Thau Lagoon Research Articles

Modelling nitrogen, primary production and oxygen in a Mediterranean lagoon. Impact of oysters farming and inputs from the watershed

An ecosystem model based on nitrogen cycling and oxygen has been developed for the Thau lagoon. It takes into account the specific features of this Mediterranean lagoon, a semi-confined system with watershed inputs and oyster farming. The ecosystem model uses currents calculated by a two-dimensional hydrodynamic model and integrated into a box model. This model is compared with a year survey data and used to estimate nitrogen and oxygen fluxes between the different ecosystem compartments. The yearly simulation shows that the ecosystem behavior is driven by meteorological forcing, especially rain which causes watershed inputs. These inputs trigger microphytoplankton growth, which is responsible for new primary production. During dry periods, nitrogen is recycled into the lagoon thanks to oysters excretion, sediment release, microzooplankton excretion and mineralization. Ammonium produced in this way is consumed by a population of pico- and nanophytoplankton causing regenerated primary production. Consequently, the ecosystem remains highly productive in summer even without external inputs. Shellfish farming also plays an important role in the whole lagoon through biodeposition. Driven by biodeposition, sediment release is the major source of nitrogen in the water column and causes oxygen reduction. The oysters contribute to the recycling activity by excretion, which supports the regenerated primary production. They are also involved in oxygen consumption by respiration which can cause local hypoxia. Further improvements are proposed before this model may become a functional environmental model for a lagoon ecosystem.

Feeding rate of the oyster Crassostrea gigas in a natural planktonic community of the Mediterranean Thau Lagoon

The Mediterranean Thau Lagoon is an important European oyster farming area. Oyster growth levels are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant in the Thau Lagoon. Moreover, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in water columns of the Thau Lagoon and to understand the particular structure of the Thau microbial food web, which may explain such a paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter with a maximum Summer abundance. On 17 August 1998, the picophytoplankton and nanophytoplankton abundances were not as high as expected and we observed the development of large diatoms. At this time, available carbon resources arose from microphytoplankton (84.5 %) and picoplanktonic cells represented only 1.27 % in terms of carbon. The heterotrophic cells were few in abundance and constitued only 5 μm flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only < 5 μm flagellates and picoeukaryotic cells, Ostreococcus tauri, were not very well retained (45 % and 2 %). The high clearance rates of Crassostrea gigas found in this experiment can be explained by a low suspended particulate matter (0.65 mg l-1). Oysters adapted their retention mechanism when they lived in oligotrophic waters. These results indicate that, under the given experimental conditions, picophytoplankton did not represent a valuable trophic resource for farmed oysters because (1) Crassostrea can not retain picoparticles and (2) the picoplankton represented a poor available carbon resource to be transferred via a weak heterotrophic protist community. In the oyster pends of the Thau Lagoon during this study, which followed a rainfall event, microphytoplanktonic primary producers, in particular diatoms, could be considered as the main food sources for bivalve suspension feeders.

Procaryotic infections in the mussel Mytilus galloprovinciallis and in its parasite the turbellarian Urastoma cyprinae

Mussels Mytilus galloprovincialis from the Thau lagoon (Mediterranean coast of France) were regularly sam pIed to determine the prevalence and intensity of parasitic infections. Microscopi­ caUy, hepatopancreatic tubules of the mussel appeared infected by a rickettsia-like organism (RLO). Each RLO were surrounded by 2 unit membranes, and colonies composed of several bacteria were enclosed within a vacuolar membrane of the host cell. ln addition, examination by transmission elec­ tron microscopy revealed that the RLO was infected by phage particles . Histological investigations of the turbeUarian Urastoma cyprinae parasitizing the mussels have shown that this ectoparasite was also infected by 2 types of procaryotes, a chlamydia-like organism (CLO) and a mollicute-like organism (MLO). The CLO displayed characteristic developmental stages of the Chlamydia les and was secon­ darily infected by ele ctron-dense particles presumed to be phage particles . The MLO exhibited sorne morphological characteristics of the mollicutes, in that the microorganisms were bounded by a single membrane sharing a trilaminar structure. Neither of these microorganisms have previously been reported in Platyhelminthes.

