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Microphytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high-resolution tri-dimensional hydrodynamic model coupled to a MPB model with or without photoinhibition. Photoinhibition leads to a 20% (-0.79 t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (-29%) than on the lower shore (-5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (-22% and -23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter-annual time scale. Plain Language Summary Benthic micro-algae or microphytobenthos (MPB) inhabiting the surficial sediment sustain the high biological production of intertidal mudflats. MPB achieve photosynthesis by aggregating into a dense biofilm at the mud surface during daytime low tides. As MPB can be exposed to short-term variations and high light levels, they change their short-term physiology and position within the sediment to protect themselves. However, such strategies can be outbalanced by a too long stressful light exposure. In this study, we explore with a numerical model the impact of photoinhibition on MPB primary production (PP) over a large and very productive mudflat (NW France). The model suggests that photoinhibition can strongly impact MPB PP. With photoinhibition, the yearly PP decreases by 20 % over the whole mudflat. The model suggests MPB PP is sensitive to the photoacclimation status of MPB cells, i.e. their light use efficiency at a given light level. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP.

International audience; Microphytobenthos (MPB) at the sediment surface of intertidal mudflats are known to show a high spatial and temporal variability in response to the biotic and abiotic conditions prevailing at the mud surface. It makes long-term and large-scale monitoring of MPB Gross Primary Production (GPP) difficult to set up. In this study, we developed the first 3D physical-biological coupled model (MARS-3D) that explicitly simulates GPP of intertidal MPB at the mudflat scale, and we compared the outputs with in situ and space remote sensing MPB GPP data. We discuss the sources of discrepancies between the modeling and the remote sensing approach in the light of future developments to be done. For instance, the remote sensing algorithm provides a very synoptic view of the mudflat GPP. It is well-suited to achieve diagnostic estimates of MPB GPP at the synoptic spatial and temporal scale. By contrast, the MARS-3D model provides a more dynamic representation of the MPB activity and prognostic estimates of MPB GPP over the mudflat. It is very relevant to resolve the seasonal and inter-annual dynamics of MPB. Getting comparable GPP estimates derived from the remote sensing algorithm and 3D physical-biological coupled model will further require a better convergence in terms of equations structure, biological constants parameterization, and source data used (i.e., vegetation index vs. chlorophyll a). Setting a common parameterization in both the numerical model and remote sensing algorithm might be challenging in a perspective of mapping MPB PP over large mudflats from a synoptic to inter-annual time scale, but it could open the door to a new way of quantifying MPB GPP over large intertidal mudflats.

Benthic macrofauna are a key component of intertidal ecosystems. Their mobility and behavior determine processes like nutrient cycling and the biogeomorphic development of intertidal flats. Many physical drivers of benthic macrofauna behavior, such as sediment grain size, have been well-studied. However, little is known about how sediment bulk density (a measure of sediment compaction and water content) affects this behavior. We investigated the effect of bulk density on the burrowing rate, burrowing depth, bioturbation activity, and oxygen consumption of bivalves (Limecola balthica, Scrobicularia plana, and Cerastoderma edule) and polychaetes (Hediste diversicolor and Arenicola marina) during a 29-day mesocosm experiment. We compared four sediment treatments consisting of two sediments of differing grain size classes (sandy and muddy) with two bulk densities (compact and soft). Overall, bulk density had a strong effect on benthic macrofauna behavior. Benthic macrofauna burrowed faster and bioturbation more intensely in soft sediments with low bulk density, regardless of grain size. In addition, L. balthica burrowed deeper in low bulk density sediment. Finally, we found that larger bivalves (both C. edule and S. plana) burrowed slower in compact sediment than smaller ones. This study shows that benthic macrofauna change their behavior in subtle but important ways under different sediment bulk densities which could affect animal-sediment interactions and tidal flat biogeomorphology. We conclude that lower bulk density conditions lead to more active macrofaunal movement and sediment reworking.

