Patrones de variabilidad de las microalgas epibentónicas en la costa este tunecina

Epiphytic microalgae were monitored on various substrates of seagrass and macroalgae and in the water column for one year (from March 2013 to March 2014) in Oued Lafrann along the eastern coast of Chebba (Tunisia) with a focus on the distribution patterns of the epibenthic toxic dinoflagellates Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis. Microalgae assemblages were dominated by diatoms and dinoflagellates both in the water column and on vegetation. High concentrations of epiphytic toxic and potentially toxic dinoflagellates were preferentially hosted by Posidonia leaves, mainly in the apical and middle regions of the leaves, and P. lima was the dominant species. A significant positive correlation was found between P. lima concentrations on Posidonia and in the water column, suggesting that macrophytes should be sampled in the framework of harmful algal species monitoring. Ostreopsis cf. ovata, exhibited low concentrations and was mainly present on the inner surface of the Posidonia leaf, whereas P. lima was mainly present on the outer surface of the leaf, suggesting a likely space competition.Las microalgas epifitas fueron monitorizadas en varios sustratos de praderas marinas, macroalgas y en la columna de agua durante un año (de marzo 2013 hasta marzo 2014) en Oued Lafrann, en la costa este de Chebba (Túnez), con especial atención a la distribución de dinoflagelados tóxicos epibénicos, como Ostreopsis cf. ovata, Prorocentrum lima y Coolia monotis. Las comunidades de microalgas estaban dominadas por diatomeas y dinoflagelados, tanto en la columna de agua como en la vegetación. Se encontraron elevadas concentraciones de dinoflagelados epífitos tóxicos preferentemente en las hojas de Posidonia, principalmente en las regiones apicales y media y Prorocentrum lima fue la especie epífita dominante. Se observó una correlación positiva significativa entre las concentraciones de P. lima en Posidonia y en la columna de agua, lo que sugiere el muestreo de esta macrófita como parte de la monitorización de especies nocivas. Las células de Ostreopsis cf. ovata mostraron concentraciones bajas y se encontraron principalmente en la superficie interior de la hoja de Posidonia contrariamente a la distribución de P. lima presente principalmente en la superficie exterior de la lámina. Este hecho probablemente sugiere una competencia espacial entre los dos microalgas tóxicas

Fonctionnement des marais maritimes atlantiques: échanges d'énergie et effets des flux aquacoles diffus

