Annual changes in abundance, biomass, and taxonomic composition of potamoplanktonic ciliates were assessed in the 139 km long, coastal (Baltic coast) river (the Słupia River, Northern Poland) having mean annual flow – 18 m³ s⁻¹and flowing through several natu ral lakes and reservoirs. The samples were taken from one site, once a week from December 2005 to December 2006, then concentrated by sedimentation and counted and measured under an inverted microscope. Ciliate production was assessed with relevant published formulas. Mean annual abundance and biomass amounted to 6448 cells l⁻¹ and 6.60 μg C l⁻¹, respectively. During the year two distinct peaks of biomass were observed: a spring peak in April and May, and a winter one in December. Spring peak was formed by prostomatids and oligotrichs, whereas winter one by peritrichs. Three aforementioned orders contributed altogether to 64% of abundance and 82% of ciliate biomass. Prevalence of prostomatids seemed to be characteristic for the studied river. Most of ciliates were surprisingly small, fraction below 20 μm contributed 77% of the mean annual abundance. Domination of small ciliates indicated high mean annual potential growth rates ranging from 0.6 d⁻¹ to 0.7d⁻¹. The river is characterised by a narrow variability of the water level and numerous through-flow lakes. As a result ciliate community in the river resembled communities encountered in lakes – free-swimming organisms dominated and sessile forms and those detached from sediments contributed not more than one fifth of ciliate biomass.
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