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Field observations in Thau Lagoon, southern France, indicate that the growth of natural populations of Alexandrium catenella during blooms is limited by nitrogen and exhibits a storage rather than a growth response to an ammonium pulse. Therefore, ammonium uptake and accumulation under transient conditions were investigated in detail in laboratory cultures. Following nitrogen exhaustion from the medium, ammonium pulses of varying magnitudes were induced, and measurements of extra- and intra-cellular ammonium were carried out for 24–72h along with measurements of ammonium incorporation (15N tracer) and inorganic carbon fixation (13C tracer). During vegetative growth, values of intra-cellular ammonium reached 30% (Strain TL01) and 2% (Strain ACT03) of cell nitrogen. When ammonium was available in the culture medium, ammonium uptake estimated by the 15N tracer technique accounted for only 65% of the decrease in external ammonium. This discrepancy is probably due to organic nitrogen excretion. Once external a...

The in situ diet of the pearl oyster Pinctada margaritifera was determined in the lagoon of Takapoto Atoll by comparing the phytoplankton composition of water and bivalve gut contents using 2 different methods, optical microscopy and HPLC pigment analysis. In order to evaluate the available food resources for pearl oysters in the water column, a new method for estimating the pigment/chloro- phyll a (chl a) ratio (based on an inverse analysis) was developed which allowed us to determine the contribution of the main phytoplanktonic groups in terms of chl a. In the water, picocyanobacteria and nanoflagellates predominated, the latter being mainly chlorophytes and prymnesiophytes. Compar- isons between the results obtained by the 2 methods of investigation indicated that most of the dinofla- gellates are unpigmented and, therefore, heterotrophic. An examination of the gut contents showed that picocyanobacteria were only weakly ingested by the oyster and, thus, nanoflagellates constituted the main food resource. Cryptophytes, although poorly represented in the water, were preferentially ingested. Chlorophytes were inefficiently hgested since they were found alive and motile in the faeces of the oyster. The ecological implications of this feeding behaviour are discussed.

Since 1998, blooms of Alexandrium catenella/tamarense in the lagoon of Thau developed regularly each autumn, reaching a maximum of several millions cells per liter in 2004. By contrast, spring blooms occurred only twice (in 2000 and 2007). During these periods, sea surface temperatures (SST) and the wind patterns appear to impact the bloom occurrences much more than the apparent limiting resources such as inorganic nutrients. The analysis of SST and wind from April to June and September to November (from 2000 to 2007) indicates first that there has to be an initial wind stress in order to resuspend the cysts buried in the sediment. Blooms then occur after a period of weak winds ( 4 m s(-1)) and of stable SST close to 20 degrees C (+/- 2 degrees C). Those conditions appear to be most favorable for germination of Alexandrium cysts and its ensuing vegetative growth. This period of stability (a few days to a few weeks) allows the development of the inoculum from the cyst's germination, its cohesion because of reduced hydrodynamics, and development of vegetative cells that are sensitive to agitation. Strong winds during 1-2 day periods can interrupt the bloom dynamics by dispersing (advection due to southeasterly winds) and/or eliminating (turbulence due to northwesterly winds) the vegetative cells. In the spring, under the same conditions of optimal SST, strong wind episodes dominate and those, as well as biological factors very likely lead to a lower occurrence of blooms relative to the fall situation. (C) 2013 Elsevier B.V. All rights reserved.

Carbon stable isotopes and fatty acids were measured in the suspended particulate organic matter (POM) of the Thau lagoon to study its qualitative temporal changes in relation to environmental factors and to identify the food sources of bivalves over a one-yr-cycle in relation to their growth. Reciprocally, the impact of shellfish farming on POM was also studied. Oysters and mussels were sampled and measured for biometry, stable isotopes and fatty acid composition. Water samples were collected at two sites, both inside and outside of the shellfish farming area, to determine concentrations in POM, chlorophyll a (Chl a) and stable isotopes. Carbon isotopes and fatty acids in bivalves reflected seasonal changes in food sources, which varied consistently with the environment. Seasonal changes in delta C-13 and fatty acids in the bivalves suggested that dietary phytoplankton contribution varied according to season. Terrestrial organic matter and bacteria can contribute to the diet of bivalves during non-bloom periods. Mussels seemed to rely more on diatoms and less on terrestrial organic matter and bacteria than oysters did, particularly when phytoplankton biomass was low during the summer. Although one- and two-yr-old oysters showed similar delta C-13, their fatty acid dynamics differed slightly. Periods of high growth rate in bivalves were mainly fuelled by diatoms, thus highlighting the importance of seasonal blooms of microphytoplankton during the critical period of bivalve growth and gamete production. Although there was no significant effect of shellfish farms on Chl a and POM delta C-13, consistent differences indicate that stable isotopes could be used successfully to investigate the effects of bivalve aquaculture. (C) 2011 Elsevier B.V. All rights reserved.

