Publications

 2008 - Relation concentration-débit et évolution temporelle du nitrate dans 25 rivières de la région Bretagne (France) 

Auteurs : Guillaud J.F., Bouriel L. (UFR Sciences et Techniques - Anglet)

Revue des Sciences de l'Eau / Journal of Water Science (INRS-Eau, Terre & environnement) (in press) (0.49 Mo)
La concentration de nitrate dans les 25 principales rivières bretonnes est, dans 90 % des cas, corrélée positivement au débit. Les caractéristiques hydrogéologiques des bassins versants jouent sur les termes de cette corrélation ; ainsi un taux d'écoulement superficiel important (caractéristique des bassins versants peu perméables) renforce cette corrélation et diminue le décalage temporel entre les pointes de crue et de concentration de nitrate.
L'évolution pluriannuelle des concentrations de nitrate dans les rivières bretonnes est caractérisée par un quadruplement durant les trente dernières années. Par contre depuis le milieu des années 90, on observe sur 80 % des rivières une tendance à la décroissance, indépendamment des évolutions pluriannuelles des débits. Le taux annuel de décroissance est d'autant plus fort que le bassin versant présente une part importante d'écoulements superficiels, et a donc une moins grande inertie hydrogéologique.
Etant donné que les temps de réaction des bassins versants sont relativement courts (2-10 ans), il se peut que l'on commence à constater l'effet sur les eaux superficielles d'une inversion de tendance concernant la pression agricole (baisse des fumures organiques et minérales azotées depuis le début des années 90). Ces tendances restent à confirmer et ne permettent pas encore d'atteindre, dans de nombreuses rivières, des concentrations d'azote inférieures au seuil à partir duquel se produisent des proliférations macroalgales en zone côtière, ou de permettre sans problème la production d'eaux destinées à la consommation humaine.
The study of nitrate concentrations in the main twenty-five rivers of Brittany shows that these concentrations are positively correlated with the river flow, in 90 % of cases. The hydrogeological characteristics of the watersheds have an influence on these relationships; in this way, river basins characterized by prevalent surficial water runoff exhibit higher correlation coefficient and lower time lag between river flow sequences and nitrate concentration series.
The pluriannual trend of riverine nitrate concentrations has shown a fourfold increase for thirty years; nevertheless eighty percent of rivers exhibit a recent decrease in nitrate concentrations, independently of river flow fluctuations. The highest annual rates of nitrate decrease correspond to river basins where the fast surficial runoff is prevalent, that is a sign of lower hydrogeological "inertia".
It is probably the visible consequence of a decreasing agricultural pressure (i.e. decrease of organic manure and mineral fertilizer uses since the nineties) in waterwheds that have fairly short reaction time (between 2 and 10 years). Nevertheless this nascent decreasing tendency will must be strengthened in order to reach weak enough nitrate concentrations to limit macroalgal eutrophication in the coastal zone and to allow everywhere the production of drinking water.
Keywords : Evolution pluriannuelle, Régression multiple, Débit fluvial, Eutrophisation, Bretagne, Nitrate

Contact : Jean-Francois.Guillaud@ifremer.fr 

 2008 - Thin layers of Pseudo-nitzschia spp and the fate of Dinophysis acuminata during an upwelling-downwelling cycle in a Galician Ría 

Auteurs : Velo-Suárez L., González-Gil S., Gentien P., Lunven M., Bechemin C., Fernand L., Raine R. and B. Reguera

Limnology and oceanography (in press)

 2008 - Using a nitrogen-tracking technique in a 3D model of the primary production to assess the fueling sources of Phaeocystis globosa blooms in the eastern English Channel and the southern North Sea 

Auteurs : Vanhoutte-Brunier A., Ménesguen A., Lefebvre A., Cugier P.

Soumis à Estuarine Coastal and Shelf Science

 2008 - Changes in phytoplankton and its’ environment between 1981 and 2005 in Killary Harbour, western Ireland 

Auteurs : Roden C.M., MacDermot G., Salas R., Farrell H., Reguera B., Gentien P., Donaghay P., Cusack C.

