Daniel Razet

Effects of high natural seston concentrations on the feeding, selection, and absorption of the oyster Crassostrea gigas (Thunberg)

Feeding, selection and absorption were determined for the Pacific oyster Crassostrea gigas cultivated in the Bay of Marennes-Oleron, over a spring/neap tidal cycle. Physiological determinations were related to the highly variable food environment with continuous recordings of turbidity and fluorescence. In this bay, resuspension processes have a major influence on food availability and quality. Seston characteristics experienced by oysters can be summarized by high turbidity levels from 20 to 350 mg·l−1 and a predominance of the detritic fraction among the organic fraction (mean C/N ratio=16.57). Food is diluted by the fine resuspended sediment, and organic content of particulate matter in the water column decreases from 30% to 10% with increasing seston loads. Significant differences (Ancova, P<0.01), due to low retention efficiencies of the smaller particle size range, were recorded between the food quality (estimated by the organic content and the total pigment content) measured in the water column and the fraction retained by the oysters gill. Below seston concentrations of 90 mg·l−1 ingestion rate was regulated by pseudofaecal production. Above 90 mg·l−1, a sharp reduction of filtration and rejection rates suggests physical constraints limiting food acquisition. The oyster selectively rejects inorganic from organic particles, enriching the ingested fraction. Amongst the potentially nutritive particles, significantly fewer particles containing phytopigments were rejected relative to organic particles (non-linear regressions, P<0.001). The negative influence, through food dilution, of high seston loads on net absorption efficiency was determined. This efficiency decreases with decreasing organic ingested fraction. Scope for growth calculations confirm the negative influence of seston loads, but show, supported by field growth measurements, that resuspended organic particles play an important role in the oysters nutrition.

Correction for particulate organic matter as estimated by loss on ignition in estuarine ecosystems

Abstract Loss on ignition (LOI) is a simple, inexpensive method widely used to estimate organic matter in the water column and sediment of marine and freshwater ecosystems. Suspended particulate matter in estuarine waters however often contains a large fraction of argillaceous minerals whose loss of structural water can introduce significant bias to the method. The accuracy of LOI, in comparison with elemental analysis, was evaluated in 105 water samples from the Marennes-Oleron Bay (French Atlantic coast). The suspended particulate matter analysed ranged from 7 to 108 mg l −1 , with a mean inorganic fraction of 81.3±s.d. 7.2%. LOI overestimated particulate organic matter (POM) concentrations by up to 300% (mean 98 ± s.d. 28%), and this overestimation was significantly related to particulate inorganic matter (PIM) concentrations. POM overestimation increased linearly with PIM, representing a mean 8.7±s.d. 2.0% of PIM. This value was compared with a theoretical correction factor based on the specific clay composition of PIM at the study site and on thermo-gravimetric analysis for standard clay. The clay fraction, which represents 90% of PIM in the Marennes-Oleron Sound, consists of 40% illite, 30% kaolinite and 30% montmorillonite. The similarity between the theoretical correction factor (9.3% of PIM) and the overestimation based on measurements (8.7%), suggests that a priori correction of POM data is possible. In fact, for three bodies of water along the French Atlantic coast, whose inorganic resuspended matter shows the same mineralogical composition, POM concentrations as estimated by LOI can be corrected by subtracting 9.3% of PIM mass.

Ecophysiologie et bilan énergétique de la palourde japonaise d'élevage Ruditapes philippinarum

Rates of filtration and respiration both follow a nonlinear model based on temperature of the form: Y = a × (T−T0)c × e −b(T−T0 with maximal values at 15 and 20°C, respectively. Quantities of seston varying from 0 to 30 mg · 1−1 have no effect in reducing the filtration rate. > 8 mg · 1−1, ingestion is regulated by the production of pseudofaeces. Maximal assimilation efficiency is ≈ 78%, but this is considerably reduced when the mineral content of the water increases. Assimilation efficiency for the Manila clam is reduced at both high (> 10 mm3· h−1) or low (< 2 mm3· h−1) values of ingested ration. The estimated value of growth efficiency (75%) and values of growth efficiency derived from the model k1 = 33%, K2 = 51% are optimized when ingested volumes are between 1 and 2 mm3. Standard metabolism is estimated as 0.11 ml O2 · h−1. Zero growth efficiency occurs at a ration level of 2 J · h−1 for an adult. The individual energy budget shows that production is dependent more on temperature than on the energy value of the food. Comparison of calculated and measured production reveals differences resulting from the higher levels of seston found in the field. In particular, during the winter when the mineral content of the seston is high (90 mg · 1−1), there is a continuous loss of weight. This results from a lower assimilation efficiency together with production of pseudofaeces. Excretion of organic nitrogen varies throughout the year, ammonia representing no more than a mean of 29.8% of the total nitrogen excretion.

