Phagotrophic potentialities of phytoflagellates in the Allal El Fassi Reservoir (Morocco)
The numerical abundances of bacterial and phytoplanktonic cells and the bacterivorous activity from phytoflagellates were measured at different depths from march to December 1998 in the Allal El Fassi Reservoir (Morocco). Bacterial community was dominated by free-living spherical cells. Their abundance fluctuated from 1.0 to 9.8·106 cells·mL-1, with the highest values occuring in spring, coinciding with the beginning of the thermal stratification and the bloom of the centric diatom Cyclotella oceallata. Bacterial number then decreased drastically in summer and increased slightly in autumn, when two chrysophytes (Dinobryon sertularia and D. cylindricum) dominated the algal community, accompagnied by two dinoflagellates (Peridinium cinctum and Ceratium hirundinella) and by the cryptophyte Cryptomonas ovata. Grazing experiments using bacterial tracers indicated that these five algal species were exhibiting a bacterivorous activity. The impact of predation from these mixotrophic community varied from 1.2 to 357·104·bacteria·L-1·h-1, about 81% of the impact being related to the activity of the two chrysophytes, vs 12% for the two dinoflagellates and 7% for the cryptophyte. We conclude that mixotrophic phytoflagellates play a significant role in the functioning of the microbial food web in the Allal El Fassi Reservoir, primarily by controlling the bacterial development in autumn.