Journal of Marine System (in press)

 2008 - Rapid enzyme-linked immunosorbent assay for detection of the algal toxin domoic acid 

Auteurs : Litaker R.W., Stewart T. N., Eberhart L., Wekell J. C., Trainer V. L., Kudela R. M., Roberts A., Hertz C., Johnson T. A., Frankfurter G., Smith G. J., Schnetzer A., Schumacker J., Bastian J. L., Odell A.

Journal of Shellfish Research (in press)

 2008 - A model-based evaluation of Marine Protected Areas for fishery management in the case of strong environmental forcing - the example of Eastern Baltic cod ( Gadus morhua callarias) 

Auteurs : Kraus, G., Pelletier D., Dubreuil J., Moellmann C., Hinrichsen H.H., Bastardie F., Vermard Y., & Mahévas S.
ICES Journal of Marine Science (accepté)

 Juin 2008 - Do we protect biological originality in protected areas ? A new index and an application to the Bonifacio Strait Natural Reserve 

Auteurs : Mouillot D., Culioli J.M., Pelletier D., Tomasini J.A.

Biological Conservation, Volume 141, Issua 6, Jd'un08 -, Pages 1569-1580

 Juin 2008 - Oligotrophication and emergence of picocyanobacteria and a toxic dinoflagellate in Thau lagoon, Southern France 

Auteurs : Collos Y., Bec B., Jauzein C., Abadie E., Laugier T., Lautier J., Pastoureaud A., Souchu P., Vaquer A.

Journal of Sea Research, on line 17/06/08

 Juin 2008 - “Classical and novel organohalogen compounds (PCBs and PBDEs) in hake (M. merluccius, L.) from Mediterranean and Atlantic coasts (France)” 

Auteurs : Bodiguel X., Tronczynski J., Loizeau V., Munschy C., Guiot N., Le Guellec A.M., Olivier N., Roupsard F., Mellon C.

Accepté pour publication dans Environmental Toxicology conference book (Wessex Institute of technology Transactions on Ecology and the environment, ISSN:1743-3541).

 Mai 2008 - In situ microcosms, a tool assessment of pesticide impacts on oyster spat ( Crassostrea gigas) 

Ecotoxicology (Springer), 2008/05, Volume 17, Issua 4, Pages 235-245 (0.54 Mo)

 Mai 2008 - Layer-by-Layer DNA film synthesis via branched hybridization 

Auteurs : Lazerges M., Perrot H., Antoine E., Compère C.

ITBM-RBM, volume 29, issuas 2-3, pp. 133-135

  Avril 2008 -   Competition for food in the larvae of two marine molluscs, Crepidula fornicata and Crassostrea gigas  

Auteurs : Blanchard M., Pechenik J.A. , Giudicelli E., Connan J.P., Robert R.

 

Compétition trophique chez les larves de mollusques marins Crepidula fornicata et Crassostrea gigas. En 2003, des expérimentations ont été menées en laboratoire pour évaluer la compétition trophique entre les larves de crépidules ( Crepidula fornicata) et celles de l’huître japonaise ( Crassostrea gigas). Les taux de consommation, de croissance et de mortalité sont comparés, en alimentant les larves de chaque mollusque avec six espèces d’algues unicellulaires, dont la taille varie de 2 à 10 µm, deux flagellées Tetraselmis chui (Prasinophycée) et Isochrysis affinis galbana (T-ISO, Haptophycée), une chlorophycée Nannochloris atomus, et trois diatomées : Chaetoceros calcitrans forma pumilum, Chaetoceros gracilis et Skeletonema marinoï. Contrairement aux larves d’huîtres, les larves de crépidules consomment toutes les cellules phytoplanctoniques quelles que soient leurs tailles, et ceci, à un taux plus élevé que celui des larves d’huîtres. Ainsi, une larve d’huître consomme 2216 cellules h?1 de N. atomus, tandis que la larve de crépidule en consomme 5159 cellules h?1, soit environ deux fois plus. Ces expériences mettent également en évidence que les larves des deux espèces grandissent plus vite quand elles sont nourries avec le mélange des algues flagellées plutôt qu’avec chacune des espèces de ces mêmes algues (12 contre 7 µm j?1 pour l’huître et 41 contre 28 µm j?1 pour la crépidule). Ainsi, en baie du Mont Saint-Michel (Manche ouest) où les larves des deux espèces apparaissent à la même période estivale, nos résultats suggèrent que la filtration des larves de crépidules abaisse notablement la concentration de phytoplancton, et tout spécialement celle des petites cellules algales, entraînant alors une compétition trophique avec les larves d’huîtres.

