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The direct amplification of length polymorphism technique (DALP) has been used to distinguish species-specific banding patterns in two marine gastropod oyster drills Ocenebra erinacea (Linnaeus, 1758) and Ocinebrellus inornatus (Recluz, 1851). Ocenebra erinacea is the European oyster drill, common along all European coasts. Ocinebrellus inornatus, the Japanese oyster drill, was recorded in oyster growing areas of the Marennes-Oleron Bay (SW France) for the first time in 1995. This new biological invasion could lead to an increase, which must be evaluated, in the predation risk for cultivated species i.e. oysters and blue mussels, and for littoral fishing resources along the French Atlantic coasts. As a result, since specific identification of early life stages of both species (egg capsules and juveniles) was previously found to be both difficult and unsure using only morphological criteria, four Ocenebra erinacea and two Ocinebrellus inornatus specific molecular markers were identified and sequenced. These markers will facilitate the assessment of respective ecological impacts (reproductive patterns, abundance and spatial distribution of juveniles), resulting from the exotic species versus the native species and will allow us to analyse with certainty demographic profiles of the two oyster drill populations.

Microphytobenthos plays an important role as primary producer in shallow embayments subject to a tidal regime. The present paper aims to characterize the spatial patterns of sedimentary chlorophyll a (chl a) biomass in 2 seasons in the wide intertidal mudflats of the Marennes-Oleron Bay (France); furthermore, it provides estimates of total microphytobenthic biomass. Accordingly, a systematic sampling scheme was performed in June 1995 and in January 1996. On average, biomass was higher in June (ca 120 mg chi a m(-2)) than in January (ca 85 mg chi a m(-2)). Moreover, the geostatistical analysis revealed that spatial similarities were propagated over larger distances in June (6 km) than in January (2.2 km). In order to account or this result, we suggest that seasonal dynamics of benthic microalgae biomass obeys a 'constant-density model' that describes the alternating spreading/shrinkage phenomenon around the few persistent patches of maximum biomass (ca 300 mg chi a m-2). Total biomass estimates were ca 13 t chl a (SE = 0.26) in June and 10 t chi a (SE = 0.28) in January. Calculating a raw approximation of the growth rate (0.1 d(-1)) of that biomass and considering that the total produced biomass is resuspended at flood tide, we found that microphytobenthos supplies about 2 mu g chi a l(-1), which corresponds to 30-90 % of the chi a suspended in the water column, depending on the season. This suggests that microphytobenthos could be a major food source for secondary producers in both pelagic and benthic ecosystems of the Marennes-Oleron Bay.

Resuspension of the top few sediment layers of tidal mud flats is known to enhance planktonic biomass of microbiota (benthic diatoms and bacteria). This process is mainly controlled by tidal shear stress and cohesiveness of mud, and is also influenced by bioturbation activities. Laboratory experiments in a race track flume were performed to test the interactive effects of these factors on both the critical entrainment and resuspension kinetics of microbiota from silt-clay sediments from the Marennes-Oleron Bay, France. The marine snail Hydrobia ulvae was used to mimic surface bioturbation activities. As expected, the kinetics of microbial resuspension versus shear stress were largely controlled by the cohesiveness of silt-clay sediments. However, our results indicate that the effect of surface tracking by H. ulvae on microbial resuspension was clearly dependent on the interaction between sediment cohesiveness and shear velocity. Evidence was also found that microphytobenthos and bacteria are not simultaneously resuspended from silt-clay bioturbated sediments. This supports the theory that diatoms within the easily eroded mucus matrix behave actively and bacteria adhering to fine silt particles eroded at higher critical shear velocities behave passively.