HYDROBIOLOGY OF THE MARENNES-OLERON BAY. SEASONAL INDICES AND ANALYSIS OF TRENDS FROM 1978 TO 1995

A hydrobiological monitoring network has been in place since 1977 in the Bay of Marennes Oleron (France). Data collected for physical variables (seawater temperature, salinity, oxygen concentration), nutrients (ammonium, nitrates, phosphates and silicates) and chlorophyll a and pheophytin, from 5 representative stations in the bay, were examined by time-series analysis (Census II method) to study seasonal variability and trends. The seasonal changes were similar over the entire Marennes Oleron Bay for all variables and were chiefly influenced by fluxes in the Charente river. The seasonal range reached 180 units for nitrates and 70 and 100 units respectively for phosphates and silicates. These values were similarly mostly correlated with the Charente River fluxes. With regard to long-term trends, seawater temperature has shown a significantly increasing trend close to 2 °C over 18 years. At the same time, a 1 °C gradient was demonstrated from the northern to the southern part of the Bay. The salinity trend varied between 30 and 34‰for all stations. The trend for oxygen concentration, ranged from 90 to 100% but during a specific two year period (1980–1982) saturation decreased to 76% in the northern part of the Bay.The trend analysis for nitrates showed a significant relationship with the water output level of the Charente. Phosphate inputs have been irregular during the two last decades which has affected primary productivity along the coastline (e.g., spring 1979–1983; 1990; 1993–1995).Since 1988, a significant increasing trend for ammonium was observed at the mouth of the Seudre river (4 μmoles l−1 ) while other stations were well below this, ranging from 1 to 3 μmoles l−1 . This should be considered as an indicator of seawater deterioration within the southern part of the Marennes Oleron Bay.

Optimisation of the traditional Pacific cupped oyster (Crassostrea gigas Thunberg) culture on the French Atlantic coastline: autumnal fattening in semi-closed ponds

Abstract Oyster farming in France is a traditional activity. Each year, 60,000 tons of C. gigas are fattened before being sold. Present-day fattening techniques for improving oyster taste and colour and increasing the meat weight are not particularly reliable. To optimize the fattening process, large phytoplanktonic blooms were induced in ponds, then distributed in oyster ponds. Despite the variability of the autumn weather conditions, diatoms ( Skeletonema costatum ) have been successfully cultured in outdoor ponds. During neap tides (when no seawater replenishes the water in the oyster beds), between mid-October and mid-December 1996, the fattening index (the weight of the meat) was constant or increased by 10–15% when a supplement of algae of 380,000 cells/oyster/day was added to the ponds. Algal supplement was clearly responsible for fattening improvement. In 1995, the addition of 110,000 cells/oyster/day, a significantly lower quantity, was not sufficient to prevent weight loss. Over two periods of 3 months (in 1995 and 1996) of the experiment, a significant reduction in weight (up to 20%) was observed in oysters grown in ponds which did not receive additional algae.

Energy budget of a Mytilus edulis L. population during its first year on bouchots in the bay of Marennes-Oléron

Abstract The energy budget of a population of Mytilus edulis cultured on bouchots in the bay of Marennes-Oleron was worked out from measurements of shell (7050 kJ m −2 year −1 ) and flesh (40 636 kJ m −2 year −1 ) production, and from field biodeposition (348 775 kJ m −2 year −1 ). The losses of energy induced by oxygen consumption (58 367 kJ m −2 year −1 ) and by NH 4 excretion (6202 kJ m −2 year −1 ) were measured in the laboratory under near field conditions. Production was negative in winter. During the phytoplanktonic bloom in June, production reached a maximum. The mean assimilation efficiency (23%) was near values recorded in the bay for other bivalve molluscs; nevertheless, the net production efficiency (47.8%) was representative of values described for this species. There was an inverse relationship between the amount of organic matter in sea water and the assimilation efficiency, and a positive relationship between food quality and the production of the mussel population.