The degree to which larvae of the invasive American slipper limpet ( Crepidula fornicata) and the Japanese oyster (C rassostrea gigas) may compete for food was examined during 2003 in the laboratory. Larval microalgae uptake, growth and mortality were compared for larvae fed each of six species of unicellular algae, ranging in length from 2 to 10 µm. Tested diets included the two flagellates Tetraselmis chui ( Prasinophyceae) and Isochrysis affinis galbana (T-ISO, Haptophyceae), one member of the Chlorophyceae ( Nannochloris atomus), and three diatom species ( Chaetoceros calcitrans forma pumilum, Chaetoceros gracilis, Skeletonema marinoï). We found that the limpet larvae ingested phytoplankton over a wider range of cell sizes and ate at higher rates on each diet than did the oyster larvae. For example, oyster larvae consumed 2216 cells h?1 of N. atomus, while limpet larvae consumed the same phytoplankton cells at approximately twice that rate, 5159 cells h?1, on the same diet. Larvae of both species grew more quickly on a mixture of flagellates than on any of the diatom alone (12 versus 7 µm d?1 for oyster larvae and 41 versus 28 µmd?1 for limpet larvae). Our results suggest that in the Bay of Mount Saint-Michel (France, Western Channel), where larvae of both species co-exist in the summer, intensive grazing by limpet larvae can potentially deplete phytoplankton concentrations to cause competition with oyster larvae, particularly for smaller sized phytoplankton species.

Mots clés : Phytoplankton, diet, feeding rate, growth, larvae, Crepidula fornicata, Crassostrea gigas 

Contact : Michel.Blanchard@ifremer.fr 

 Avril 2008 - Models and indicators for assessing conservation and fisheries-related effects of marine protected areas 

Auteurs : Pelletier D., Claudet J., Ferraris J., Benedetti-Cecchi L., García-Charton J.A.

 
Two kinds of approaches have been used for assessing conservation and fisheries-related effects of marine protected areas (MPAs): ( i) statistical modelling based on field data and ( ii) mathematical modelling quantifying the consequences of MPAs on the dynamics of populations, communities, and fisheries. Statistical models provide a diagnostic on the impact of MPAs on the ecosystem and resources; they are also needed for devising and assessing sampling designs for monitoring programs. Dynamic models enable exploration of the consequences of MPA designs and other management policies. We briefly review how each of these approaches has been implemented up to now in the literature and identify potential indicators of MPA effects that can be obtained from each approach to provide scientific advice for managers. Methodological gaps that impede the assessment of MPA effects and the construction of appropriate indicators are then discussed, and recent developments in this respect are presented. We finally propose ways to reconcile the two approaches based on their complementarity to derive suitable indicators to support decision making. In this respect, we suggest in addition that MPA managers should be associated from the beginning to the design and construction of indicators.
Deux sortes d’approches sont utilisées pour évaluer les effets des aires marines protégées (AMP) sur les pêcheries et la conservation biologique : ( i) la modélisation statistique de données de terrain et ( ii) la modélisation mathématique qui quantifie les conséquences des AMP sur la dynamique des populations, des peuplements et des pêcheries. Les modèles statistiques fournissent un diagnostic sur l’impact des AMP sur les écosystèmes et les ressources; ils sont également nécessaires pour mettre au point et évaluer des protocoles d’échantillonnage pour les programmes de suivi. Les modèles dynamiques permettent d’explorer les conséquences de différentes configurations de AMP et d’autres mesures de gestion. Nous faisons une brève revue bibliographique des applications de chacune de ces approches et identifions des indicateurs potentiels des effets des AMP, tels qu’ils peuvent être obtenus par chaque approche, dans le but de fournir un avis scientifique aux gestionnaires. Nous discutons ensuite des faiblesses méthodologiques qui nuisent à l’évaluation des effets des AMP et à la construction d’indicateurs appropriés et nous présentons des développements récents en la matière. Pour finir, nous proposons des pistes pour réconcilier les deux approches sur la base de leur complémentarité, afin d’en déduire des indicateurs adéquats pour aider à la prise de décisions. Dans cette optique, nous suggérons de plus que les gestionnaires de AMP soient associés dès le départ à la mise au point et à la construction des indicateurs.