Phosphorus (P) supplies from the sediment are important when evaluating the impacts of nutrient dynamics on the ecosystem. Many studies have reported P dynamics based on gradients of pore water (PW)-phosphate (Formula presented) concentrations at the sediment?water interface and iron-related abiotic reactions in response to redox conditions. However, the aim of the present study is to evaluate the contributions of intracellular (IC)-P and marine organism-mediated reactions to P-dynamics under variable environmental conditions. The focus of this study is the intertidal mudflat habitat of the innermost part of Ariake Bay, Kyushu, Japan, during winter. The vertical distribution of several forms of P fraction [i.e. PW- and IC-dissolved inorganic/organic-P (DIP/DOP), and particulate inorganic/organic-P (PIP/POP)], the number of microalgae and bacteria, and chlorophyll a concentrations in the sediment were measured. The results obtained revealed that the IC-P pool, especially IC-DOP, accounted for 76% of the total dissolved P in the surface sediment layer, mainly associated with the microphytobenthos. Additionally, phytoplankton, which comprised more than 60% of the surface microalgae in tidal-flat sediments, are considered to carry river-derived-P to the intertidal flats via inner bay-specific physical movements and disturbances. Incubation experiments on mud sediment under oxic/anoxic conditions suggest that dissolved P fluxes between the IC pool and PW occur actively in response to redox conditions, according to the balance between microalgal uptake and bacterial degradation. These results indicate that evaluation of the IC-P pool is indispensable to understanding P cycling in intertidal mudflats, and that microalgae could play important roles not only as food sources, but also as P reservoirs and sources of (Formula presented)., Plankton and Benthos Research, 10(1), pp.1-10; 2015

International audience; Intertidal mudflats are rich and fluctuating systems. The upper 20 cm support a high diversity and density of microorganisms that ensure diversified roles. The depth profiles of microbial abundances and activities were measured in an intertidal mudflat (Marennes-Oléron Bay, SW France) at centimeter-scale resolution (0-10 cm below the sediment surface). The aim of the study was to detect microbial stratification patterns within the sediments and how this stratification is shaped by environmental drivers. Two sampling dates, i.e. one in summer and another in winter, were compared. The highest activities of the microbial communities were observed in July in the surface layers (0-1 cm), with a strong decrease of activities with depth. In contrast, in February, low microbial bulk activities were recorded throughout the sediment. In general, prokaryotic abundances and activities were significantly correlated. Variation partitioning analysis suggested a low impact of predation and a mainly bottom-up-controlled prokaryotic community. Hence, in the top layer from the surface to 1–3.5 cm depth, microbial communities were mainly affected by physicochemical variables (i.e., salinity, phosphate and silicate concentrations). Below this zone and at least to 10 cm depth, environmental variables were more stable and prokaryotic activities were low. The transition zone between both layers probably represents a rather smooth gradient (environmental ecocline). The results of our study provide a better understanding of the complex interactions between micro-organisms and their environment in a fluctuating ecosystem such as an intertidal mudflat.

Knowledge of the trophic position of meiofauna in marine sediments is still contradictory primarily due to the scarcity of experimental data. The present study analyses diatom-feeders among the meiofauna. The assimilation rate of carbon by meiofauna was determined through a grazing experiment using in situ collected diatoms pre-labelled with 14 C. The assimilation rates of diatom carbon were estimated for the bulk meiofaunal community (0.84 ng C.ind − 1 .h − 1 ), for the bulk harpacticoid copepods (1.92 ng C.ind. − 1 h − 1 ) and for two dominant nematode species ( Metachromadora remanei 1.26 ng C ind − 1 h − 1 , Daptonema oxycerca 1.35 ngC.ind. − 1 h − 1 ). Experimental results were extrapolated to compute the consumption of meiofauna in situ that would attain up to 12% of primary benthic production in the Brouage mudflat (Marennes-Oleron Bay, southwest Atlantic coast).

