| Summary: | The seasonal distribution (summer-autumn) of microphytoplankton biomass coupled with environmental factors was studied at four ponds of increasing salinity (A5, A16, C4-1, M2) in the solar saltern of Sfax. Subsamples for microphytoplankton (20–200 μm) counting were analysed under an inverted microscope using the Utermöhl method after settling for 24 to 48 hr (Utermöhl, 1958). The results obtained during this study explain the importance of salinity in the microphytoplankton communities’ structure. Microphytoplankton biomass values show a large range of variations since they fluctuate from 23.47 μgC l-1(M2) to 753.35 μgC l-1(C4-1) during the autumn season. Low microphytoplankton biomasses were recorded during the summer (min = 35.89 -max = 293.48 μgC l-1) at ponds C4-1 and A5 respectively.The microphytoplankton community consists of 68 taxa belonging to five groups composed of diatoms, dinoflagellates, cyanobacteria, chlorophyceae and euglenophyceae. Diatoms and dinoflagellates account for more than 80% ofthe total microphytoplankton biomass at ponds A5,A16 and C4-1. Microphytoplankton was characterised by the proliferation of pinnate diatoms(Navicula sp. et Nitzschia sp.) andGymnodinium sp., Prorocentrum micans, Ackashiwo sanguineaand Peridiniumsp. for dinoflagellates. In the hypersaline ponds (M2), cyanobacteria (Oscillatoriasalina, Aphanothece halophytica, Phormidiumversicolorand Spirulina subsalsa)and mainly chlorophyceae (Chlamydomonas rubrifilumand Dunaliella salina) showed the highest biomasses 49% for cyanobacteria and 16% for chlorophyceae during autumn. There was a clear spatial and seasonal variation in microphytoplankton communities and it was noted that species richness and biomass decreased with the increasing salinity gradient.
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