French estuaries, from the Atlantic coast to the English channel, have been characterized by a poor phytoplankton biomass. The turbidity has always been considered as the limiting factor for phytoplankton. Estuarine ecosystems are situated at the boundary between the land and the sea. Consequently, they are subject to an enrichment by nutrient loadings from upland ecosystems. Although it has been demonstrated that a part of the nutrient loadings support an important benthic production. The part, which escapes to the estuarine benthic and planktonic food web and is exported to adjacent estuarine ecosystems, still remains undetermined. Quantifying the nu trient and particulate matter exchange between a macrotidal Bay, the Fier d'Ars, and the adjacent ocean was among the objectives of this study. The technique already performed was based on the regression estimation, usi ng the hydrometeorological parameters as predictor variables of instantaneous nutrjent fluxes . This method allowed us to establish the direction of net flux with tight confidence bounds on net annual flux. It also takes into account a large variability, inherent to the periodic phenomenon, such as hydrodynamics and also to nonperiodic variations, relative to the meteorological fluctuations. The assessment of nonpoint source loadings was performed using a simple model that accounts for the spatial pattern in topography and land use. This model was based on spatial analyses using Geographie Information System (GIS ) databases. The spatial distribution of nitrogen and phosphorus loadings was performed using the kriging interpolator. An 8-direction-model, particularly its derivative function, the weighted accumulation model, allowed the assessment of nutrient loading in each instream location. The study of the phytoplankton standing crops, highlighted a low biomass throughout the year. Showing a pattern of recurring variations within factors regulating phytoplankton biomass, which incriminated both light and nutrient availability (nitrogen and phosphorus). It was demonstrated that the study system produced more inorganic nitrogen than can be consumed, the excess was exported to the adjacent coastal ecosystem. A net export of inorganic dissolved carbon was also assessed. The system imported particulate matter, more than 70% was minerai forrn, which engendered a progressive sedimentation of the system. The GIS- based assessment of nu trient loading highlighted the major contribution of intensive activities to the enrichment of the coastal area with nitrogen loading. Atmospheric loading represented low proportions ( 4% - 12%) of the potential aquacultural loading. Nevertheless, internal processes, such as denitrification and nitrogen fixation, susceptible to reduce the quantities exported to the ocean, are not enough weil studied to establish the total balance of the system. Perspectives are proposed in this way, they concern the spatialisation of the different internaI processes with the determination of their weight locally.De faibles biomasses phytoplanctoniques caractérisent les systèmes estuariens des côtes atlantiques jusqu'à la manche. La turbidité a été souvent considérée comme le facteur limitant pour le phytoplancton. De part leur situation géographique, à la charnière entre la terre et la mer, les écosystèmes estuariens sont sujet à des enrichissements en matières nutritives dissoutes provenant des systèmes amonts. Bien qu'on a déjà démontré qu'une partie de ces apports supporte une forte production benthique, la part qui échappe au réseau trophique estuarien aussi bien benthique que planctonique, et qui est exportée aux systèmes océaniques adjacents, reste toutefois indéterminée. L'un des objectifs de ce travail est de quantifier les échanges de nutriments dissous et de matière particulaire entre la baie macrotidale du Fier d'Ars et l'océan côtier. L'approche utilisée a été basée sur la recherche de corrélations multiples à partir de paramètres hydrométéorologiques comme variables prédictives des flux instantanés. Le modèle de régression a pour but de déterminer un flux annuel net avec une estimation très fiable de la moyenne annuelle, tenant compte de la plus large variabilité, inhérente aux phénomènes périodiques tel que l'hydrodynarnisme, ainsi qu'aux variations non périodiques relatives à des changements météorologiques. La quantification des apports d'origine diffuse a été étudiée à l'aide d'un modèle stationnaire basé essentiellement sur le modèle spatial de la topographie et de l'utilisation du sol. Cette étude a fait appel à des outils d'analyses spatiales pour le développement de cartes thématiques, tels que les Systèmes d'Information Géographiques. La répartition spatiale des apports diffus aussi bien de l'azote que des phosphates a été réalisée à l'aide de l'interpolateur du krigeage. Un modèle d'écoulement de pente, en particulier une approche dérivée de ce modèle, le modèle d'accumulation pondérée, a permis d'estimer les flux diffus au niveau du réseau hydraulique. L'étude de la capacité trophique du système a permis d'évaluer de faibles biomasses phytoplanctoniques. Une complexe succession de facteurs (lumière, azote, phosphore) assure le contrôle et la limitation de la production phytoplanctonique n a été montré que le système étudié produit plus de matière inorganique azotée qu'il ne peut en consommer, l'excédant est donc exporté vers la zone côtière connexe. Une nette exportation de carbone organique dissous a été aussi mise en évidence. Le système importe de la matière particulaire dont plus de 70% est sous la forme minérale, engendrant ainsi une sédimentation progressive du système. Le modèle de quantification des apports diffus au sein de l'écosystème a permis de mettre en évidence la contribution des activités intensives dans l'enrichissement de la zone côtière en charges azotées. Les apports atmosphériques ne représentent que de faibles proportions 4%-12% de ce que peuvent produire les activités aquacoles. Cependant, les processus internes au système, susceptibles de diminuer la charge exportée vers l'océan, telle que la dénitrification ou la fixation d'azote ne sont pas encore assez bien étudiés afin de dresser le bilan global du système. Des perspectives sont proposées pour y remédier, qui nécessitent la spatialisation des différents processus afin de déterminer leurs poids localement

Ingestion of a bacterivorous ciliate by the oyster Crassostrea gigas: Protozoa as a trophic link between picoplankton and benthic suspension-feeders

The linked concepts of 'microbial loop' and 'protozoan trophic link' have been very well documented in filter-feeding microzooplankton such as copepods, but have not been applied to energy transfer to benthic suspension-feeding macrofauna, with the exception of the recent demonstration of heterotrophic flagellate assimilation by mussels. The oyster Crassostrea gigas obtains energy resources by filtering microalgae (similar to 5 to 100 mu m). However, in turbid estuaries, light-limited phytoplanktonic production cannot entirely account for oyster energy requirements. Conversely, picoplankters (<2 mu m), which are main effecters of coastal energy flow and matter cycling, are not efficiently retained by oyster filtration. Ciliate protozoal as both micro-sized cells (similar to 5 to 100 run) and bacteria grazers, may represent a major intermediary in trophic transfer between picoplankton and metazoa. The ciliate Uronema was intensely cultured and labelled, using the cyanobacteria Synechococcus as an auto-fluorescent biomarker. The labelled ciliates were offered as potential prey to oysters. We report here the first experimental evidence of a significant retention and ingestion of ciliates by oysters, supporting the role of protozoa as a realistic trophic link between picoplankters and filter-feeding bivalves and thus enhancing their potential importance in estuarine microbial food webs