Influence d'un élevage ostréicole sur les flux de nutriments et d'oxygène dans un écosystème lagunaire

The impact of suspended oyster culture ( Crassostrea gigas, Thunberg) on oxygen and nutrient fluxes has been studied in situ, in a coastal lagoon (Thau, France), during a seasonal cycle. On the first plan of the multiple factorial correspondences analysis (MCA), seasons were well discriminated. The fluxes were maximum in summer and minimum in winter. However, this seasonal pattern was not only linked to the water temperature, as autumn and spring (similar temperatures of about 12 °C) were distinct in the second factorial plan (2.3). Oxygen uptake by the oyster cultures varied between 0 μmol m −2 h −1 (January) and 11 823 ±377 μmol m −2 h −1 (July). Ammonia and nitrate-nitrites were released into the water column respectively at a rate of 2905 ± 327 μmol m −2h −1 and 891 ± 88 μmol m −2 h −1 in the summer and 0 μmol m −2 h −1 and 177 ± 97 μmol m −2 h −1 in the cold season. During the summer, the nitrate-nitrites flux was about 20 % of the total dissolved inorganic nitrogen production. Phosphate release was low except for two periods during which an important release was measured; in May (1686 ± 44 μmol m −2 h −1) and in November (2691 ± 800 μmol m −2 h −1). No linear relation between water temperature and phosphate flux was found. In Thau Lagoon, oyster cultures (oysters and epibiota) by producing 2 × 10 7 mol-N y −1 play a central role in nitrogen renewal in the water column.

Oxygen fluxes involving the benthic micro- and macrophytic components in the Thau Lagoon under pre-anoxic conditions

Measurements of oxygen fluxes at the water-sediment interface (transparent and opaque enclosures) were made on bare sediments inhabited by microphytes on the one hand, and on sediments diversely colonized by macrophytes (macroalgae and seagrasses) on the other hand. Five stations, typical of different biotopes of a Mediterranean shallow lagoon were studied from May to July 1993 in order to observe an anoxic event which usually occurs at that time of the year. Average diurnal respiration of benthic communities ranged from 40 mg (or 1.25 mmol) O 2 m 2 h −1 in bare sediments (31 % of the lagoon area) to 76 mg (or 2.37 mmol) O 2 m −2 h −1 in sediments with a medium coverage of macroalgae (37 % of the total area) and, finally, to 100 mg (or 3.12 mmol) O 2 m −2h −1 in the denser macrophytic area (32 % of the total area). The highest diurnal gross production was observed in the zone colonized by macroalgae and seagrasses, and especially in corridors between shellfish-cultivation tables (300 mg O 2 m −2 h −1, or 9.37 mmol O 2m −2 h −1, equivalent to 113 mg C m −2 h −1). Overall, during this period, net bottom oxygen production was close to nil in ca. 30 %, and positive in 70 % of the lagoon area. The average net oxygen production for the whole lagoon in summer is thus in the order of 100 mg O 2 m −2h −1. In 1993, at the end of July, an anoxic event was avoided due to a period of strong wind.

PHYLOGENETIC ANALYSIS AND GENOME SIZE OF OSTREOCOCCUS TAURI (CHLOROPHYTA, PRASINOPHYCEAE)

Ostreococcus tauri Courties et Chrétiennot‐Dinet is the smallest described autotrophic eukaryote dominating the phytoplanktonic assemblage of the marine Mediterranean Thau lagoon (France). Its taxonomic position was partly elucidated from ultrastructure and high‐pressure liquid chromatography (HLPC) pigment analysis. The sequence analysis of the 18S rDNA gene of O. tauri measured here is available in EMBL Nucleotide Sequence Database (accession number: Y15814) and allowed to clarify its phylogenetic position. O. tauri belongs to the Prasinophyceae and appears very close to Mantoniella, a typical scaly Prasinophyceae, morphologically very different from the naked and coccoid Ostreococcus. An electrophoretic analysis of O. tauri shows that the nucleus contains 10.20 mbp. This small genome, fragmented into 14 chromosomes ranging in size from 300 to 1500 kbp, confirms the minimalist characteristics of Ostreococcus tauri.

Geochemical and Isotopic Investigation of Thermal Fluids — Thau Lagoon (Hérault, South of France)

Geochemical and Isotopic Investigation of Thermal Fluids — Thau Lagoon (Hérault, South of France)

