Isochrysis galbana (small flagellates) and Phaeodactylum tricornutum (diatoms) to spectra measured using an experimental spectrophotometric setup. Our study revealed that the total transmittance spectra simulated for the quasi-spherical cells of Isochrysis galbana were in good agreement with the measured spectra over the whole spectral range. For Phaeodactylum tricornutum, large differences between simulated and measured spectra were observed over the blue part of the transmittance spectra, probably due to non-spherical shape of the algal cells. Prediction of the algal cell density, mean size and pigment composition from the total transmittance spectra measured on algal samples was also investigated using the reversal of the method. Mean cell size was successfully predicted for both species. The cell density was also successfully predicted for spherical Isochrysis galbana, with a relative error below 7%, but not for elongated Phaeodactylum tricornutum with a relative error up to 26%. The pigments total quantity and composition, the carotenoids:chlorophyll ratio in particular, were also successfully predicted for Isochrysis galbana with a relative error below 8%. However, the pigment predictions and measurements for Phaeodactylum tricornutum showed large discrepancies, with a relative error up to 88%. These results give strong support for the development of a promising tool providing rapid and accurate estimations of biomass and physiological status of a dense microalgal culture based on only light transmittance properties.

Contamination of coastal environments is often due to a complex mixture of pollutants, sometimes in trace levels, that may have significant effects on diversity and function of organisms. The aim of this study was to evaluate the short-term dynamics of bacterioplankton exposed to natural and artificial mixtures of contaminants. Bacterial communities from a southwestern Mediterranean ecosystem, lagoon and the bay (offshore) of Bizerte were exposed to i) elutriate from resuspension of contaminated sediment, and ii) an artificial mixture of metals and herbicides mimicking the contamination observed during sediment resuspension. Elutriate incubation as well as artificial spiking induced strong enrichments in nutrients (up to 18 times), metals (up to six times) and herbicides (up to 20 times) relative to the in situ concentrations in the offshore station, whereas the increases in contaminants were less marked in the lagoon station. In the offshore waters, the artificial mixture of pollutants provoked a strong inhibition of bacterial abundance, production and respiration and significant modifications of the potential functional diversity of bacterioplankton with a strong decrease of the carbohydrate utilization. In contrast, incubation with elutriate resulted in a stimulation of bacterial activities and abundances, suggesting that the toxic effects of pollutants were modified by the increase in nutrient and DOM concentrations due to the sediment resuspension. The effects of elutriate and the artificial mixture of pollutants on bacterial dynamics and the functional diversity were less marked in the lagoon waters, than in offshore waters, suggesting a relative tolerance of lagoon bacteria against contaminants.

International audience; Coral reef environments are generally recognized as being the most threatened of marine ecosystems. However, it is extremely difficult to distinguish the effects of climate change from other forcing factors, mainly because it is difficult to study ecosystems that are isolated from human pressure. The five Iles Eparses (Scattered Islands) are located in the Western Indian Ocean (WIO) and can be considered to be "pristine" ecosystems not subject to anthropogenic pressure. This study characterized their plankton assemblages for the first time, by determining the abundances of microbial (virus, bacteria, heterotrophic protists and phytoplankton) and metazooplankton communities in various lagoon and ocean sites around each island. The Europa lagoon has extensive, productive mangrove forests, which have the highest nutrient concentrations (nitrogen forms, dissolved organic carbon) and whose microbial communities present a peculiar structure and functioning. By means of bioassay experiments, we observed that bacterial production and growth rates are higher in Europa than those reported for the other islands. Tromelin, which lies outside the Mozambique Channel, had the lowest biological productivity, nutrient concentrations, and bacterial growth rates. Multivariate analysis indicated that distinct microbial assemblages occur in association to varying nutrient concentrations. Molecular fingerprinting showed clear discrimination of the structure of the archaea, bacteria and eukaryotes community between the sites. Our results suggest that the geographical distance can influence the diversity of dominant microbial taxa in the WIO.

Virus-prokaryote interactions were investigated in four natural sites in Senegal (West Africa) covering a salinity gradient ranging from brackish (10‰) to near salt saturation (360‰). Both the viral and the prokaryote communities exhibited remarkable differences in their physiological, ecological and morphological traits along the gradient. Above 240‰ salinity, viral and prokaryotic abundance increased considerably with the emergence of (1) highly active square haloarchaea and of (2) viral particles with pleiomorphic morphologies (predominantly spindle, spherical and linear shaped). Viral life strategies also showed some salinity-driven dependence, switching from a prevalence of lytic to lysogenic modes of infection at the highest salinities. Interestingly, the fraction of lysogenized cells was positively correlated with the proportion of square cells. Overall, the extraordinary abundance of viruses in hypersaline systems (up to 6.8 × 10(8) virus-like particles per milliliter) appears to be partly explained by their high stability and specific ability to persist and proliferate in these apparently restrictive habitats.