Keywords : Marine Protected Areas, indicators, modelling, fisheries management, ecosystem conservation

 Avril 2008 - Sedimentary processes in a shellfish farming environment, Mont Saint Michel Bay, France. In Sediment and Ecohydraulics: INTERCOH 2005 

Auteurs : Cayocca F., Bassoullet P., Le Hir P., Jestin H. and Cann P.

Elsevier, Chapitre 29, 431-446 available on line 15 April 2008

 Avril 2008 - Competition for phosphorus between a toxic Alexandrium minutum and a non toxic Heterocapsa triquetra  

Auteurs : Labry C., Erard-Le Denn E., Chapelle A., Fauchot J., Youenou A., Crassous M.P., Le Grand J., Lorgeoux B.

Journal of Experimental Marine Biology and Ecology 358: 124-135 (0.23 Mo)

The understanding of the dominance of one species with respect to others is a pertinent challenge in HAB growth dynamics studies and the nutrient supply mode is one of the factors potentially involved. The competition for phosphorus (P) between a toxic species, Alexandrium minutum, and a non-toxic species, Heterocapsa triquetra, was studied (1) along a gradient of P depletion, (2) testing different P depletion degrees before a single PO4 supply and (3) experimenting different PO4 supply frequencies. In conditions Of PO4 depletion, H. triquetra stopped growing after two days both in monospecific and mixed batch cultures whereas A. minutum grew progressively from day 2 until the end of the experiment. This time-lag growth of A. minutum is associated to its ability to store P intracellularly and then mobilize it for cell division when P depletion becomes severe. Heterocapsa triquetra outcompeted A. minutum when it was submitted to less than three days of P depletion before the pulse. In contrast, A. minutum outcompeted H. triquetra after more than three days of depletion. This transition was related to the capacity for A. minutum to increase its cell PO4 uptake rate in a higher proportion to face potential PO4 supply. As a result of this physiological acclimatation to P starvation, A. minutum consumed the whole PO4 pulse supplied after 3 to 10 days of P depletion. This resulted in a reduction of H. triquetra growth. These two acclimatations were confirmed in a P limited semi-continuous culture experiment testing several PO4 supply frequencies (1, 2, 4, 6 day intervals). These experiments revealed that A. minutum is a "storage specialist" species for P, which uptakes PO4 pulses for luxury consumption, survives depletion periods and, then, utilizes P for cell growth. In contrast, H. triquetra is more a "velocity adapted" species, which utilizes PO4 just after supply to increase their cell division rate. (C) 2008 Elsevier B.V. All rights reserved.
Keywords : Pulse, Ecophysiology, Batch-culture, Semi-continuous-culture, Alexandrium minutum, Heterocapsa-triquetra, Dinoflagellates, Phosphorus-storage
Contact : Claire.Labry@ifremer.fr 

 Mars 2008 - Surface plasmon resonance in chalcogenide glass-based optical system 

Auteurs : Le Person J., Colas F., Compère C., Lehaitre M., Anne M.-L., Boussard-Pléde C.l, Bureau B., Adam J.-L., Deputier S., Guilloux-Viry M.

Sensors & Actuators: B. Chemical, 130(2), 771-776 (0.15 Mo)

The surface plasmon resonance phenomenon has been studied in a chalcogenide glass-based optical system. IR transmission properties of these materials combined to their high refractive indices lead to advantageous properties for sensing. In this study, numerical simulations have been carried out to investigate the potentialities of sulfide glass from the GeGaSbS system as a coupling prism material. Then, an angular modulation SPR biosensor has been set up in the Kretschmann-Raether arrangement. Experimental data are consistent with numerical calculation and the detection limit of the sensor is 3 x 10(-5) RIU. These preliminary results are promising. Further investigations have to be carried out to confirm the great potentialities of those materials for SPR-based biosensor. (c) 2007 Elsevier B.V. All rights reserved.
Keywords : Biosensor, Infrared, Chalcogenide-glass, Surface-plasmon-resonance