The fluorochrome calcein has been used in numerous growth studies of molluscs to internally mark calcified structures. Because of interspecific variations in marking success and possible effects on growth performance, methodological assessments of the suitability of calcein as a growth marker, especially in field contexts, remain necessary. Here we report on the effects of different calcein concentrations (100, 200, 400 and 800 mg l(-1) on fluorescent mark deposition, growth rate, density, body condition and size-frequency distribution of an intertidal infaunal bivalve species, Loripes lacteus (Linnaeus, 1758), using an outdoor immersion technique. To avoid stress caused by handling and transportation, in situ enclosures were placed at seagrass-covered patches during low tide, to which calcein solutions were added. After a 1.3- to 2.6-h period of exposure to calcein, the enclosures were removed. Sites were sampled three months later. All calcein concentrations produced live L. lacteus with a clear fluorescent shell mark, but the percentage successfully marked tended to increase with higher calcein concentrations. Furthermore, marking success and growth rate decreased significantly with shell size (i.e., age). Calcein concentration did not measurably affect shell growth rate, body condition and size-frequency distribution, but the numerical densities were lower for sites treated with calcein concentrations >= 400 mg l(-1). Our results suggest that in situ calcein-marking of burrowing lucinid bivalves with low concentrations (100-200 mg l(-1)) is a non-invasive and rapid method to determine growth rate, provided that the lucinids are not too old. (c) 2011 Elsevier B.V. All rights reserved.

Intertidal mudflats are a dominant feature in many estuarine systems and may be a significant component of the feeding grounds available for many fish and bird species. Therefore, it is crucial to determine the importance and role that this particular habitat plays for the different estuarine communities. Spatial and temporal dynamics of macrobenthic communities in an intertidal mudflat of the Tagus estuary were assessed in order to determine the role of this habitat in the whole estuarine functioning. Benthic macroinvertebrate communities were sampled monthly in two intertidal areas (upper and lower) and in the adjoining subtidal area for one year. Macroinvertebrate density and biomass in the intertidal mudflat were higher than in the subtidal area, but no clear trends were found between the lower and upper intertidal area. Spatial patterns in the community were more pronounced than seasonal patterns. This benthic community was characterised by high densities of Pygospio elegans, Scrobicularia plana, Cyathura carinata, Hydrobia ulvae and Nereis diversicolor. Abundance and biomass values in this intertidal mudflat were considered low in comparison with other estuarine habitats, namely seagrass beds. Nevertheless, this habitat plays an important role for the main species present in the community, acting as a key area for recruitment, with high concentrations for many invertebrate species., Las marismas intermareales son características de muchos sistemas estuáricos y pueden abarcar un componente significativo de las áreas de alimentación disponibles para muchas especies de peces y aves. Por tanto, es crucial determinar la importancia y el papel que juega este hábitat para las diferentes comunidades estuarinas. Se evaluaron las dinámicas espaciales y temporales de las comunidades macrobentónicas en una marisma intermareal del estuario del Tajo, determinando el papel de este hábitat en el conjunto del funcionamiento de estuario. Las comunidades de macroinvertebrados bentónicos se muestrearon, mensualmente durante un año, en dos áreas intermareales (superior e inferior) y en las áreas submareales adyacentes. La densidad y la biomasa de los macroinvertebrados en las marismas intermareales fueron superiores que en el área submareal, y no se encontraron claras tendencias entre el intermareal superior e inferior. Los patrones espaciales en la comunidad fueron más pronunciados que los estacionales. Esta comunidad bentónica estuvo caracterizada por altas densidades de Pygospio elegans, Scrobicularia plana, Cyathura carinata, Hydrobia ulvae y Nereis diversicolor. Los valores de abundancia y biomasa en esta marisma intermareal fueron considerados bajos cuando se comparan con otros hábitats estuarinos como las praderas de fanerógamas. No obstante, este hábitat juega un importante papel para las principales especies presentes en la comunidad, actuando como un área clave para el reclutamiento, con altas concentraciones de muchas especies de invertebrados.

