Seasonal variations of mixotrophic ciliates in the northwest Mediterranean Sea

The seasonal distribution of mixotrophic oligotrichous ciliates was compared to that of the strict heterotrophic oligotrichs and of the autotrophic haptorid Mesodinium rubrum, in surface coastal waters of the Ligurian Sea, from March 1988 to December 1989.The results show that ( l ) mixotrophic cili a t e ~, which were observed year-round, average 51 % of the overall biovolume (from 45% for cells 30 to 50 pm to 57 O/u for cells > 5 0 pm); (2) mixotrophs dominate the ciliate populations in spring and in suinmer (average >60 %), maximum 100%); (3) pigmented ciliates, mixotrophic oligotrichs and M. rubrum dominate the biovolume of ciliates in the size ranges <30 and >50 pm; (4) besides numerous Strombidium spp., S mixotrophic species belonging to the genera Tontonia, Laboea and Lohmaniella were observed; and (5) a positive correlation was found between biovolume of pigmented ciliates and light and between biovolume of mixotrophic ciliates and biovolume of cyanobacteria.