 Mars 2008 - Phosphorus dynamics and bioavailability in sediments of the Penzé Estuary (NW France): in relation to annual P-fluxes and occurrences of Alexandrium Minutum  

Auteurs : F. Andrieux-Loyer, X. Philippon, G. Bally, R. Kerouel, A. Youenou, J. Le Grand

 

Abstract : The macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) has been subjected to recurrent annual toxic blooms of Alexandrium minutum since 1988. This study aims to specify the phosphorus dynamics and bioavailability in sediments in order to improve our understanding of Alexandrium occurrences. Sediment-P pools and diffusive phosphate fluxes were studied under similar hydrodynamic conditions, in the intermediate estuary in May, Jd'unand July 2003 and along the salinity gradient from August 2004 to Jd'un08 5. The results highlight a decrease in bioavailable phosphorus (iron and organic bound) from the inner part of the estuary seaward. The ratio of iron-bound phosphorus to ironoxyhydroxides is lower in the inner and intermediate estuaries (5–8) than in the outer site (15), suggesting a saturation of sorption sites and greater phosphorus bioavailability in this area. Pools of bioavailable phosphorus in surficial sediments are about eight times higher than the annual net-export of P (7 ton year-1). Phosphate releases from sediments are always lower than 5 lmol m-2 d-1 in March. The highest supplies occur in Jd'unand August in the intermediate area (up to 400 lmol m-2 d-1) where they represent up to 50% of river loadings. These results further suggest that phosphate pulses coincide with occurrences of Alexandrium reported in Jd'u.

Keywords : Alexandrium minutum, Estuary, Phosphate fluxes, Pore water, Sediment phosphorus, Sequential extraction

Contact : Francoise.Andrieux@ifremer.fr 

 Mars 2008 -  A deterministic population dynamics model to study the distribution of a benthic bivalve with planktonic larvae (Paphia rhomboides) in the English Channel (NW Europe) 

Auteurs : Marie Savina (Doctorante Ifremer), Alain Ménesguen (Ifremer)

Journal of Marine Systems (Elsevier), 2008/03, Volume 70, Issua 1-2, Pages 63-76  (0,27 Mo)

This paper describes the development of a modelling framework to study the distribution in the English Channel of a benthic bivalve with planktonic larvae, Paphia rhomboides. Age-structured population dynamics and growth models are linked together to a compartmental transport and NPZ (Nutrients-Phytoplankton-Zooplankton) model. Used in a spin-up mode, i.e. with identical environmental forcing from a year to the next, this deterministic model converges to the same steady state whatever the initial conditions, after 15-30 years of simulation. Important features of the P. rhomboides distribution are reproduced, particularly its high abundance in the GNB (Golfe Normand-Breton) and its almost complete absence in the Western Channel. The results are discussed in relation with the modelling strategies, especially with regard to the advantages and drawbacks which could result from changing the box-model approach into a fine 3D hydrodynamic approach. (C) 2007 Elsevier B.V. All rights reserved.

Keywords : Meroplanktonic-invertebrates, Spatial-distribution, English-Channel, Paphia-rhomboides, Population-dynamics-model, Growth-model

Contact : Alain.Menesguen@ifremer.fr 

 Février 2008 -  PCDD/F and PCB multi-media ambient concentrations, congener patterns and occurrence in a Mediterranean coastal lagoon (Etang de Thau, France) 

Auteurs : Castro-Jimenez J., Deviller G., Mariani G., Skejo H., Umlauf G., Guiot N., Héas-Moisan K., Léauté F., Munschy C., Tixier C., Tronczynski J.

Environmental Pollution, In Press, Corrected Proof, Available online 20 February 2008

 Février 2008 -  Remaniements sédimentaires superficiels sur l'estran, occidental de la baie du Mont-Saint-Michel 

Auteurs : Bonnot C., Bassoullet P., Le Hir P., Tessier B., Cayocca F., Baltzer A.

Revue Européenne de Génie Civil Génie Côtier, Hermès-Lavoisier Ed., Vol. 12, n° 1-2, pp. 51-65.