To assess the role of macrofauna-mediated sediment changes on nematode community recovery, we examined the temporal development of macrobenthos, nematode communities and sediment properties following hypoxia in 16 m2 replicated plots over a 6 mo period. Hypoxia drastically changed nematode community composition (i.e. reduced diversity and abundances of all dominant nematodes, except Odontophora spp.), but complete mortality, as was the case for the macrobenthos, did not occur. Macrofauna diversity recovered slowly, but community composition approached that of control communities after several months. In contrast, nematode diversity recovered to control values within 1 mo but, subsequently, decreased again; hence, no clear convergence towards the control community composition was apparent. This diversity decline and lack of community recovery was mainly attributed to abundance overshoots of the epistrate feeding nematodes Chromadora spp., Daptonema spp. and Ptycholaimellus ponticus in the treatments, which dominated the treatment community after 2 mo. Nematode community reassembling was strongly related to the coupled macrobenthos- environmental temporal development. The dynamics of 2 sediment characteristics, which were both mediated by the colonizing macrobenthos, are presented as possible determinant factors for this relationship: (1) low nematode post-settlement resuspension resulting from stable sediments at early macrofauna recovery stages and (2) enhanced nematode reproduction and settlement success in a dense microphytobenthos mat in relation to the temporal variation in macrobenthos grazing pressure and bioturbation. In conclusion, the strong relationships between macrobenthos recovery, environmental development and nematode community development after hypoxia highlight the importance of macrobenthos-sediment interactions in the recovery and structuring of nematode communities.

Lacking a propensity to emerge over the mud surface, the eel goby, Odontamblyopus lacepedii, survives low tide periods by continuously breathing air in burrows filled with hypoxic water. As with most marine air-breathing fishes, O. lacepedii does not possess an accessory air-breathing organ, but holds air in the buccal-opercular cavity. The present study aimed to clarify how the respiratory vasculature has been modified in this facultative air-breathing fish. Results showed that the gills apparently lacked structural modifications for air breathing, whereas the inner epithelia of the opercula were richly vascularized. Comparison with two sympatric gobies revealed that the density of blood capillaries within 10μm from the inner opercular epithelial surface in O. lacepedii (14.5 ± 3.0 capillaries mm-1; mean ± s.d., n = 3) was significantly higher than in the aquatic non-air-breathing Acanthogobius hasta (0.0 ± 0.0) but significantly lower than in the amphibious air-breathing mudskipper, Periophthalmus modestus (59.1 ± 8.5). The opercular capillary bed was supplied predominantly by the 1st efferent branchial arteries (EBA1) and drained by the opercular veins, which open into the anterior cardinal vein. Deep invaginations at the distal end of the EBA1 and the junction with EBA2 are suggestive of blood flow regulatory sites during breath-holding and apnoeic periods. It remains to be investigated how blood flow through the gills is maintained during breath holding when the buccal-opercular cavity is filled with air., Environmental Biology of Fishes, 82(4), pp.341-351; 2008