Ultraphytoplankton community structure in subsurface waters along a North-South Mediterranean transect

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Diazotrophic cyanobacteria signatures and their relationship to hydrographic conditions in the Gulf of Gabes, Tunisia

International audienceChanges in the planktonic cyanobacteria structure, composition and diversity were followed over three consecutive years (2005–2006–2007) in the Gulf of Gabes (Eastern Mediterranean Sea, Tunisia). Cyanobacteria abundances, biomasses and cell lengths were measured together with selected environmental variables (pH, salinity, temperature and nutrients). The space and time variations of the cyanobacteria in relation to the environmental factors showed a close relationship between these plankton communities and the hydrographic structure of the water column. Cyanobacteria developed over semi-mixed conditions (May–June 2006) and during the thermal stratification (July 2005). The cyanobacterial abundance and biomass was evident between 20 and 35 m in inshore stations and between 20 and 25 m in deeper stations during the semi-mixing conditions and stratification. This thermocline level coincided with the euphotic layer (21.85 ± 3.76 m) allowing access of light radiation. The cyanobacteria bloom occurred during May–June 2006 when the N/P ratio (<10) was clearly below the accepted standard molar ratio of N/P = 16/1. Commonalities among cyanobacterial genera include being highly competitive for low concentrations of inorganic P (DIP) and the ability to acquire organic P compounds. Our study showed that both diazotrophic (N2-fixing) cyanobacteria such as Anabaena sp., Chroococcus sp., Trichodesmium erythraeum, Spirulina sp. and Spirulina subsalsa and non-diazotrophic cyanobacteria such as Pseudoanabaena sp. and Microcystis display a great flexibility in the N sources which allow formation of blooms

Ultraphytoplankton community structure in subsurface waters along a North-South Mediterranean transect

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Picoeukaryotic diversity in the Gulf of Gabès: variability patterns and relationships to nutrients and water masses

17 pages, 7 figures, 3 tables, supplementary material https://doi.org/10.3354/ame01857Marine picoeukaryotes show high phylogenetic diversity worldwide, notably in oligotrophic waters. In the Gulf of Gabès (south-eastern Mediterranean), characterized by oligotrophic conditions and a complex water mass circulation, information on picoeukaryotic diversity is still lacking. In this study, we investigated the diversity and spatial variability of picoeukaryotic assemblages in relation to nutrient availability, physical parameters and water masses in 3 cruises carried out in the Gulf of Gabès in June of 2008, April of 2009 and November of 2009. High-throughput sequencing revealed a dominance of sequences from non photosynthetic picoeukaryotes, mostly represented by the presumably parasitic marine alveolate MALV-II (33.20%) and the bacterivorous Bicosoecida (13.56%). Differences in picoeukaryotic assemblages were higher between coastal and open-sea stations, and depth in the water column also affected community differences, with surface (5 m), intermediate (25-100 m) and mesopelagic (>200 m) samples forming separate groups. A clear temporal variability was also evident, particularly for communities collected from the surface layer and open-sea stations. Co-inertia analysis revealed that picoeukaryotic groups were more affected by salinity in deep waters, whereas at the surface, they were dependent on nutrients and temperature. During the November cruise, samples that shared similar water mass properties generally clustered together. The Levantine water mass, observed for the first time in this area, was characterized by the presence of Acantharia and Polycystinea. Our study highlights the role of physical and chemical features, such as water mass origin, the wide continental shelf and trophic status, in determining the diversity of marine picoeukaryotesThis study was partially supported by the European funded project ‘Improving National Capacities in Observation and Management of Marine Environment in Tunisia’ (INCOMMET, 295009)Peer Reviewe

Prokaryotic Diversity and Distribution Along Physical and Nutrient Gradients in the Tunisian Coastal Waters (South Mediterranean Sea)

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Karenia selliformis bloom dynamics and growth rate estimation in the Sfax harbour (Tunisia), by using automated flow cytometry equipped with image in flow, during autumn 2019

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