 Février 2008 -  Comet assay in phytoplankton as biomarker of genotoxic effects of environmental pollution 

Auteurs : F. Akcha, G. Arzul, S. Rousseau, M. Bardouil

Marine Environmental Research (2008). Sous presse doi: 10.1016/j.marenvres.2008.02.022

The alkaline comet assay was tested on different microalgae: the dinoflagellates, Karenia mikimotoi and Alexandrium minutum, and the diatom, Chaetoceros gracilis. The microalgae were exposed during their exponential growth to the model direct genotoxicant, hydrogen peroxide (1h, 5 and 100 µM H2O2). Following H2O2 exposure, the comet assay was validated only for K. mikimotoi for which genotoxicity was observed from the lowest tested concentration of 5 µM with a concentration-dependent effect. C. gracilis was too small in size (4 µm) to be correctly analysed. For A. minutum, our lysis buffer was not strong enough to digest the cellulosic thecal plates. For K. mikimotoi, the comet assay was thus applied for the study of the genotoxic effects of different pesticides: epoxiconazole (as Opus??formulation), chlorpyriphos-ethyl (as Dursban??formulation) and endosulfan at 1, 10 and 100 µg of active substance /L for 24h. Exposure to epoxiconazole in formulation resulted in an increase in the extent of DNA strand breaks at the highest tested concentration of 100 µg/L. Genotoxicity was also observed for chlorpyriphos-ethyl in formulation from 1 µg/L (p<0.05), with a significant increase at 10 µg/L. Endosulfan exposure resulted in DNA damage for K. mikimotoi nuclei. Genotoxicity was observed from 1 µg/L of endosulfan and was not concentration dependent.

 Février 2008 -  Modelling the Karenia mikimotoi bloom that occurred in the western English Channel during summer 2003  

Auteurs : Alice Vanhoutte Brunier (Ifremer) , L. Fernand (CEFAS, UK) , Alain Ménesguen (Ifremer) , S. Lyons (National University of Ireland) , Francis Gohin (Ifremer) , Philippe Cugier (Ifremer)

Ecological Modelling (Elsevier), 2008/02, Volume 210, Issua 4, Pages 351-376 (4,45 Mo)

Observations from space and in situ from the R.V Corystes 8/03 Cruise show that a massive Karenia mikimotoi bloom occured during summer 2003 in the western English Channel. Due to exceptional climatoligical conditions that occured in Jd'u 2003, the installation of a very strong thermocline enhanced the development of a massive bloom over 1 million cells 1(-1) in the Central English Channel. This paper presents the application of a mathematical model of this species, previously developed in for the Bay of Biscay, into a general 3D model of the primary production of the English Channel and southern Bight of the North Sea. Allelopathic interactions exerted by K. mikimotoi on other phytoplankton species and the role of agitation in the mortality of this species are taken into account. The model includes the dynamics of the bloom and consequently reproduces with good agreement the geographical distribution of the K. mikimotoi bloom both surface and subsurface. The model suggests that the apparent transport of the bloom towards the French coasts as inferred from the satellite observation was not due to advection but was only caused by the establishment of suitable conditions. The sensivity of the K. mikimotoi distribution to boundary conditions, initialization and the role of turbulence is discussed. (c) 2007 Published by Elsevier B.V.

 

Keywords : Coupled-physical-biogeochemical, 3D-modelling, Karenia-mikimotoi, Primary-production, SeaWiFS, Western-English-Channel

Contact : Alain.Menesguen@ifremer.fr 

 Janvier 2008 - Competition for phosphorus between two dinoflagellates: A toxic Alexandrium minutum and a non-toxic Heterocapsa triquetra  

Auteurs : C. Labry, E. Erard-Le Denn, A. Chapelle, J. Fauchot, A. Youenou, M.P. Crassous, J. Le Grand, B. Lorgeoux
Ifremer Brest, Dyneco/Pelagos, BP 70, 29280 Plouzané, France

 

Abstract : The understanding of the dominance of one species with respect to others is a pertinent challenge in HAB growth dynamics studies and the nutrient supply mode is one of the factors potentially involved. The competition for phosphorus (P) between a toxic species, Alexandrium minutum, and a non-toxic species, Heterocapsa triquetra, was studied (1) along a gradient of P depletion, (2) testing different P depletion degrees before a single PO4 supply and (3) experimenting different PO4 supply frequencies. In conditions of PO4 depletion, H. triquetra stopped growing after two days both in monospecific and mixed batch cultures whereas A. minutum grew progressively from day 2 until the end of the experiment.