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L−1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l−1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 104 cells l−1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

International audience; Size fractionation was performed using water from the Great Reef of Toliara (Madagascar) taken from two different habitats (ocean and lagoon) during the dry and wet seasons, to study the growth and mortality rates of bacterioplankton. Experiments were conducted with 1 and 100% of heterotrophic nanoflagellate (HNF) concentrations and virus-free water was obtained by tangential filtration (10 kDa). During the dry season, in both environments, bacterial abundance and production were significantly lower than values recorded during the wet season. Bacterial growth rates without grazers were 0.88 day–1 in the lagoon and 0.58 day–1 in the ocean. However, growth rates were statistically higher without grazers and viruses (1.58 day–1 and 1.27 day–1). An estimate of virus-induced bacterial mortality revealed the important role played by viruses in the lagoon (0.70 day–1) and the ocean (0.69 day–1). During the wet season, bacterial growth rates without grazers were significantly higher in both environments than were values obtained in the dry season. However, the bacterial growth rates were paradoxally lower in the absence of viruses than with viruses in both environments. Our results suggest that changes in nutrient concentrations can play an important role in the balance between viral lysis and HNF grazing in the bacterial mortality. However, virus-mediated bacterial mortality is likely to act simultaneously with nanoflagellates pressure in their effects on bacterial communities.

Coupling algal biomass production and anaerobic digestion is one of the most promising bioprocesses for economically viable algal production. This study assesses the production rates of some native microalgae growing in media supplemented with algal digestate, urban wastewater or digested sludge. Native microalgal populations isolated from temperate freshwaters ( Scenedesmus spp.) and marine ecosystems ( Nannochloris spp.) had the highest potential production rates (about 100 mg DW L −1 d −1 ) with algal digestate at about 20% loading ratio. However, no growth was measured for Nannochloris spp., when the ammonium concentration exceeded 100 mg L −1 although Scenedesmus spp. appeared to be tolerant to higher NH 4 + concentrations. Very low production rates, or no growth, were measured when microalgae isolated from high salinity waters ( Dunaliella salina , Lyngbya aestuarii ) were used, suggesting that populations well adapted to extreme environmental conditions are not suitable candidates for growing on wastewater or anaerobic digestate.

International audience; Natural plankton communities from a tropical freshwater reservoir (Combani Reservoir, Mayotte Island, Mozambique Channel) were exposed, in 20-l nutrient-enriched microcosms, to two nominal concentrations of three pesticides: the herbicides diuron (2.2 and 11 mu g/l) and paraquat (10 and 40.5 mu g/l) and the insecticide fenitrothion (10 and 100 mu g/l), commonly used in the tropics for agriculture and disease vector control. Bacterioplankton, phytoplankton, and zooplankton communities were monitored for 5 days after exposure, and the concentrations of toxicant and major nutrients were measured. Bacterioplankton growth was noticeable in all systems and was slightly affected by pesticide at any concentration. A transitory increase in thymidine-based bacterial production was observed in diuron- and fenitrothion-treated microcosms, followed by a marked decrease in all microcosms after 5 days. The functional diversity of bacterioplankton, evaluated using BIOLOG ECO (R) microplates, was reduced by exposure to the highest pesticide concentrations. Phytoplankton was affected by pesticides in different ways. Chlorophyll biomass and biovolumes were increased by diuron addition and decreased by paraquat, whereas fenitrothion-treated microcosms remained unaffected relative to controls. Phytoplankton taxonomic diversity was decreased by paraquat and high doses of fenitrothion but was unaffected by addition of diuron. The decrease in diversity was due to a reduction in the number of species, whereas the density of small cells increased, especially after addition of paraquat. Heterotrophic flagellates were sensitive to paraquat and to the highest diuron concentration; a reduction in biomass of up to 90% was observed for 40.5 mu g/l paraquat. Zooplankton, dominated by Thermocyclops decipiens and Diaphanosoma excisum, was slightly sensitive to diuron, and very sensitive to paraquat. High concentrations of the insecticide fenitrothion were effective only on young stages. The potential direct and indirect effects of pesticide contamination on such a simplified plankton food web, typical of newly constructed reservoirs, appear to differ significantly depending on the biological compartment considered. The overall sensitivity of tropical plankton is comparable to the sensitivity for temperate systems, and direct and indirect effects appeared rapidly, within 5 days of exposure.