Time series data have been examined in Thau lagoon (Southern France) from 1972 to 2006 for water temperature, salinity, nutrients and from 1987 to 2006 for phytoplankton. A first main trend identified is an increase in mean annual water temperature (1.5 degrees C over 33 years or 0.045 degrees C/year) that was not evenly distributed among seasons. The highest rate of increase was in the spring (+ 3.0 degrees C over 33 years), followed by summer (+ 2.0 degrees C and fall (+ 1.7 degrees C. In winter, no significant increase over the 33 year period could be found. A second clear trend is a large decrease in soluble reactive phosphorus (SRP) concentration over the same 33 year period (summer values decreased from 10 mu M to 1 mu M. while winter values decreased from 3 mu M to undetectable at present). Nitrate concentrations depended mainly on rainfall events and watershed runoff. Ammonium data were too fragmentary to be useful. N/P ratios expressed the traditional way of DIN/SRP cannot be used for phytoplankton that are not strict autotrophs. The recent and almost simultaneous appearance of both picocyanobacteria (mostly Synechococcus) and the toxic dinoflagellate Alexandrium catenella in Thau seem to be related to reduced nutrient loading and the increase in water temperature. A. catenella blooms occur either in the spring or the fall when water temperature is near 20 degrees C and remains so for several weeks with winds speeds below 2-3 m s(-1). Picocyanobacterial growth is stimulated by increased summer temperatures, and lowered SRP levels provide picocyanobacteria an ecological advantage over other phytoplankton classes, in particular diatoms such as Skeletonema costatum whose cell densities have decreased over the last 8 years in summer and fall, but not in winter. An hypothesis is presented according to which A. catenella is not stimulated by increased temperatures, but is able to use picocyanobacteria for growth, and this provides this organism an additional resource over other strictly autotrophic phytoplankton. On a more general level, our data do not support the hypothesis that increased nutrient loading leads to harmful blooms of dinoflagellates. Instead, a combination of habitat disturbance and species displacement seems to lead to such blooms. (C) 2008 Elsevier B.V. All rights reserved.

The effect of starvation at selected low temperatures was studied in sea bass,Dicentrarchus labrax. The work focused on proximate chemical constituents and blood glucose. Appetite and growth of starved, refed specimens were also examined. Two experimental sets were designed. In one set the fish were exposed to 3 °C, 7 °C and 11 °C and in the other to 3 °C, 8 °C and 13 °C. Survival of the fish in the experimental conditions was 99.7%. Appreciable loss in body weight occurred. The reduction was more marked in the first week. No significant difference in chemical composition occurred in the first two weeks of starvation. Prolongation of food deprivation resulted in decline of lipid content with a corresponding increase in water content. This trend was the more marked at high temperature. Liver glycogen was the most readily utilizable source of energy. Lipid yielded energy after which proteins were mobilized to save further loss of lipid. Lipid exerted a sparing role for glycogen. The preference for different energy sources depended on exposure temperature and duration of starvation, Lipid chiefly supplied the energy needs of the 3 °C test groups, whereas liver glycogen provided the calorific requirements of fish at 7 °C, 8 °C, 11 °C and 13 °C. In the 3 °C test groups gluconeogenesis occurred after 2 weeks. The products of lipid breakdown may have served as substrates for glycogen biosynthesis. High tolerance of sea bass to low temperature was attributed to high fat content and hyperglycemia. The study also pointed to the involvement of some organs other than liver in generating glucose from their glycogen stores following exhaustion of hepatic glycogen. Starvation stress at the lowest temperatures adversely affected the return of the fish's appetite and growth characters. Compensatory growth occurred in sea bass refed after starvation. The findings have implications for aquaculture management procedures.

Condition of overwintering European sea bass, Dicentrarchus labrax , was studied in salt marshes along the French Atlantic coast. The somatic and liver condition indices (K and LSI, respectively) and RNA : DNA ratio were determined over a period of 5 months (October '89–February '90). K, LSI and RNA : DNA were high in October and this was attributed to the fish's physiological preparation in autumn for the overwintering phase. In the age-I teleost studied here this preparatory growth was linked to anticipated inclement temperature and not to gonad maturation. Decline in K and LSI started in November and continued till January but improvement was noticed in February. RNA : DNA ratio decreased till December, rose in January and remained stable in February. Magnitude of change was greater in LSI than in K. RNA : DNA was more sensitive compared to K and LSI. Low temperature in winter resulted in decrease in K, LSI and RNA : DNA through decline of food intake and slowing of metabolism. Discordancy in the pattern of change in the midst of winter demonstrated a more prompt response of RNA : DNA to environment. Recovery of the three parameters in the second half of winter was attributed to sea bass undergoing compensatory growth with a favourable turn in the thermal regime and, of course, after developing adaptation to cold.