Biogeochemical aspects of bottom anoxia in a Mediterranean lagoon (Thau, France)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 164:135-146 (1998) - doi:10.3354/meps164135 Biogeochemical aspects of bottom anoxia in a Mediterranean lagoon (Thau, France) Philippe Souchu1,*, Anne Gasc1,**, Yves Collos2, André Vaquer2, Henri Tournier1, Bertrand Bibent2, Jean-Marc Deslous-Paoli1 1Laboratoire Ecologie, Station IFREMER, 1 rue Jean Vilar, F-34200 Sète, France 2Laboratoire d'Hydrobiologie (UMR 5556 CNRS), Université de Montpellier CC093, F-34095 Montpellier Cedex 5, France *E-mail: psouchu@ifremer.fr **Present address: Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098, USA Physical and chemical characteristics (temperature, salinity, dissolved oxygen), nutrients [dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and silicate], dissolved organic nitrogen (DON) and phosphorus and particulate matter [particulate organic carbon (POC) and nitrogen (PON) and chlorophyll a] were measured at a station located in the deepest part of the Thau lagoon (8.5 m), France, during a 10 d period of bottom anoxia in summer 1994. The upper 8 cm of sediment were also analyzed for ammonium (NH4+) and SRP concentrations in the porewater. The study period was characterized by mean wind speed under 4 m s-1 which induced an increase of surface temperature from 18 to 29°C with the formation of a thermocline. The correlation (p < 10-4) between the wind speed averaged over the previous day and the difference between bottom and surface temperatures showed that the wind constituted the main vector of vertical mixing. The lack of wind led to dissolved oxygen depletion in the bottom 2 m and to a strong porosity increase in the upper 10 cm of sediment (80 to >95%). Anoxic conditions increased NH4+ and SRP concentrations in porewater from 231 ± 89 to 1305 ± 305 (± SD) µM and from 6.6 ± 0.8 to 108 ± 43 µM respectively. The potential increase of NH4+ concentrations in porewater estimated from the mineralization of the microphytobenthos explained 30% of the measured increase in the upper 8 cm of porewater. The study period was characterized by an increase in NH4+, SRP and dissolved Si concentrations in the bottom water column (maxima respectively 24.2, 4.9 and 57 µM). Linear regressions between nutrient concentrations in the water column and temperature revealed a strong enhancement of bottom fluxes during anoxia. Nitrate + nitrite (NO3- + NO2-) were absent in anoxic waters and remained below 0.5 µM in oxic waters. Monthly concentrations of SRP in the water column of the Thau lagoon from 1970 to 1994 revealed anoxia events by summer peak values corresponding to strong bottom anoxia. Nevertheless, the general trend was a decrease due to the gradual control of eutrophication which should lead to the disappearance of bottom anoxia within the next decade. A fraction of the nutrients of benthic origin was transformed into planktonic particulate matter via primary production which increased the concentrations of chl a from about 1 to 15 µg l-1. Concentrations of DON and PON in the water column were significantly correlated, suggesting that DON compounds were released by the pelagic food web but not directly by the sediment. The increase of total nitrogen (DIN + DON + PON), considering the water column as a closed box during 10 d of winds under 5 m s-1, was estimated at 1.7 µmol N l-1 d-1 and would correspond to a benthic flux of 600 µmol N m-2 h-1 for 10 d. Anoxia · Nitrogen · Phosphorus · Eutrophication · Thau lagoon · Mediterranean Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 164. Publication date: April 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Water Movements in a Lagoon and a Harbour as Constrained by Lead Isotopes in Mussels

Mussels, through their ability to concentrate metals from seawater (103 to 104 ) are generally used as bioindicators in many programmes of coastal survey (NOAA, French Mussel Watch). They have many advantages: they are widely distributed, easy to analyse and allowing weekly-integrated records. The previous studies generally used concentration variations in the mussels soft parts to determine anthropogenic impact, but concentrations alone do not allow us to constrain the metal origin(s). We use lead isotopes as tracers of human activities both on fleshes and shells (Labonne et al.). The comparison of the data with the various sources of lead already identified and geochemically characterized in the catchment allow us not only to determine the origins and proportions of natural and anthropogenic leads, but also trace water movements. The Thau lagoon is located in the South of France near Montpellier. This lagoon was chosen because of the various sources of both natural and anthropogenic inputs. The surrounding rocks are principally Jurassic limestones and Miocene marls (Petelet et al.). Along the lagoon coast we find the S~te harbour which presents a high density of industries: fertilisers and cement factories, boat workshops. An important highway crosses the watershed and several outputs of water treatment plants reach the lagoon (Monna et al.). Young mussels (Mytilus galloprovincialis) from the sea were introduced at several locations of the Thau lagoon: in the Sete harbour, in the lagoon centre, at the exutory of the main river feeding the lagoon and near a local city. Sampling was done in January 96, May 96 and July 96. The water movements inside the Harbour were also studied by the sampling of wild mussels directly in the channels (July 1996). Four locations were chosen: one close to a boat workshop in the main channel and three on a smaller channel near industrial piers.

