Growth rates of dominant planktonic ciliates in two freshwater bodies of different trophic degree

Abstract

The in situ growth rates of dominant ciliate species were studied during and shortly after phytoplankton peaks in two water bodies: the eutrophic Římov Reservoir (South Bohemia, Czech Republic) and the oligo-mesotrophic Piburger See (Tyrol, Austria). Growth rate estimates based on changes in ciliate abundances in incubated pre-screened samples (EN) were compared with those derived from the ciliate cell volume and ambient temperatures (ET). The values of EN were always rather lower than those of ET. During the studies, the food supply limited the ciliate growth depending on the ciliate feeding mode. An ecological grouping into filter feeding versus raptorial feeding ('hunting') species, on the one hand, and attached/crawling (browsing) versus free swimming species, on the other hand, clearly affected experimental estimation. Both fine filter feeders (namely attached) and browsers exhibited a calculated EN closer to the theoretical (maximum) ET than did hunters and coarse filter feeders. It was apparent, for example, comparing EN and ET (day-1) of the following species: filter feeders Halteria grandinella (EN = 0.42; ET > 1.4), Strobilidium hexakinetum (0.34;>1.9), Pelagohalteria viridis (0.27; >0.9), Vorticella aquadulcis complex (0.75; >1.0); raptorial Balanion planctonicum (0.65; >1.5), Urotricha furcata (in Římov Reservoir 0.65; >2.1; in Piburger See 0.20; >1.5), Rhabdoaskenasia minima (0.22; >1.0), Askenasia acrostomia (0.12; >0.6); opportunistic Cyrtolophosis mucicola (0.42; >1.6) and Cinetochilum margaritaceum (0.86; >1.4). Predation by rotifers apparently affected measurements in several samples containing ∼400 rotifers I-1; however, it seemed to be of little importance in the water column.