Ben Othman, H., Lanouguère, É., Got, P., Sakka Hlaili, A., & Leboulanger, C. (2018). Structural and functional responses of coastal marine phytoplankton communities to PAH mixtures. Chemosphere, 209, 908–919.
Résumé: The toxicity of polycyclic aromatic hydrocarbons (PAHs) mixtures was evaluated on natural phytoplankton communities sampled from lagoons of Bizerte (South-western Mediterranean Sea) and Thau (North-western Mediterranean Sea). PAHs induced short-term dose and ecosystem-dependant decreases in photosynthetic potential. Chlorophyll a was negatively affected by increasing PAHs concentrations, together with dramatic changes in phytoplankton community composition. Size classes were strongly affected in the Bizerte compare to the Thau lagoon, with a decrease in nano- and microphytoplankton densities compare to picophytoplankton. In both locations, the diatom Entomoneis paludosa appeared favoured under PAH exposure as evidenced by increase in cell density, whereas autotrophic flagellates and dinophytes were strongly reduced. Smaller cells were more tolerant to exposure to highest PAHs concentrations, with persistent picophytoplankton carbon biomass at the end of the incubations. Apparent recovery of photosynthetic potential, accompanied with a regrowth of chlorophyll a under the lowest PAH doses, coincided with a significantly altered community composition in both lagoons. Furthermore, sensitivity to PAHs was not related to the phytoplankton cell size, and toxicity-induced modification of top-down control by grazers during the experiment cannot be excluded.
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Caillibotte, R., Leredde, Y., Vidussi, F., Ulses, C., Marsaleix, P., Estourne, C., et al. (2020). Impact of oysters as top predators on microbial food web dynamics: a modelling approach with parameter optimisation. Mar. Ecol.-Prog. Ser., 641, 79–100.
Résumé: Aquaculture is becoming a relevant and productive source of seafood, and production is expected to double in the near future. However, bivalve activities can significantly impact coastal ecosystem functioning. To study the direct and indirect impacts of oysters on the microbial food web, a OD biogeochemical modelling approach was adopted. The model was adjusted by parameter optimisation, assimilating data from several mesocosm observations of concentrations of nitrate, phosphate, silicate, dissolved organic carbon, chlorophyll, and bacterial biomass. The optimisation method provided a set of optimal parameters to fit the experimental observations of 'control' (i.e. natural water without oysters) and 'oyster' (i.e. natural water with oysters) mesocosms. The modelling results showed good accordance with the experimental observations, suggesting that the oysters directly reduced phytoplankton community biomass, thus constraining the ecosystem to a more heterotrophic state. Oysters also reduced competition between bacteria and phytoplankton for nutrient uptake, favouring higher bacterial biomass than in the control experiment. Additionally, the presence of oysters strongly increased large micro-zooplankton biomass (50-200 pm; mainly ciliates and large flagellates). This was a consequence of bacterivory by small zooplankton (5-50 mu m; mostly flagellates and small ciliates), providing a trophic link between bacteria and larger zooplankton. In conclusion, parameter optimisation showed good capacity to manage experimental data in order to build a more realistic model. Such models, in connection with future developments in aquaculture and global change scenarios, could be a promising tool for exploited ecosystem management and testing different environmental scenarios.
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Cecchi, P., Garrido, M., Collos, Y., & Pasqualini, V. (2016). Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part II: Mixotrophy of dinoflagellates as an adaptive strategy? Marine Pollution Bulletin, 108(1–2), 120–133.
Résumé: 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.
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Chahinian, N., Bancon-Montigny, C., Caro, A., Got, P., Perrin, J. L., Rosain, D., et al. (2012). The role of river sediments in contamination storage downstream of a waste water treatment plant in low flow conditions: Organotins, faecal indicator bacteria and nutrients. Estuarine Coastal and Shelf Science, 114, 70–81.
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Clavier, J., Chauvaud, L., Carlier, A., Amice, E., van der Geest, M., Labrosse, P., et al. (2011). Aerial and underwater carbon metabolism of a Zostera noltii seagrass bed in the Banc d'Arguin, Mauritania. Aquatic Botany, 95, 24–30.
Résumé: Community respiration and primary production were measured in a dense intertidal Zostera noltii bed on the Banc d'Arguin, Mauritania (West Africa) under aerial and submerged conditions. Metabolism was studied in situ in dark and transparent benthic chambers. CO(2) fluxes in the air were measured over a series of short-term incubations (3 min) using an infrared gas analyzer. Dissolved inorganic carbon fluxes were calculated from concentration changes during one-hour underwater incubations. Air and underwater irradiance levels were measured every minute throughout the experiments. Carbon respiration was lower in the air (2.2 mmol m(-2) h(-1)) than underwater (5.0 mmol m(-2) h(-1)); similarly, a production-irradiance model fitted to the data indicated that gross maximal photosynthetic rate was markedly lower during emergence (6.0 mmol C m(-2) h(-1)) than under water (42.7 mmol C m(-2) h(-1)). The delta(13)C values observed in shoots indicated a decrease in atmospheric CO(2) contribution, compared to dissolved inorganic carbon, in Z. noltii metabolism along a depth gradient within a single location. As the seagrass bed remains under a thin layer of water at low tide at the studied site, the large difference in primary production can be mainly attributed to photosynthesis inhibition by high pH and oxygen concentration, as well as to the negative feedback of self-shading by seagrass leaves during emersion. The observed differences in respiration can be explained by the oxygen deficit at night during low tide near the sediment surface, a deficit that is consistent with the abundance of anoxia-tolerant species. (C) 2011 Elsevier B.V. All rights reserved.
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