Effect of Environmental Parameters on the Growth of Ulva rigida (Thau Lagoon, France)

Effect of Environmental Parameters on the Growth of Ulva rigida (Thau Lagoon, France)

Seasonal changes of nutrients in water and sediment in a Mediterranean lagoon with shellfish farming activity (Thau Lagoon, France)

The French Mediterranean lagoon of Thau is characterized by an important eutrophication dominated by shellfish farming (ca. 15 times the terrestrial inputs). On the basis of increasing eutrophication, three areas were identified and monitored for one year (overlying and sediment pore water nutrients, macrophytic biomass and water column chlorophyll a). Though some parameters show similar changes in the three areas (salinity, temperature), others are elevated in eutrophicated sites, e.g. organic content and siltation of the sediments, dissolved inorganic nitrogen (136.1 μmol l−1 overlying water, and 1185 μmol l−1pore water), dissolved reactive phosphorus (DRP) (20.9 μmol l−1overlying water), the summer oxygen depletion (1.1 mg l−1), the peak of macrophytic biomass (8 kg w · wt m−2) and phytoplanktonic bloom (14 μg l−1). Differences in DRP levels arise from sediment release during the summer anoxia; DRP appeared to play a key role as a limiting factor, and regulates competition between macrophytes and phytoplankton in spring. The macrophytes (seagrass Zostera and seaweeds Gracilaria and Ulva) may sustain the environment they are living in, acting as additional eutrophication sources.

Biogeochemical Composition of Mediterranean Waters Outside Thau Lagoon

Physicochemical characteristics (temperature, salinity, dissolved oxygen), nutrients (NO 3 −, NO 2 −, NH 4 +, soluble reactive phosphorus, Si), dissolved organic matter with bacterial cell counts (DON, DOP, DOC, BACT) and particulate matter (POC, PON, Chl a) were measured weekly from January 1993 to March 1994 in Mediterranean surface waters, sampled 2·5 km offshore in Thau Lagoon (Sète, France). Waters outside the lagoon displayed salinity reductions below 29 which corresponded to flood periods of the Rhône River and/or to winds oriented in a southeasterly direction. Levels of nutrients were increased by the influence of the Rhône river plume. Concentrations of NO 3 −and NH 4 +were rather close to the theoretical dilution line, while SRP and Si seemed to be rapidly taken up along their dilution gradient, especially in spring. The influx of NO 3 −enriched waters into the lagoon in October showed that the Rhône river plume can potentially fertilize this lagoon. The flux of nutrient from the Rhône River led to Chl amaxima above 4 μ g l −1in spring. The concentration of DOC and BACT reached peak values at the decay of the spring bloom and decreased to their lowest level afterwards, suggesting that primary production was an important source of DOC but also that DOC was repackaged in the microbial loop rather than being transported to deeper layers. Yearly averaged atomic C:N and C:P ratios in DOM were 15 and 1000, respectively, which probably compelled bacterioplankton to compete with phytoplankton for NH 4 +and SRP. No influence of the Rhône River was observed during summer. This period was characterized by SRP, NO 3 −and NO 2 −concentrations below the limits of detection and by the lowest levels in DOC, DON, BACT and Chl a. However, NH 4 +and DOP concentrations remained, respectively, above 0·3 and 0·15 μ M, and the summer period corresponded to oxygen supersaturation (up to 122\\%). These observations suggest the dominance of picophytoplanktonic communities with low biomass and high productivity associated with a heterotrophic component providing NH 4 +directly to phytoplankton, and phosphorus dominated by organic forms.

Impact of shellfish farming eutrophication on benthic macrophyte communities in the Thau lagoon, France

In a large marine lagoon (Thau lagoon, southern France) with a shellfish farming dominant eutrophication, the macrophyte communities were sampled by six transects of three depths (1.5, 2.5 and 5 m) and their characteristics (species composition, diversity and biomass) were described in relation to environmental and sediment parameters. With increasing eutrophication (total inorganic nitrogen, 0.140-0.295 mg l-1; dissolved reactive phosphorus, 0.045-0.110 mg l-1; and N/P atomic ratio, 3-22), silt fraction and shell fragments in sediments increased (12-93 and 0-65% dry wt respectively). Different types of macrophytic communities could be defined in the shallow zone (1.5-2.5 m) corresponding to four main and successive stages of degradation. A pure eelgrass stand (Zostera marina and Z. noltii) and an eelgrass community colonized by macroalgae were observed in SW sites and could be distinguished by their sedimentary features. In sites (NE) more affected by eutrophication (fine-textured sediment), available incident light determined two main seaweed communities: an Ulva rigida community, outside the shellfish tables, and a Gracilaria bursa-pastoris community in the shellfish tables (lower incident light).