Semi-diurnal and fortnightly surveys were carried out to quantify the effects of wind- and navigation-induced high-energy events on bed sediments above intertidal mudflats. The mudflats are located in the upper fluvial part (Oissel mudflat) and at the mouth (Vasiere Nord mudflat) of the macrotidal Seine estuary. Instantaneous flow velocities and mudflat bed elevation were measured at a high frequency and high resolution with an acoustic doppler velocimeter (ADV) and an ALTUS altimeter, respectively. Suspended particulate matter concentrations were estimated by calibrating the ADV acoustic backscattered intensity with bed sediments collected at the study sites. Turbulent bed shear stress values were estimated by the turbulent kinetic energy method, using velocity variances filtered from the wave contribution. Wave shear stress and maximum wave-current shear stress values were calculated with the wave-current interaction (WCI) model, which is based on the bed roughness length, wave orbital velocities and the wave period (T-S). In the fluvial part of the estuary, boat passages occurred unevenly during the surveys and were characterized by long waves (T-S > 50 s) induced by the drawdown effect and by short boat-waves (T-S < 10 s). Boat waves generated large bottom shear stress values of 0.5 Nm(-2) for 2-5 min periods and, in burst of several seconds, larger bottom shear stress values up to 1 N m(-2). At the mouth of the estuary, west south-west wind events generated short waves (T-S < 10s) of H-S values ranging from 0.1 to 0.3 m. In shallow-water environment (water depth < 1.5 m), these waves produced bottom shear stress values between 1 and 2 N m(-2). Wave-current shear stress values are one order of magnitude larger than the current-induced shear stress and indicate that navigation and wind are the dominant hydrodynamic forcing parameters above the two mudflats. Bed elevation and SPM concentration time series showed that these high energy events induced erosion processes of up to several centimetres. Critical erosion shear stress (tau(ce)) values were determined from the SPM concentration and bed elevation measurements. Rough tau(ce) values were found above 0.2 N m(-2) for the Oissel mudflat and about 1 N m(-2) for the Vasiere Nord mudflat. These results demonstrate the advantages of combining the measurement of instantaneous velocity and bed elevation to determine in situ the erosion and deposition processes as a function of bottom shear stress variations. (c) 2006 Elsevier Ltd. All rights reserved.

The spatio-temporal dynamics of the microphytobenthic biofilm was analysed at microscale over a complete day-time emersion period in an intertidal mudflat. The structures of the biofilm were monitored by analysing the variability of the epipelic diatom cover in the horizontal dimension and the kinetics of biomass dispersion in the vertical dimension. Colonisation of the sediment surface by diatoms was rapid (within 15 min); 75% of the biomass contained in the top 1 mm was concentrated in the upper 200 mum, but the cover never reached 100%. This biomass had a higher chlorophyll a/pheopigment ratio than deeper in the sediment, suggesting a better physiological state and a higher photosynthetic potential. The dynamics of the biomass in the upper 200 mum turned out to be independent of that deeper in the sediment, supporting the concept of an independent surface compartment, mainly responsible for primary production. In the second part of emersion, the horizontal dispersion of the microphytobenthic biofilm was concomitant with the onset of downward migration. The process was slow, showing that the diatoms were not phased in their downward movement. The migration process appeared to be asymmetrical: rapid upward migration and biofilm formation versus slow downward movement and dispersion of the biofilm. To take into account these structural characteristics of the biofilm and to simulate its dynamics, we developed a new mathematical model that provides simulations consistent with our observations. We have also shown for the first time that the constitution of the biofilm at the sediment surface (0 to 200 mum) is connected to an increase of biomass in the top 1 cm.

In the present paper, we list and document the relevant behavioral and physiological processes controlling primary productivity of epipelic microalgae on intertidal mudflats in order to develop a simplified model. We first propose, in an attempt to characterize the 'photosynthetically active biomass' of the epipelic community, a new approach to describe the photic environment at the sediment surface, by substituting a discrete a-layer model in place of continuous vertical light distribution. This concept thus allows us to build a functional representation of the distribution of the photosynthetically active biomass in the sediment and, by then integrating the light and temperature forcing of the latter biomass, to predict the dynamics of the whole epipelic community on short-term time scales. The model then clearly reveals an oscillatory behavior of the total biomass at the surface of the sediment, consistent with field biomass daily measurements performed during a spring-neap tidal cycle. Biomass increases occur during the diurnal emersion phase, in alternation with decreases during the submersion and nocturnal emersion phases; the dynamics of intertidal microphytobenthos is thus controlled by the night/day cycle and tidal hydrodynamic forcing, which determines the fast changes in environmental conditions (light and nutrient availability, temperature) which epipelic microalgae experience. The similarity between model simulations and field observations leads us to conclude that the conceptual framework of the model is valid.

