Jean-Marc Deslous-Paoli

Allometric relationships and effects of temperature on clearance and oxygen consumption rates of Crassostrea gigas (Thunberg).

Abstract Clearance and oxygen consumption rates of Crassostrea gigas were investigated with animals of 5–200 g total wet weight (0.1–3 g dry tissue weight), and at different temperatures (5–32 °C) after 10 days acclimation. During this period significant mortalities were observed at 32 °C, which may be close to the upper thermal limit for this species. For each temperature, allometric relationships between physiological rates and the dry weight (DW, g) of the animal were estimated. Clearance rate (CR, 1·h−1) was maximal at 19 °C; oxygen consumption rate (VO2, mgO2·h−1) increased over the range of experimental temperatures (T, °C). Two statistical models are proposed: CR = [a − (b ∗ (T − c) 2 )] ∗ DW d and VO 2 = [a + (b ∗ c T )] ∗ DW d . However, neither model is appropriate during the reproductive period.

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.

Seagrass (Zostera marina L.) bed recolonisation after anoxia-induced full mortality

Recolonisation of Zostera marina, following complete destruction caused by an anoxic crisis, was studied in the Thau lagoon (French Mediterranean Sea) from February 1998 to September 1999. The recolonisation took place surprisingly rapidly as biomasses similar to those from untouched areas were reached only nine months after seed germination. The recolonisation success was partly due to a high seedling survival rate as well as a rapid vegetative recruitment (ranging from 0.012 to 0.042 per day). Two phases of recovery could be observed: a rapid multiplication of shoots during the first 3 months was followed by an increase in biomass due to elongation of leaves. During the first year of recolonisation no flowering shoot was observed whilst reproductive effort was considerable during the second year. In case of two consecutive anoxic crises at the same site, the recovery would have probably been much slower, since the annual seedbank would have been depleted.

Nutrient and oxygen exchanges at the water–sediment interface in a shellfish farming lagoon (Thau, France)

The Etang de Thau (France) is a shallow lagoon characterised by the semi-intensive farming of oysters (Crassostrea gigas, Thunberg) cultured in suspension on frames. Analysis of the benthic fluxes of inorganic nutrients and oxygen over a period of a year has provided a basis for describing the dynamics of the water–sediment interface in the lagoon. Monthly measurements of fluxes at the water–sediment interface at two stations have been compared. One station (UC) is located under a culture table, and is subject to intensive accumulation of organic matter (biodeposition); the other (OC) is located outside the area directly under the impact of the culture activities. Oxygen consumption ranged from 288.24 to 1026.85 μmol·m−2·h−1 according to the season and the station. Ammonium production was maximal at station UC in Summer (600 μmol·m−2·h−1) and minimal at station OC in the Autumn (30 μmol·m−2·h−1). In general, the fluxes recorded at station UC were 1.8–3 times higher than those recorded at station OC for oxygen and 1–5 times higher for ammonium. Nevertheless, the variability between stations was lower than the seasonal variability. Using a Multiple Correspondence Analysis (MCA), it was possible to point out the occurrence of an atypical event that was responsible for the disruption of the seasonal cycle. This event was a state of hypoxia known locally under the generic name of malaigue. The dystrophic crisis consists of a major perturbation of the ecosystem, responsible for a massive mortality affecting both the benthos and the reared stocks.

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.

Modelling seasonal dynamics of biomasses and nitrogen contents in a seagrass meadow (Zostera noltii Hornem.): application to the Thau lagoon (French Mediterranean coast)

Abstract Anumerical deterministic model for a seagrass ecosystem (Zostera noltii meadows) has been developed for the Thau lagoon. It involves both above- and belowground seagrass biomasses, nitrogen quotas and epiphytes. Driving variables are light intensity, wind speed, rain data and water temperature. This seagrass model has been coupled to another biological model in order to simulate the relative contributions of each primary producer to: (i) the total ecosystem production, (ii) the impact on inorganic nitrogen and (iii) the fluxes towards the detritus compartment. As a first step in the modelling of seagrass beds in the Thau lagoon, the model has a vertical structure based on four boxes (a water box on top of three sediment boxes) and the horizontal variability is neglected until now. This simple box structure is nevertheless representative for the shallow depth Z. noltii meadows, spread over large areas at the lagoon periphery. After calibration, simulation results have been compared with in situ measurements and have shown that the model is able to reproduce the general pattern of biomasses and nitrogen contents seasonal dynamics. Moreover, results show that, in such shallow ecosystems, seagrasses remain the most productive compartment when compared with epiphytes or phytoplankton productions, and that seagrasses, probably due to their ability in taking nutrients in the sediment, have a lower impact on nutrient concentration in the water column than the phytoplankton. Furthermore, in spite of active mechanisms of internal nitrogen redistribution and reclamation, the occurrence of a nitrogen limitation of the seagrass growth during summer, already mentioned in the literature, have also been pointed out by the model. Finally, simulations seems to point out that epiphytes and phytoplankton could compete for nitrogen in the water column, while a competition for light resources seems to be more likely between epiphytes and seagrasses.