Influence of shellfish farming activities on nitrification, nitrate reduction to ammonium and denitrification at the water-sediment interface of the Thau lagoon, France

The seasonal patterns of nitrification, denitrification and dissimilatory ammonium production (DAP) rates were studied in the sediment of 2 stations in the Thau lagoon (south of France). The station ZA was located within the shellfish farming zone and the station B was the reference site. A marked effect of shellfish fanning on bacterial activities was observed. Spatial differences were associated with discrepancies in the organic content and the reduction state of sediments, i.e. highest reductive processes (denitrification and DAP) were noted in shellfish farming area, whereas the oxidative process (nitrification) was predominant outside the farming zone. At both stations, the DAP activity increased in September (autumn) concomitant with an increase of the C/N ratio in the sediment due to the sedimentation of the summer phytoplanktonic production. Nitrification and denitrification rates exhibited maxima in November (winter) corresponding to dissolved inorganic nitrogen inputs from the surrounding land. In the shellfish farming site, 98 % of nitrate was reduced to NH4+ and 2 % to N2O showing that the most of the NO3- was reduced to ammonium and remained available for the ecosystem.

Modélisation de l'impact des mollusques cultivés sur la biomasse phytoplanctonique de l'étang de Thau (France)

A model coupling a 2D transport equation and a simple model of the nitrogen cycling was developed for the Thau lagoon (France). The model took into account the wind forcing on the hydrodynamism, the effect of the temperature on the biological processes, the input of the watershed area, the light dependence of the pelagic primary production and the prédation of the phytoplankton by the cultivated filter feeders. The model ran under two steady-state situations of wind with constant temperature and light values corresponding to spring weather conditions. Simulations showed that the biological flows were very sensitive to the wind. The budget of the nitrogen flows were computed for the three areas devoted to the filter feeder cultivation and the results highlighted the impact of cultivated oysters as a source of nutrients and an important nitrogen flow through the predation of phytoplankton.

Standing stock and dynamics of picophytoplankton in the Thau Lagoon (northwest Mediterranean coast)

Eucaryotic picophytoplankton (PEUC), picocyanobacteria, and larger phytoplanktonic cells from the Thau Lagoon (northwest Mediterranean coast) were numbered by flow cytometry from November 1991 to February 1994. PEUC cells dominated the phytoplanktonic assemblage and exhibited seasonal dynamics. Monthly mean abundances of larger phytoplanktonic cell and PEUC were significantly correlated with temperature and solar irradiance values, with the highest correlations having been obtained for PEUC abundances. The mean abundance of total picophytoplankton (3.5 × 104 cells ml‒1) was among the highest recorded in marine waters. The dominance of picophytoplankton over larger phytoplanktonic cells (average abundance, 5 × 103 cells ml‒1) in the nutrient‐rich waters of the lagoon was related to grazing from large‐scale shellfish breeding (oyster’s ≈35,000 t), which seems to act preferentially on the largest cells. Several hypotheses, including the potential selective effect of copper, were proposed to explain the dominance of the PEUC form (average abundance, 3.4 × 104 cells ml‒1).

Karyotypes of Cerastoderma glaucum (Bivalvia) from Baltic and Mediterranean populations

Chromosomes of C. glaucum from Baltic (Gdansk Bay) and Mediterranean (Thau lagoon, Sete) populations were studied using karyometric analysis and silver-staining. The karyotype of the Gdansk population consists of three metacentric, ten submetacentric and six subtelocentric chromosome pairs. The karyotype of the Thau population shows four metacentric, nine submetacentric and six subtelocentric pairs. Nucleolus organizer regions were found terminally on the second largest submetacentric chromosome pair of the Gdansk population and on the second largest metacentric chromosome pair of the Thau population. This suggests a cytotaxomomic difference between these two geographically isolated populations. Striking differences exist between C. glaucum and C. edule karyotypes, probably resulting from a long divergence time.

