Gerard Blanchard

Dynamic behaviour of benthic microalgal biomass in intertidal mudflats

The dynamic behaviour of microphytobenthic biomass in a European intertidal mudflat has been analysed by comparing field and laboratory measurements. In an experimental mesocosm, where the effects of grazing by deposit-feeders and resuspension by tides had been significantly decreased, the benthic microalgal biomass followed a logistic-type growth curve, and thus converged towards a maximum value at which production is theoretically equal to zero. In the field, the kinetics of microalgal biomass—measured during 14 consecutive days at the beginning and at the end of every daytime exposures—exhibited a different pattern with biomass increases during daytime exposures and biomass decreases during the other periods, thus describing a series of oscillations. It is suggested that in the field, the effect of grazing and resuspension prevents biomass from holding steady at its maximum level. Moreover, it has been found a significant negative relationship which states that the higher the biomass level at the beginning of daytime exposures, the lower the net production during that daytime exposure. In other words, it turned out that in the field, the biomass also tends to converge towards a “field maximum” where net production is equal to zero. Taken together, these observations allow to hypothesize that the high productivity of the microphytobenthic community in intertidal mudflats is due to the tight coupling between physical and biological processes.

Measurement of ingestion rate of Hydrobia ulvae (Pennant) on intertidal epipelic microalgae: the effect of mud snail density

The individual mean ingestion rate of Hydrobia ulvae was measured experimentally in controlled microcosms, in the dark to avoid primary production during measurement and at constant temperature. The experimental design was based on the addition of prelabelled epipelic microalgae to microcosms in a constant proportion with unlabelled diatoms, and in such a way that algal food availability was not a limiting factor within the range of tested densities (0.3 to 4.1 snails cm(-2)). Results show that the individual mean ingestion rate decreased significantly from 26.6+/-1.1 ng Chl a snail(-1) h(-1) to 22.4+/-1.0 ng Chl a snail(-1) h(-1) between 0.7 and 3 snails cm(-2). We hypothesize that this sharp decrease (the threshold density was between 1.4 and 2.5 snails cm(-2)) may account for a density-dependent effect. We have tested this hypothesis by using a simple random walk model including basic behavioural processes such as a break in feeding activity when two individuals contact each other. The model represents quantitatively well the threshold effect, suggesting that behavioural processes have to be taken into account for estimating a global feeding activity of H. ulvae populations.

Carbohydrate production in relation to microphytobenthic biofilm development: an integrated approach in a tidal mesocosm.

Experiments were performed to evaluate short-term changes in sediment extracellular carbohydrates for a multispecific assemblage of benthic diatoms in relation to physiological status, endogenous migratory rhythms, and environmental conditions. For this purpose, a mesocosm was used, which simulated both tidal and dark: light alternating cycles under controlled conditions. Scanning electronic microscopy in combination with picture analyses indicated that natural diatom migration patterns were reproduced in the mesocosm. Two EPS fractions were operationally separated in colloidal carbohydrate measurements: alcohol-soluble EPS (termed “soluble EPS”) and alcohol-insoluble EPS (termed “bound EPS”). Microphytobenthic biomass followed a logistic-type curve and converged toward a maximal value termed the “biotic capacity of the local environment.” Both EPS fractions showed oscillations with production during photosynthetic periods and sharp decreases during night immersion periods. Productions of both EPS fractions increased with Chl a production during light periods suggesting a light dependence in relation to migratory patterns. The decreases in both EPS fractions, which occurred during night immersion periods suggest that carbohydrate hydrolysis and/or washaway affected both EPS fractions similarly in benthic environments. Our results confirm the theory according to which the two distinct fractions are under different metabolic controls. No change in soluble EPS release was obtained during the transition from logarithmic to stationary phase. On the other hand, a metabolism modification of microalgae, probably related to ammonium depletion, occurred when cells entered the stationary phase, since there was a high enhancement in bound EPS production. Mesocosm results can serve as a system of reference useful to characterize biofilm development in field investigations and to revisit the effective implication of each EPS fraction in sediment stability.

CHARACTERIZING AND QUANTIFYING PHOTOINHIBITION IN INTERTIDAL MICROPHYTOBENTHOS1

This study characterizes the short‐term influence of the sustained saturating irradiance encountered by the microphytobenthos inhabiting intertidal mudflats. The kinetics of photoinhibition in epipelic microalgae from intertidal mudflats were investigated in the laboratory. Previously isolated benthic microalgae were exposed to a saturating photon flux density (PFD) for periods ranging from 0 to 180 min; every 30 min, a photosynthesis‐irradiance curve was established to quantify the effect of the saturating PFD on both parameters αB, the photosynthetic efficiency, and PmB, the photosynthetic capacity. The αB decreased from the beginning of light exposure until the end, whereas PmB first slightly increased and then diminished from 90 min exposure onward. It turned out that epipelic microphytobenthos undergoes photoinhibition after about 90 min of saturating PFD. The possible ecological consequences of these ecophysiological results are discussed.

Properties of the dynamics of intertidal microphytobenthic biomass

The short-term variability of intertidal microphytobenthic biomass was characterized by analysing a 30-day time series encompassing two spring-neap tide cycles. Chlorophyll-a concentration was measured every day at the beginning and at the end of every diurnal exposure periods. Microalgal biomass exhibited predictible net increases during diurnal exposures due to a positive production-loss balance. In addition, our data series shows that after an environmental disturbance, even though biomass decreased strongly, it converged quickly back to its initial steady-state oscillation pattern. This study supports some fundamental properties of the dynamics of microphytobenthic biomass, as previously predicted in our mathematical model of primary production.

Spatio-temporal differentiation in the population structure of Hydrobia ulvae on an intertidal mudflat (Marennes-Oleron Bay, France)

distributed in the upper half part of the mud£at and no individuals were found in the lower part.The breeding cycle extended from March to December and showed two annual peaks, in spring and in autumn.Three cohorts were recruited during the year and showed high growth rates during summer; the parameters of the von Bertalaniy model describing the growth curves were equal to kmax¼0.47 � 0.5 mm month 71 and L1¼5.4 � 0.2 mm. The snail population had similar size^ frequency structure along the transect at the beginning of the survey but summer recruitment initiated spatial diierentiation.Reproduction occurred in the middle part of the mud£at but recruits mainly settled down at the upper level of the mud£at; new cohorts appeared with increasing individual densities.The middle part of the mud£at was rather dominated by adult individuals which showed large density £uctuations.

Recent invasion of the Japanese oyster drill along the French Atlantic coast: identification of specific molecular markers that differentiate Japanese, Ocinebrellus inornatus, and European, Ocenebra erinacea, oyster drills

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.

Trophic shift in young‐of‐the‐year Mugilidae during salt‐marsh colonization

This study investigated the trophic shift of young-of-the-year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable-isotope signatures (δ(13) C and δ(15) N) of the fishes followed a pattern, having enrichments in (13) C and (15) N with increasing fork length (LF ): δ(13) C in fishes < 30 mm ranged from -19.5 to -15.0‰, whereas in fishes > 30 mm δ(13) C ranged from -15.8 to -12.7‰, closer to the level in salt-marsh food resources. Large differences between the δ(15) N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm LF. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.