Effects of the feeding behavior of Crassostrea gigas (Bivalve Molluscs) on biosedimentation of natural particulate matter

Some of the particulate matter found in bottom sediment comes from material rejected as pseudofaeces or produces as faeces by Crassostrea gigas. The amount of pseudofaeces rejected is proportional to the amount of seston for total seston values up to 4.6 mg 1−1. For this values up to pseudofaeces production, the amount of faeces is mainly limitated by pseudofaeces production and reaches a constant mean value of 8.9 mg 1−1 g DW−1 when seston concentration increases.The proportion of faeces versus pseudofaeces is over 1 when seston concentrations are below 10 mg 1−1, and remains less than 1 when seston concentrations are higher than 10 mg 1−1.The pseudofaeces show a lower organic content (13 ± 3%) than the material trapped through the gills (27 ± 4%), as well as a lower percentage of proteins and lipids, suggesting a selection by the labial palps. On the contrary, an enrichment with labile carbohydrates is recorded in pseudofaeces, probably due to mucus agglutination of rejected particulate matter.The feeding of Crassostrea gigas can package and transform (agglutinate, change the organic content, change the biochemical composition) suspended matter, thus influencing both the quality and quantity of organic matter sedimentation.

Factors influencing primary production of seagrass beds (Zostera noltii Hornem.) in the Thau lagoon (French Mediterranean coast)

The primary production and the respiration of Zostera noltii beds in the Thau lagoon were studied by means of the benthic bell jar technique. Concurrently, environmental data (temperature, light and nutrients) as well as morphological data of seagrass meadows (leaf width and height, density of shoots, above/below-ground biomass ratio) were collected with the purpose of explaining most of the observed variations in metabolism. Seagrass plus epiphyte respiration rates were influenced mainly by the water temperature, showing a typical exponential response to an increase in temperature. Surprisingly, measurements of production rates were not related to incoming light intensities recorded at the seagrass canopy level. An equation frequently used for terrestrial standing crops, involving the leaf area index (LAI) and the characteristics of the canopy architecture (parameter K, depending on leaves optical and geometrical properties), was applied to the seagrass ecosystem in order to estimate the light energy actually available for the plants, i.e. the light intercepted by the seagrass canopy (Q(abs)). Linear relationships were then validated between gross production rates and calculated Q(abs) for Z. noltii beds, and the best fits were obtained with K values nearing 0.6, confirming the similarities between terrestrial graminaceae and seagrasses. A linear regression model for primary production is proposed, involving the calculated Q(abs), the water temperature and the leaf nutrient content.

Relations milieu-ressources · impact de la conchyliculture sur un environnement lagunaire méditerranéen (Thau)

Abstract Shellfish farming leaves its mark on the environment in which it has developed, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased water clarity caused by the uptake of particulate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations increase ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low level. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermoclines limit oxygen transfer from surface water, the organically enriched substrate induces oxygen depletion and ammonium and nitrogen sulfide accumulation in the water column. This ecosystem dysfunction kills benthic populations, and sometimes reaches pelagic populations and affects the shellfish farming economy.

Production of Mytilus edulis L. reared on bouchots in the bay of Marennes-Oleron: Comparison between two methods of culture

Abstract In the bay of Marennes-Oleron, Mytilus edulis were cultivated on bouchots in intertidal areas. Two methods of culture were used. In the first, common one, the larvae settled naturally during May and June on recently cleaned stakes. In the second one, the larvae were collected on cocofibre rope and then transferred to clean stakes. In the first year of culture, the production and the eliminated biomass were higher for naturally settled mussels (P = 27.76 kg m−1, E = 10.38 kg m−1) than for transplanted mussels (P = 19.23 kg m−1, E = 8.46 kg m−1). Nevertheless, the higher growth of the transplanted mussels, due to a lower density, induced a production of commercial mussel (> 40 mm) higher (10.78 kg m−1) than for naturally settled mussels (6.07 kg m−1). Thus, to obtain mussels of commercial size quickly and with less loss, it is important to limit spatial competition either by using the transplanting method or by decreasing the density of natural culture by regular thinning.