Numerical and functional responses of choreo- and oligotrich planktonic ciliates

Abstract

A total of 6 planktonic naked ciliates were isolated from the Oslofjord, Norway. Numer- ical and functional responses were investigated using bottle incubations at 11 to 14°C, with the prasinophyceae Nephroselmis pyriformis and cryptophyceae Hemiselmis sp. as food. Growth rates followed hyperbolic curves, which levelled off at about 15 000 algal cells ml -1 (152 µg C l -1 ). Highest growth rate was recorded for the large heterotroph Strobilidium spiralis (μmax = 1.38 ± 0.08 d -1 (mean ± SE). A small mixotroph, Strombidium vestitum, displayed high growth rates (μmax = 1.04 ± 0.05 d -1 ), compared to the larger strombidiids Strombidium conicum, S. acutum and Strombidium sp. (μmax = 0.55 to 0.80 d -1 ). Food thresholds for positive net growth rates were estimated to be in the range of 600 to 3300 algal cells ml -1 , or 6 to 34 µg C l -1 . Specific ingestion rates increased at increasing food concentrations, and maximum measured rates were generally in the range of 2 to 4 d -1 . S. spiralis increased its clearance rate to a maximum of 18.3 ± 0.69 µl h -1 as algal concentrations were enhanced from 900 to 2600 algal cells ml -1 . Gross growth efficiency ranged from 7 to 52% above threshold food levels, with a mean and median of 22 and 23%, respectively.