A monitoring program with a weekly sampling frequency over a 15-month period indicates that urea concentrations above a certain threshold level may trigger the blooms of Alexandrium catenella in Thau lagoon. However, urea concentrations were also sometimes related to ammonium and dissolved organic nitrogen concentrations, indicating that the role of urea may not be a direct one. An original approach is used to assess the relative contribution of several nitrogen sources (nitrate, nitrite, ammonium, urea) to growth of A. catenella by comparing nitrogen uptake rates to nitrogen-based growth rates estimated from dilution experiments during four blooms over a 4-year period (2001-2004) in Thau lagoon. Nitrate and nitrite contributed 0.1-14% and 0.1-5% respectively of growth requirements. Ammonium and urea were the main N sources fueling growth of A. catenella (30-100% and 2-59%, respectively). Indirect estimates indicated that an unidentified N source could also contribute significantly to growth at specific times. Concerning ammonium and urea uptake kinetics, half-saturation constants varied between 0.2 and 20 mu gat NL-1 for ammonium and between 0.1 and 44 mu gat N L-1 over the 4-year period, indicating that A. catenella can have a competitive advantage over other members of the phytoplankton even under low concentrations of ammonium and urea. However, the observed large changes in ammonium and urea uptake kinetics on a short time scale (days) during blooms preclude more precise estimates of those contributions to growth and require further investigation. (C) 2007 Elsevier B.V. All rights reserved.

The study proposes methodological developments to optimize sampling strategy of resting cysts of Alexandrium catenella to estimate their abundance with a predefined error. This work also aims to provide information on spatial distribution of resting cysts in sediments. The distribution mode of A. catenella resting cysts related to the abundance variability was studied through sediment cores sampling on four different spatial scales and using Ludox CLX gradient density method. The quantification method underestimates by a factor of 2 the resting cysts abundance in one gram of sediment. Application of Taylor's power law allowed us to define a compromise between sampling effort and abundance estimation error. In the case of A. catenella resting cysts from Thau lagoon, the optimal sampling strategy consists of sampling 10 stations on a surface of 2 km(2) for a given coefficient of variability (C) of 15%, sampling 3 sediment cores at each station (C = 30%) and counting only one replicate by core (C = 18%). Results related to the application of Taylor's power law are closely dependent on resting cyst density and aggregation in a given sediment. In our area, A. catenella resting cysts are mainly observed in the upper 3 cm of sediment. Horizontally, their heterogeneity is lower on 10 cm(2) surface and tends to stabilize itself beyond a surface of 10 m(2). Each author has to carry out this pre-sampling effort for his own resting cysts-fonning species, in his own area, in order to increase accuracy of resting cyst mapping. (C) 2007 Elsevier B.V. All rights reserved.

International audience; During a 4-week experiment on oyster grazing in coastal ponds (claires) near La Rochelle (France), a red coloration was observed in the digestive gland of oysters. This pigmentation was attributed to an intensive grazing on cryptophyte nanoflagellates. Three different techniques led to this conclusion, mentioning for the first time free-living cryptophytes as responsible for such a phenomenon. Spectrofluorometry was used to demonstrate that the red coloration was due to the presence of a phycoerythrin, characteristic of cryptophytes. Microscopic observations and accessory pigments analyses in water ponds confirmed that the source of this pigment was of cryptomonadal origin. Oyster grazing on these algae is evidenced by large differences in alloxanthin concentrations and flagellate abundances between the two experimental ponds. The winter occurrence of this phenomenon is of importance for oyster commercialization and a short-term depuration is suggested to remove the undesirable pigmentation. D