Relationships between bacteria, microphytobenthos and extracellular polymeric substances (EPS) that make up microbial biofilms over bare mudflats were investigated at an hourly frequency during two 14-day spring– neap cycles in winter and summer 2008. Bacterial abundance and total chl a concentration were lower in summer (0.78 × 108 ± SD 0.39 × 108 cell.m−2 and 59.0 ± SD 10.42 mgchla.m−2) than in winter (3.7 × 108 ± SD 1.9 × 108 cell.m−2 and 106.64 ± SD 11.29 mgchla.m−2), coinciding with a high abundance of the gastropod Peringia ulvae in summer, which subsequently impacted 1st-cm chl a concentration by intense grazing. Bound and colloidal EPS carbohydrate temporal patterns were similar in winter (5.71 ± SD 3.95 and 4.67 ± SD 3.45 μg.g−1, respectively) butwere different in summer (14.9±SD 4.05 and 5.60±SD 4.50 μg.g−1, respectively). Carbohydrate colloidal EPS appeared to be related to light and salinity, while 1st-mm chl a concentration was negatively affected by strong salinities and predation pressure by P. ulvae. The fluctuations of colloidal carbohydrates were remarkably similar in the two seasons with peaks just after spring tideswhen the highest irradiance was received by microphytobenthic cells. Apparently, colloidal EPS carbohydrates can protect cells against the high salinity values ranging from 32.3 to 50.4 PSU. The presence of bound EPS carbohydrates may be linked to sediment colonization and resistance of biofilm activity. Proteins in EPS were absent in winter and represented a small proportion in summer (10%), but they appeared to be a good indicator of potential synergistic effects between MPB and bacteria in summer. Conversely, bound EPS carbohydrates reached high levels in winter, while the number of bacteria decreased simultaneously, suggesting a negative effect on bacterial growth in the absence of proteins in EPS. There was a lower proportion (31%) of low molecular weight EPS in summer than in winter (83%), possibly in relation to desiccation.

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.

Network analysis was used to analyse steady-state models of the food webs of two intertidat mudflat ecosystems: Aiguillon Cove and Brouage Mudflat, on the South-Western Atlantic Coast of France. The aim was to highlight emergent properties of food-web functioning in these two ecosystems and to compare these properties with other coastal ecosystems. Both ecosystems imported detritus in parallel to a high benthic primary production. They were characterised by a high diversity of resources. Both also exported living material, leading to a high quality production, quantified as export of Exergy. This export was mainly composed of cultivated bivalves during the cold season for Brouage Mudflat, and of the migration of grazing fish in Aiguillon Cove during the warm season. Their internal organization was characterised by short pathways, high recycling, high redundancy and low net ecosystem production, compared to the other systems selected. These characteristics, encountered in many estuaries, presented less extreme values. (c) 2007 Elsevier Ltd. All rights reserved.

International audience; The composition and seasonal dynamics of biofilm-associated eukaryotic communities were analysed at the metre and kilometre scale along a salinity gradient in the Westerschelde estuary (The Netherlands), using microscopy and a genetic fingerprinting technique (PCR-DGGE). Microphytobenthic biomass, measured as chlorophyll a (chl a), varied seasonally over 2 orders of magnitude, being highest in spring. Communities were dominated by epipelic diatoms, in particular by members of the genus Navicula. In spring, a few smaller epipelic diatom species dominated during biomass peaks, while during the rest of the year, communities were more diverse and were characterised by larger species. The microphytobenthic community collapsed when grazers appeared, which happened concomitantly with a rise in temperature. Spring biomass development was associated with marked changes in porewater nutrient concentrations, especially towards the estuary mouth. In the DGGE data, diatoms, ciliates, amoebae, copepods, nematodes, annelids and platyhelminthes were detected. Ordination analysis of the species counts and DGGE data were largely congruent and indicated that on the scale of the whole estuary (i.e. km scale), taxonomic turnover in microphytobenthos composition was mainly associated with the salinity gradient. At smaller spatial scales, the position of sampling localities along the tidal exposure gradient appeared to be the main determinant of species turnover, in particular in the brackish reaches of the estuary.

