Spatiotemporal changes in microphytobenthos assemblages in a macrotidal flat (Bourgneuf Bay,France)1

Spatiotemporal changes in microphytobenthos species composition were investigated in relation to structural variables—biomass; life‐forms; detritus ratio, pheopigmenta(pheoa):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 wereNavicula perminuta,Achnanthes hauckiana, Fallacia tenera, A. lanceolatavar.elleptica, Amphoraspp., Plagiogrammopsis vanheurkii, andPlagiogramma 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 byScolioneis tumidaorPlagiotropis vitreaandNavicula 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 Syvertsen Plagp vanh Epipsammic Nanobenthic 82.50 Amphoraspp. Amph spp. Epipsammic Nanobenthic 81.25 Achnanthes hauckianaGrunow Achn hauc Epipsammic Nanobenthic 78.75 Navicula perminutaGrunow Navi perm Epipelic Nanobenthic 76.25 Thalassiosira decipiens(Grunow) Jörg. Thal deci Epipsammic Nanobenthic 76.25 Fallacia tenera(Hust.) D. G. Mann Fall tene Epipelic Nanobenthic 73.75 Achnanthes lanceolatavar.ellepticaCleve Achn lanc Epipsammic Nanobenthic 71.25 Cymatosira belgicaGrunow Cyma belg Epipsammic Nanobenthic 68.75 Navicula spartinetensisM. J. Sullivan et Reimer Navi spar Epipelic Nanobenthic 68.75 Plagiogramma tenuissimumHust. Plagg tenu Epipsammic Nanobenthic 66.25 Cocconeis hoffmanniSimonsen Cocc hoff Epipsammic Nanobenthic 65.00 Biremis ambiguaD. G. Mann Bire ambi Epipsammic Nanobenthic 58.75 Odontella aurita(Lyngb.) C. Agardh Odon auri Epiphytic Microbenthic 57.50 Navicula anglicaRalfs Navi angl Epipelic Nanobenthic 56.25 Cocconeis scutellumEhrenb. Cocc scut Epiphytic Microbenthic 55.00 Fragilaria pinnataEhrenb. Frag pinn Epipsammic Nanobenthic 53.75 Cocconeis peltaA. W. F. Schmidt Cocc pelt Epipsammic Nanobenthic 51.25 Occasional Raphoneis amphicerosEhrenb. Raph amph Epipsammic Microbenthic 41.25 Actinoptychus undulatus(J. W. Bailey) Ralfs Acti undu Epipsammic Nanobenthic 38.75 Odontella rhombus(Ehrenb.) Kütz. Odon rhom Benthic Microbenthic 37.50 Navicula rostellataKütz. Navi rost Epipelic Microbenthic 37.50 Brockmaniella brockmanii(Grunow) Hasle, Stosch et Syvertsen Broc broc Epipsammic Nanobenthic 36.25 Plagiotropis vitrea(W. Smith) Cleve Plagt vitr Epipelic Microbenthic 35.00 Paralia marina(W. Smith) Heib. Para marina Epipsammic Nanobenthic 35.00 Delphineis minutissima(Hust.) Simonsen Delp minu Epipsammic Nanobenthic 32.50 Pleurosigma aestuari(Bréb.) W. Smith Pleu aest Epipelic Microbenthic 31.25 Dimerogramma minor(W. Greg.) Ralfs Dime minor Epipsammic Nanobenthic 30.00 Podosira stelliger(J. W. Bailey) D. G. Mann Podo stel Epipsammic Microbenthic 30.00 Staurophora amphioxys(W. Greg.) D. G. Mann Stau amph Epipelic Microbenthic 23.75 Nitzschiacf.frustulum(Kütz.) Grünow Nitz frus Epiphytic Nanobenthic 23.75 Gyrosigma fasciola(Ehrenb.) J. W. Griff. et Henfr. Gyro fasc Epipelic Microbenthic 22.50 Scolioneis tumida(Bréb.) D. G. Mann Scol tumi Epipelic Microbenthic 22.50 Gyrosigma distortum(W. Smith) Cleve Gyro dist Epipelic Microbenthic 20.00