Fluxes of nutrients (NH sub(4) super(+), NO sub(3) super(-), PO sub(4) super(3-) and Si(OH) sub(4)) were studied on an intertidal mudflat in Marennes-Oleron Bay, France, at two different seasons and at different times of the emersion period. Fluxes through the sediment-water interface were both calculated from vertical profiles of nutrient concentration in pore-water (diffusive fluxes, JD) and measured in light and dark benthic mini-chambers (measured fluxes, J sub(0)). Results indicate that ammonia was mainly released in summer while nitrate was mainly taken up in late winter. This uptake from the overlying water was probably due to the coupling of nitrification-denitrification within the sediment. The J sub(0) /J sub(D) ratio further indicates that bioturbation likely enhanced ammonia release in summer. Concerning phosphate, the comparison of diffusive and measured fluxes suggests that PO sub(4) super(3-) could be assimilated by the biofilm in winter while it was released in summer at a high rate due to both bioturbation and desorption because of the relative summer anoxic conditions. Silica was always released by the sediment, but at a higher rate in summer. Statistically significant differences in measured fluxes were detected in dark chambers at different times of low tide, thus suggesting a short-term variability of fluxes. Microphytobenthos preferred ammonia to nitrate, but assimilated nitrate when ammonia was not available. It also turned out that benthic cells could be limited in nitrogen during low tide in late winter. In summer, ammonia was not limiting and microphytobenthic activity significantly decreased the measured flux of NH sub(4) super(+) in the middle of low tide when its photosynthetic capacity was highest., La minéralisation des éléments nutritifs (NH sub(4) super(+), NO sub(3) super(-), PO sub(4) super(3-), and Si(OH) sub(4)) est étudiée sur un estran vaseux dans le bassin de Marennes-Oléron, France, à deux saisons différentes et à différents moments de la période d'exondation. Les flux qui ont leur siège à l'interface eau-sédiment ont été à la fois calculés à partir des profils verticaux des teneurs en sels nutritifs dans les eaux interstitielles (flux diffusifs ; JD) et mesurés dans des mini-enceintes claires et opaques (flux mesurés ; J0), placées sur la vasière. Ainsi l'ammonium est principalement relâché en été, les nitrates sont essentiellement absorbés par le sédiment à la fin de l'hiver. Cette absorption est probablement due à un couplage des processus de nitrification-dénitrification dans le sédiment. Le rapport J0/JD indique que la libération d'ammonium a probablement été accrue par la bioturbation. Les phosphates sous forme d'ions PO sub(4) super(3-) seraient assimilés par le biofilm en hiver alors qu'ils ont été relâchés dans l'eau à un fort taux en été, dû à l'action conjuguée de la bioturbation et de la désorption consécutive aux conditions d'anoxie relative en été. La silice présente toujours des flux dirigés du sédiment vers la colonne d'eau, avec une intensité plus forte en été. Des différences statistiquement significatives ont été décelées dans les enceintes opaques au cours de l'exondation, suggérant ainsi l'existence d'une variabilité à l'échelle horaire. Les résultats montrent également que le microphytobenthos a une forte affinité pour l'ammonium, bien qu'il assimile les nitrates en l'absence d'ammonium. Il s'est aussi avéré que les cellules algales benthiques pouvaient être potentiellement limitées en azote pendant la marée basse en hiver. En été, l'ammonium n'est pas un facteur limitant et l'activité microphytobenthique entraîne une baisse des flux d'ammonium à marée basse lorsque la photosynthèse est la plus forte.

The mud surface temperature (MST) of an intertidal mudflat in Marennes-Oléron Bay (France) and the biomass-specific photosynthetic capacity (PBmax) of benthic microalgae were modelled to investigate their spatio-temporal dynamics. Simulations were performed over 2 different periods during and after the microphytobenthos spring bloom (April and June, respectively) and under 2 different tidal conditions (spring and neap tides). The deterministic MST model is based on thermodynamic processes. Comparison at different periods between measured data series and simulations clearly establishes the reliability of the model, thus allowing extrapolations over time and space. The spatio-temporal dynamics of MST is primarily controlled by the immersion-emersion alternation combined with the solar cycle, with a strong influence of the phase difference in their respective oscillations: the highest MSTs are achieved in summer in the highest parts of the mudflat, when spring low tide occurs at midday. Three relevant time scales characterize the MST dynamics: long-term (seasonal cycle), medium-term (lunar cycle) and short-term (solar and tidal cycles). Within that framework, the response of PBmax to changes in MST depends upon Topt, the optimum temperature for photosynthesis (Topt = 25°C all year round). In April, when the MST values are below Topt, PBmax varies exponentially with MST at short time scales. Conversely, in June, when the range of MST partially exceeds Topt, PBmax is inhibited on most of the mudflat surface area (up to 75%). This thermo-inhibition is highest in summer, when low tide occurs at midday.

The present study aims at assessing the relationship between in situ mud temperature and microalgal photosynthetic capacity on intertidal mudflats. Both mud surface temperature and P-max were measured during low tide in different tidal conditions: when low tide occurred either in the morning, at midday or in the afternoon to select presumably known changes in temperature. These measurements were performed at 2 different dates to contrast meteorological conditions (March and September). P-max was always higher when low tide coincided with midday, and most of the time changes of the photosynthetic capacity reproduced changes of the temperature within a single emersion period. So, temperature appeared to be a factor determining P-max at the hourly scale and at the level of the tidal cycle on temperature intertidal mudflats. An experiment was performed to confirm the exogenous control of temperature on P-max independently of the influence of light. Finally, as primary production is fundamentally determined by the photosynthetic capacity, it is concluded that temperature of the mud surface is an important factor explaining the dynamics of microphytobenthic primary production., Le travail proposé a pour objectif l'étude des relations entre la température de surface de la vase et la capacité photosynthétique du microphytobenthos sur les vasières intertidales. Dans ce but, la température de surface de la vase et la capacité photosynthétique (Pmax) ont été mesurées pendant la basse mer dans différentes conditions de marée : lorsque la basse mer est apparue le matin, à midi ou l'après-midi, de manière à sélectionner des évolutions de température de la vase connues a priori. Toutes ces mesures ont été réalisées à 2 dates différentes pour contraster les conditions météorologiques (mars et septembre). Les valeurs de Pmax étaient toujours les plus élevées lorsque la basse mer coïncidait avec le midi solaire ; en outre, les variations de la capacité photosynthétique reproduisaient, dans chacun des cas étudiés, celles de la température pendant la durée des périodes d'exondation. Il en résulte que la temrpérature peut être considérée comme un facteur déterminant Pmax à l'échelle horaire et au niveau du cycle tidal. Une expérience a aussi été réalisée pour confirmer le contrôle exogène de la température sur Pmax, indépendamment de l'influence de la lumière. Finalement, comme la production primaire est fondamentalement déterminée par la capacité photosynthétique, il apparaît que la température de la surface de la vase est un facteur important contribuant à expliquer la dynamique de la production primaire microphytobenthique.

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