Evidence for short term retreat of the barrier shorelines

In many places on the present shoreface, offshore bar or beach sands with a sharp-basal contact overlie thin-bedded fine sands and muds. Ancient analogues are commonly interpreted as lowstand shoreface deposits lying on an erosional surface cut into the shelf during a relative sealevel fall. New observations may challenge this interpretation by providing evidence that such sequences develop during a recent shoreline retreat under highstand conditions without any significant change in sea level. Two examples of such a superimposition are provided by wave-dominated clastic shorefaces in the microtidal environment from the Senegal coast (Saloum delta, Sangomar spit) and in a non-tidal environment on the French Mediterranean coast in the Gulf of Lions. In the Saloum, a littoral cross-section across the Sangomar beach barrier shows a dune sand deposit overlying a man-made shell accumulation, dated 600 BP, and a layer of green muds formed in a lagoon at the back of an ancient beach barrier whose remnants are dated 3150 BP. The retreat of this ancient beach barrier is demonstrated by mapping the former offshore extension of a littoral sand unit which is defined clearly in the region by a grain-size signature. Other observations in the region of the Saloum delta give evidence of the widespread occurrence of coastal recession along the whole Sangomar sedimentary spit. In the Gulf of Lions, vibrocores were used to sample the beach and the shoreface of the Thau lagoon lido. The geological record of recent and present sedimentation exhibits varied littoral sands overlying ancient deposits of fine muddy sand and, at the very bottom of the sections, shoreface sands including fragments of beachrock. Radiocarbon datings give ages of 2050–6700 BP for the underlying muddy fine sand which has been interpreted as a lagoonal deposit enriched in organic matter, typical of lagoon environments in the region. An evolution similar to that proposed for the Sangomar region is supported by additional evidences of a very recent (some centuries) landward displacement of the beach barrier. Lines of wrecks of different ages are observed along this part of the Gulf of Lions, showing that the shoal determined by the offshore bars has migrated successively landwards as the littoral sedimentary prism has receded.

Modelling the impact of a cultivated oyster population on the nitrogen dynamics: The Thau Lagoon case (France)

Abstract The Thau lagoon (France) is an important site for the cultivation of Crassostrea gigas. The relationship between the oyster population and the environment was assessed through a model of trophic relationships. The results represent the initial step towards a more precise assessment of the biological fluxes in the lagoon. This preliminary model was based on the nitrogen dynamics among the following compartments: phytoplankton, zooplankton, oysters, detritus and dissolved inorganic nitrogen in the water column. Two other compartments were also considered in the sediment: detritus and dissolved nitrogen. The model considered the watershed input and seawater exchange between the lagoon and the open sea. The parameters were estimated from experiments on oyster ecophysiology, in situ primary production and biomass measurements, and by calibration of simulations against data series. The importance of vertical exchange of material between the water column and the sediment due to sedimentation, biodeposition by the cultivated oyster populations, and nutrient regeneration from the sediment, was supported by the model. Therefore, the model emphasized the impact resulting from oyster culture practices and the sediment contribution to nitrogen dynamics. Oysters could be considered as a nitrogen well that stabilizes the ecosystem by removing nitrogen over a longer time scale than zooplankton. Since grazing was dominated by the oyster compartment, zooplankton had a limited effect on phytoplankton dynamics. Moreover, model calculation demonstrated the critical role of detritus in oyster food ration. For instance, the sedimentation rate of particulate matter was doubled by the deposition by oysters. The model was sensitive to parameters controlling the primary production. For example, modifying these parameter values resulted in large winter accumulation of dissolved inorganic nitrogen, triggering a first phytoplankton bloom at the end of winter. This sensitivity stressed the importance of using experimental data for calibration of the model.

A new marine picoeucaryote: Ostreococcus tauri gen. et sp. nov. (Chlorophyta, Prasinophyceae)

A new picoplanktonic alga, Ostreococcus tauri Courties et Chrétiennot-Dinet, gen. et sp. nov. (Chlorophyta, Prasinophyceae) is described from the Thau Lagoon on the Mediterranean coast, France. Almost undetectable by light or fluorescence microscopy in field studies, the cells were discovered by their flow cytometric signature and appeared numerically as the main component of the phytoplankton. Their ultrastructure is described, with additional information on cell size, pigment and DNA content. Each cell contains a nucleus, a chloroplast, one mitochondrion, one Golgi body and a very reduced cytoplasmic compartment. A starch granule and Chi a and b demonstrate its affinity to the Chlorophyta and the presence of a Chi c-like pigment, Mg 3,8 DVP (=Mg 2,4 DVP) argues for placing it in the Prasinophyceae. It differs from other coccoid taxa in ultrastructural details, mode of division and detailed pigment composition. The size and DNA content make Ostreococcus tauri the smallest eucaryote known.