A portable erosion device (ERIS) was deployed in November 2000 and May 2001 to investigate the spatio-temporal variation of fine-grained sediment erosion over a transect on the intertidal mudflats of Marennes-Oleron Bay, western France. The objective was to relate changes in bed erodability over diurnal emersion periods to bioturbation, rapid changes in sediment characteristics, and microphytobenthos biomass. Macrofaunal assemblages were dominated by the bivalve, Scrobicularia plana, on the upper mudflat, and by the gastropod, Hydrobia ulvae. on the middle mudflat. A power relationship was obtained where tau(cr) was plotted as a function of sediment density. Results from the upper part of the mudflat deviated from this relationship due to bioturbation by S. plana. No influence of microphytobenthos (either chlorophyll a or extracellular polymeric substances (EPS) content) was found on tau(cr).The bivalve feeding activities altered consolidated sediments (sediment dry mass concentration of similar to 700 g1(-1)) so that they became resuspended easily (tau(cr) < 1 Pa). Fluff layer erosion was observed, with an erosion rate dependent upon the biomass of H. ulvae. It was also found that bioturbation by H. ulvae was implicated (combined with grazing activities) in chl a resuspension during the erosion of the fluff layer. Snail bioturbation stimulated the chl a enrichment of eroded material. whilst the high snail biomass produced a decrease in suspended chl a due to higher grazing rates. (c) 2007 Published by Elsevier Ltd.

Spatio-temporal changes in taxonomic composition and structure of an epipelic diatom assemblage from an intertidal mudflat on the French Atlantic coast was studied over an annual cycle along a cross-shore transect. The assemblage structure was described by estimating both relative abundance and contribution to biovolume of each species. Results showed that the assemblage was numerically dominated by small-sized species (mean relative abundance of 91%). Large species, however, significantly contributed to the total biovolume (mean contribution to biovolume of 49%). A factorial correspondence analysis indicated that the epipelic assemblage was relatively homogeneous along the cross-shore transect but emphasized the seasonal succession of diatom species. In relative abundance, the assemblage structure was characterized by the dominance of the small species Navicula phyllepta throughout the year with a seasonal succession of secondary species, including only one large diatom (Gyrosigma peisonis). In biovolume, small (N. phyllepta and Navicula gregaria) and large species (Pleurosigma angulatum and G. peisonis) alternatively dominated the assemblage. Describing the epipelic assemblage using cell volume emphasized the contribution of large species and revealed that the assemblage contained two diatom fractions, characterized by different biological and physiological behaviours, which may alternatively represent a large proportion of the biomass. (c) 2005 Elsevier Ltd. All rights reserved.

Flume experiments were designed to study how sediment erodibility was modified by the facultative suspension-feeder Scrobicularia plana living at different densities in sediment beds of various degrees of compaction. Two separate erosion phases were identified from the resuspension kinetics: (1) erosion of an unconsolidated surficial layer (i.e. fluff layer) and (2) the subsequent bed erosion. S. plana were found to influence both erosion phases: (1) erosion rates of the fluff layer were controlled by bioturbation activities, the extents of which were influenced by bivalve density and the degree of compaction of the sediments, and (2) critical thresholds of the subsequent bed erosion decreased as clam density increased, likely due to depressions in the sediment bed that were created by siphon activity. A 1-dimensional vertical model was modified to incorporate the effects of S. plana on fluff layer formation through pseudofaeces production. The modifications to the model and results are compared to a similar approach for Hydrobia ulvae published earlier, and are discussed with respect to the definition of functional groups of bioturbators in sediment transport models. By discussing effects of spatial grouping and analogies to other organisms, this study shows how the influence of one species on sediment resuspension may be generalised and used for model descrip- tions of other species of the same behavioural type. Model developments on pseudofaeces production show that there are asymmetric effects generated over time and by bivalve density for S. plana, which do not occur for H. ulvae.