National audience; Les communautés planctoniques naturelles du réservoir de Combani (Mayotte) ont été exposées expérimentalement à 2 herbicides (diuron, paraquat) et un insecticide (fénitrothion). Ces molécules font partie des pesticides en usage dans la zone intertropicale, que ce soit en agriculture ou pour le contrôle des insectes vecteurs de maladies. Les organismes ont été exposés à deux concentrations différentes de chaque molécule pure pendant 6 jours dans des microcosmes de 20L en conditions semi-naturelles, au cours desquels leur abondance et leur activité ont été évaluées, en parallèle avec certains paramètres physico-chimiques. Les deux herbicides sont restés stables en concentration au cours de l’expérimentation, alors que l’insecticide fénitrothion s’est dégradé (demi-vie calculée d’une quarantaine d’heures). Le bactérioplancton a été peu affecté sur le plan de la biomasse et de l’activité totale par les ajouts de pesticides, alors que la diversité fonctionnelle était relativement réduite par les plus fortes concentrations (diuron 11.4 µg/L, paraquat 22.5 µg/L et fénitrothion 34.7 µg/L). Le phytoplancton a été relativement peu affecté par l’ajout de pesticide, essentiellement par l’ajout de paraquat (diminution de la biomasse totale) ou de diuron (augmentation du contenu en chlorophylle). Cependant, la diversité taxinomique du phytoplancton a diminué sous l’effet du paraquat et du fénitrothion, accentuant la dominance des chlorophycées dans la communauté de producteurs primaires. En présence de pesticide, les communautés microbiennes eucaryotes présentent des trajectoires d’évolution de diversité (empreinte génétique) différentes au cours du temps. Ces trajectoires dépendent de la nature du pesticide et dans une moindre mesure de la dose appliquée. Les flagellés hétérotrophes ont été particulièrement sensibles au paraquat et à la plus forte dose de diuron, alors que le zooplancton montrait une sensibilité aux molécules dépendante à la fois de l’espèce et de la classe d’âge considérée. De façon inattendue le zooplancton, dominé en nombre et biomasse par le cyclopide carnivore Thermocyclops decipiens et le cladocère herbivore Diaphanosoma excisum, s’est avéré sensible aux herbicides, alors que l’insecticide n’a affecté que les stades les plus jeunes de ces organismes. Les effets directs et indirects ont pu être mis en évidence, dépendant à la fois de la molécule et de la dose appliquée, sur ces communautés naturelles issues de milieux a priori vierges de contaminations antérieures. Ce type d’étude peut contribuer à compléter les connaissances encore parcellaires en matière d’évaluation des risques en milieu tropical.

Vibrio species have been associated with recurrent mass mortalities of juvenile oysters Crassostrea gigas threatening oyster farming worldwide. However, knowledge of the ecology of pathogens in affected oyster farming areas remains scarce. Specifically, there are no data regarding (i) the environmental reservoirs of Vibrio populations pathogenic to oysters, (ii) the environmental factors favoring their transmission, and (iii) the influence of oyster farming on the persistence of those pathogens. This knowledge gap limits our capacity to predict and mitigate disease occurrence. To address these issues, we monitored Vibrio species potentially pathogenic to C. gigas in 2013 and 2014 in the Thau Lagoon, a major oyster farming region in the coastal French Mediterranean. Sampling stations were chosen inside and outside oyster farms. Abundance and composition of phyto-, microzoo-, and mesozooplankton communities were measured monthly. The spatial and temporal dynamics of plankton and Vibrio species were compared, and positive correlations between plankton species and vibrios were verified by qPCR on isolated specimens of plankton. Vibrio crassostreae was present in the water column over both years, whereas Vibrio tasmaniensis was mostly found in 2013 and Vibrio aestuarianus was never detected. Moreover, V. tasmaniensis and V. crassostreae were found both as free-living or plankton-attached vibrios 1 month after spring mortalities of the oyster juveniles. Overall, V. crassostreae was associated with temperature and plankton composition, whereas V. tasmaniensis correlated with plankton composition only. The abundance of Vibrio species in the water column was similar inside and outside oyster farms, suggesting important spatial dispersion of pathogens in surrounding areas. Remarkably, a major increase in V. tasmaniensis and V. crassostreae was measured in the sediment of oyster farms during cold months. Thus, a winter reservoir of pathogenic vibrios could contribute to their ecology in this Mediterranean shellfish farming ecosystem.