Dynamics of ciliate abundance, biomass and community composition in an oligotrophic coastal environment (NW Mediterranean)

The importance of ciliates as components of the microbial community of the oligotrophic coastal area of the Bay of Blanes (NW Mediterranean Sea) was examined based on a 3 yr, high resolution study focused on the composition, abundance and biomass of the ciliate community. The most abundant components of the ciliate community were 'oligotrich' ciliates. Naked oligotrichs included heterotrophic genera represented by Halteria, Strombidium, Strobilidium, and Lohmaniella, as well as mixotrophic genera represented by Laboea and Tontonia and loricate ciliates represented by the group of tintinnids. Autotrophic ciliates were represented by the genus Mesodinium. Other, less abundant groups encountered throughout the study period included the orders Scuticociliatida, Pleurostomatida and Prorodontida. Ciliate community abundance and biomass did not show a simple seasonal pattern. Maximum values were observed in spring, following the winter phytoplankton blooms, throughout the study period. Ciliate communities showed significant interannual differences in abundance and cell size. However, total ciliate biomass (μg C I^-1) was similar among years. Changes in ciliate abundance and biomass were independent of temperature. Periods with persistent, heavy rainfall, which promotes pulses of allochthonous material from flushed rivers, were characterized by a reduced abundance of ciliates, and increased pico- and nanoplanktonic populations. Cross correlation analysis revealed that bacterial abundance and chlorophyll a (chl a) concentration were both significantly negatively correlated with ciliate abundance, with time lags of 15 d, suggesting a role for ciliates in the control of these communities. Examination of the variability of ciliate abundance and biomass at different time scales revealed a dominant scale of temporal variation in ciliate abundance at about 50 d, similar to that of chl a in the Bay of Blanes, whereas total ciliate biomass (μg C I^-1) did not show any dominant scale of variation.