This time-lag growth of A. minutum is associated to its ability to store P intracellularly and then mobilize it for cell division when P depletion becomes severe. Heterocapsa triquetra outcompeted A. minutum when it was submitted to less than three days of P depletion before the pulse. In contrast, A. minutum outcompeted H. triquetra after more than three days of depletion. This transition was related to the capacity for A. minutum to increase its cell PO4 uptake rate in a higher proportion to face potential PO4 supply. As a result of this physiological acclimatation to P starvation, A. minutum consumed the whole PO4 pulse supplied after 3 to 10 days of P depletion. This resulted in a reduction of H. triquetra growth. These two acclimatations were confirmed in a P limited semi-continuous culture experiment testing several PO4 supply frequencies

(1, 2, 4, 6 day intervals). These experiments revealed that A. minutum is a “storage specialist” species for P, which uptakes PO4 pulses for luxury consumption, survives depletion periods and, then, utilizes P for cell growth. In contrast, H. triquetra is more a “velocity adapted” species, which utilizes PO4 just after supply to increase their cell division rate.

Keywords : Alexandrium minutum, Heterocapsa triquetra, Dinoflagellates, Phosphorus storage, Pulse, Ecophysiology, Batch culture, Semi-continuous culture.

Contact : Claire.Labry@ifremer.fr 

 Janvier 2008 -  Estimation de la matière en suspension à partir de l’intensité rétrodiffusée des courantomètres acoustiques à effet Doppler (ADCP) 

Auteurs : Caroline Tessier, Pierre Le Hir, Xavier Lurton, Patrice Castaing

Comptes Rendus Geosciences, Volume 340, Issua 1, January 2008, Pages 57-67

À partir du signal rétrodiffusé des ADCP, une information « turbidité » est obtenue en calculant un indice de rétrodiffusion volumique, dont la dynamique est corrélée au forçage par la marée et la houle. Cet indice est calculé en corrigeant le signal reçu des pertes de transmission, et en prenant en compte les caractéristiques acoustiques propres à notre appareil, déterminées par calibration en bassin. Pour quantifier les matières en suspension (MES), une calibration empirique de cet indice a été faite à partir de mesures indépendantes, obtenues avec un turbidimètre optique. Les résultats montrent qu’une calibration à un seul niveau suffit pour obtenir des profils verticaux de concentration massique par acoustique. De plus, la période de mesure de calibration peut être réduite à deux jours, si la dynamique des MES est importante (vives-eaux et houles).

 

Keywords : ADCP, rétrodiffusion, turbidité, concentration massique, marée, houles, zone côtière

Contact : Pierre.Le.Hir@ifremer.fr 

 Janvier 2008 - Interactions between a natural food web, shellfish farming and exotic species: The case of the Bay of Mont Saint Michel (France) 

Auteurs : F. Arbach Leloup, N. Desroy, P. Le Mao D. Pauly, O. Le Pape

 

To ensure sustainable uses of the coastal zone, an integrated ecosystemic approach and ecosystem models are required to frame ecological processes and evaluate environmental impacts. Here, a mass-balance trophic (Ecopath) model of the Mont Saint Michel Bay (MSMB) was developed, to analyze the bay’s functioning as an ecosystem. This bay, intensively exploited by fishing and for shellfish farming, is also suffering from the proliferation of the gastropod Crepidula fornicata, an exotic species. The MSMB model has 18 compartments, from the primary producers to top predators, and emphasizes the large biomass of filter feeders. The model identified the MSMB as a highly productive ecosystem controlled largely from the bottom-up, and strongly impacted by huge biomasses of filter feeders. However, the low transfer efficiency rates imply that a large part of the primary production is not transferred upward to higher trophic levels, but is lost in high hydrodynamic exchanges and in the trophic impasse represented by a large biomass of Crepidula fornicata.

Contact : Nicolas.Desroy@ifremer.fr