International audience; To assess the in situ impact of oysters, Crassostrea gigas, on planktonic protist and bacteria communities and the potential contribution of protozoa to their food resource intake, the abundance and the diversity of protists and bacteria were followed in 2 Atlantic coastal ponds, with and without oysters. The protist biomass in such ponds was high, with a maximum in spring of 982 μg C l-1 and a minimum in winter of 179 μg C l-1. Whatever the season, the presence of oysters (20 m-2 corresponding to on average 23 mg dry weight m-2) induced a significant decrease in > 5 μm protist abundance. On the contrary, planktonic organisms < 5 μm, such as Chlorophyta flagellates and bacteria, developed similarly in both ponds. It can be assumed that such a depletion in micro-sized protists was especially related to the grazing activity of C. gigas, which efficiently retains > 5 μm particles. In spring, oyster grazing triggered dramatic changes in the protist community by lowering the taxonomic diversity. In autumn and winter, the presence of oysters deeply influenced the taxonomic structure of the protist communities: > 5 μm protists could only develop in the control pond, whereas they were removed by filtration into the oyster pond; on the contrary, < 5 μm protists that were not retained, were favoured in the oyster pond. The set of results showed that hetero/mixotrophic protists represent an important potential resource in coastal ponds: flagellates > 5 μm were the main protist resource for C. gigas; ciliates represented the second resource with a substantial contribution in autumn; diatoms and dinoflagellates, though efficiently removed represented a weak carbon resource. Our study supports the hypothesis that oysters may access to the strong bacterioplanktonic production through hetero/mixotrophic protists, which would thus allow the transfer of carbon from the microbial loop towards C. gigas.

Pearl oysters are farmed in oligotrophic tropical atoll lagoons where planktonic communities are dominated by production from cyanobacteria smaller than 2 pm. Paradoxically, the pearl oyster Pinctada margaritifera only retains particles larger than 2 Pm. In this study, we assess the relative contribution of hetero/mixotrophic microbiota to the available planktonic resource. In Takapoto Atoll, picocyanobacteria are the dominant biomass (20 mug C l(-1)). The carbon biomass of ciliates and dinoflagellates ranges from 1 to 24 and 0.5 to 5 mug C l(-1) respectively, with a mean of 6 mug C l(-1) for ciliates and 2 mug C l(-1) for dinoflagellates. The possible retention by P. margaritifera on a natural protist suspension was investigated. Due to its high clearance rates (ca 20 l h(-1) g(-1)) the pearl oyster retained 85 mug C h(-1) g(-1) from ciliates and 55 mug C h(-1) g(-1) from dinoflagellates. Conversely, cyanobacteria were not efficiently retained by the bivalve and did not efficiently contribute to its diet. From our experiments, we concluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonic resource towards filter-feeders, particularly pearl oysters.

The activities of digestive enzymes were studied during the course of larval development (from hatching to 30 days) in the sole, Solea solea . No glycosidase activity was present, but alkaline proteolytic activity increased from hatching to day 5, then decreased until day 15. In the acid range, proteolytic activity was relatively stable from the 10th to the 30th day. There were also changes in the number of esterase-like isozymes which increased between day 20 and 30, with minimum activity observed during metamorphosis.

The methionine (MET) requirement of sea-bass was studied by the nutrient-response relationship using growth rate data and free MET levels in serum and whole blood after feeding. Two series of diets were employed, sharing a common basal diet. This basal diet contained a mixture of soybean, yeast and fish meals as protein source, and was formulated to be deficient in MET whereas the other essential amino acids levels corresponded to the pattern found in sea-bass muscle proteins. Graded levels of MET were obtained by supplementing the basal diet with crystalline MET (S series) or by substituting soybean and yeast meals by fish meal (F series). Fish were fed for 85 days on an optimal ration (1.5%/day). The MET requirement could be determined only with the F series diets using sigmoidal mathematical models. Inflexion points from curves obtained with the data for weight gain and for free MET in the blood were very similar. The MET requirement was estimated at about 1% of dry diet or 2% of dietary protein or 15 mg/day per 100 g live weight. The S series diets allowed us to determine, from growth data, the optimal level of MET supplementation: optimal growth rate occurred with 1.3% total MET content in the diet, with 0.6% in the crystalline form. Further supplementation depressed growth rate. Choices of types of diet, advantages of the biochemical method and use of sigmoidal models for computer fitting of experimental data are discussed.

Growth, food conversion, and body composition of sea bass fingerlings, Dicentrarchus labrax, exposed to different temperatures (15 and 22°C) and salinities (6, 11, 24 and 37‰) were studied. Temperature was found to have the more significant influence on these parameters. At the higher temperature, fingerlings grew faster in high and low salinities (6 and 37‰). At the lower temperature, growth was slightly faster at 11 and 24‰. Body composition was affected mainly by temperature; salinity had no significant effect. Trypsin-like activity in the digestive tract was very low at 6‰ and 15°C. Sea bass fingerlings seem to be very tolerant to changes in temperature and salinity of their environment. The results are discussed in the context of fish culture.