The structure of intertidal benthic diatoms assemblages in the Tagus estuary was investigated during a 2-year survey, carried out in six stations with different sediment texture. Nonparametric multivariate analyses were used to characterize spatial and temporal patterns of the assemblages and to link them to the measured environmental variables. In addition, diversity and other features related to community physiognomy, such as size-class or life-form distributions, were used to describe the diatom assemblages. A total of 183 diatom taxa were identified during cell counts and their biovolume was determined. Differences between stations (analysis of similarity (ANOSIM), R = 0.932) were more evident than temporal patterns (R = 0.308) and mud content alone was the environmental variable most correlated to the biotic data (BEST, ρ = 0.863). Mudflat stations were typically colonized by low diversity diatom assemblages (H' ~ 1.9), mainly composed of medium-sized motile epipelic species (250-1,000 μm(3) ), that showed species-specific seasonal blooms (e.g., Navicula gregaria Donkin). Sandy stations had more complex and diverse diatom assemblages (H' ~ 3.2). They were mostly composed by a large set of minute epipsammic species (<250 μm(3) ) that, generally, did not show temporal patterns. The structure of intertidal diatom assemblages was largely defined by the interplay between epipelon and epipsammon, and its diversity was explained within the framework of the Intermediate Disturbance Hypothesis. However, the spatial distribution of epipelic and epipsammic life-forms showed that the definition of both functional groups should not be over-simplified.

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Spatiotemporal changes in microphytobenthos species composition were investigated in relation to structural variables—biomass; life‐forms; detritus ratio, pheopigment a (pheo a):chl a; and sediment characteristics—at mesoscale in a shellfish macrotidal ecosystem. To characterize algae assemblages, multivariate analyses were performed (multidimensional scaling [MDS] ordination and clustering), supported by correlations between structural variables. Microphytobenthos, dominated by 97% diatoms belonging to 89 taxa, was constituted by two main assemblages: The first one, composed of common species (occurrence >50%), mainly nanobenthic (size <30 μm) and epipsammic (sand‐fixed cells), was associated with mixed sediments. The major species contributing to this assemblage were Navicula perminuta, Achnanthes hauckiana, Fallacia tenera, A. lanceolata var. elleptica, Amphora spp., Plagiogrammopsis vanheurkii, and Plagiogramma tenuissimum (see Table 2 for taxonomic authors). Species diversity was high (N1∼ 11), often linked to high biomass values (>70 mg chl a · m−2) and low detritus ratio. Conversely, the second assemblage comprised occasional species (occurrence from 10% to 50%), mainly microbenthic (>30 μm) and epipelic (moving cells), occurring during summer at muddy sites. This assemblage, characterized by low diversity, high biomass variability, and high detritus ratio, was dominated by Scolioneis tumida or Plagiotropis vitrea and Navicula spartinetensis. Whereas hydrodynamics globally explained the gradual assemblage changes throughout the entire mudflat and the year, oyster beds and ridge and runnel features appeared to be local spatial structuring factors allowing the establishment of specific assemblages. This study suggests a significant role for epipsammon biomass, until now underestimated, in the functioning of this turbid coastal ecosystem. Microphytobenthic taxa identified from samples from the first 2 mm of sediment from Bourgneuf Bay (France) mudflat between May 2000 and July 2001. Taxa Abbreviation Ecologya Size Occurrence (%) Common species  Plagiogrammopsis vanheurkii (Grunow) Hasle, Stosch et SyvertsenPlagp vanhEpipsammicNanobenthic82.50  Amphora spp.Amph spp.EpipsammicNanobenthic81.25  Achnanthes hauckiana GrunowAchn haucEpipsammicNanobenthic78.75  Navicula perminuta GrunowNavi permEpipelicNanobenthic76.25  Thalassiosira decipiens (Grunow) Jörg.Thal deciEpipsammicNanobenthic76.25  Fallacia tenera (Hust.) D. G. MannFall teneEpipelicNanobenthic73.75  Achnanthes lanceolata var. elleptica CleveAchn lancEpipsammicNanobenthic71.25  Cymatosira belgica GrunowCyma belgEpipsammicNanobenthic68.75  Navicula spartinetensis M. J. Sullivan et ReimerNavi sparEpipelicNanobenthic68.75  Plagiogramma tenuissimum Hust.Plagg tenuEpipsammicNanobenthic66.25  Cocconeis hoffmanni SimonsenCocc hoffEpipsammicNanobenthic65.00  Biremis ambigua D. G. MannBire ambiEpipsammicNanobenthic58.75  Odontella aurita (Lyngb.) C. AgardhOdon auriEpiphyticMicrobenthic57.50  Navicula anglica RalfsNavi anglEpipelicNanobenthic56.25  Cocconeis scute...

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Présenté par Lucien Laubier RésuméL'objectif de ce travail est d'évaluer l'intérêt de la télédétection multispectrale (SPOT) dans le domaine visible-infrarouge, pour l'étude des peuplements du microphytobenthos à l'échelle d'un écosystème côtier turbide, la baie de Bourgneuf (France), baie à caractère conchylicole. Cette étude repose sur l'étalonnage d'images satellitales à partir de mesures radiométriques de terrain, étape indispensable avant toute analyse d'image. Après étalonnage, le calcul de l'indice de végétation normalisée (NDVI) a permis la localisation du microphytobenthos sur la vasière de la baie de Bourgneuf. Sur les six images SPOT traitées, prises de 1986 à 1998, quatre d'entre elles montrent des peuplements de microphytobenthos se développant le long d'une bande au niveau de l'isobathe +2 m des cartes marines, orientée nord-sud, occupant une superficie variant entre 420 et 550 ha, soit entre 19 et 25% de la surface de la zone d'étude de la vasière. Les facteurs semblant intervenir dans la structuration spatiale des peuplements microphytobenthiques sont la bathymétrie, les apports de sels nutritifs en provenance de la rivière et du chenal du Falleron, qui est un des plus important exutoire des marais avoisinants, et la localisation des parcs ostréicoles confortant l'hypothèse d'une rétroaction entre les peuplements et les huîtres cultivées. Bien que cette étude fasse apparaître les limites des résolutions spectrale et spatiale de la télédétection multispectrale, elle laisse entrevoir de nombreuses perspectives quant à l'utilisation de la télédétection hyperspectrale, notamment l'obtention de cartes de biomasses.