Many scientists consider that the top centimetre of the sediment on intertidal mudflats contains the photosynthetic competent biomass (PCB). Part of this biomass migrates upward to the surface of the sediment during diurnal emersion periods to form a temporary biofilm: the photosyn- thetic active biomass (PAB). The present study tests the hypothesis that the size of the biofilm (PAB) is functionally dependent on PCB. Therefore, we have plotted PAB as a function of PCB for a range of different environmental conditions: 3 seasons and 5 tidal situations, representing a total of 600 cores. This investigation points out that, in spring and autumn, there was a simple linear relationship between PAB and PCB, thus indicating that the maximum size of the biofilm represents a constant fraction (between 13 and 18%) of the available biomass in the top centimetre of the mud. The rela- tionship was more complex in winter, but the general trend remained the same. The dependence of the size of the biofilm on PCB might have important ecological implications in terms of primary productivity and also the monitoring of intertidal microalgal biomass at large spatial scales.

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvac (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m(2) h(-1) snail(-1)), Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1 - 15 mug g(-1). Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvac, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment-water interface for various in situ conditions. (C) 2002 Elsevier Science B.V. All rights reserved.

International audience; 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.

International audience; 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.

International audience; 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-Oléron 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 chl a m-2) than in January (ca 85 mg chl 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 for 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 chl a m-2). Total biomass estimates were ca 13 t chl a (SE = 0.26) in June and 10 t chl 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 µg chl a l-1, which corresponds to 30-90% of the chl 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-Oléron Bay.

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 effect of physical disturbance in the form of trampling on the benthic environment of an intertidal mudflat was investigated. Intense trampling was created as unintended side-effect by benthic ecologists during field experiments in spring and summer 2005, when a mid-shore area of 25 × 25 m was visited twice per month by on average five researchers for a period of 8 months. At the putatively-impacted location (I) (25 × 25 m) and two nearby control locations (Cs) (25 × 25 m each), three sites (4 × 4 m) were randomly selected and at each site, three plots (50 × 50 cm) were sampled after 18 and 40 days from the end of the disturbance. Multivariate and univariate asymmetrical analyses tested for changes in the macrofaunal assemblage, biomass of microphytobenthos and various sediment properties (grain-size, water content, NH4 and NO3 concentrations in the pore water) between the two control locations (Cs) and the putatively-impacted location (I). There were no detectable changes in the sediment properties and microphytobenthos biomass, but was observed. Microphytobenthos and NH4 were correlated at I to the number of footprints, as estimated by the percentage cover of physical depressions. This indicated that trampling could have an impact at small scales, but more investigation is needed. Trampling, instead, clearly modified the abundance and population dynamics of the clam Macoma balthica (L.) and the cockle Cerastoderma edule (L.). There was a negative impact on adults of both species, probably because footsteps directly killed or buried the animals, provoking asphyxia. Conversely, trampling indirectly enhanced recruitment rate of M. balthica, while small-sized C. edule did not react to the trampling. It was likely that small animals could recover more quickly because trampling occurred during the growing season and there was a continuous supply of larvae and juveniles. In addition, trampling might have weakened negative adult-juvenile interactions between adult cockles and juvenile M. balthica, thus facilitating the recruitment. Our findings indicated that human trampling is a relevant source of disturbance for the conservation and management of mudflats. During the growing season recovery can be fast, but in the long-term it might lead towards the dominance of M. balthica to the cost of C. edule, thereby affecting ecosystem functioning.