AME 20:167-181 (1999)  -  doi:10.3354/ame020167

ABSTRACT: The growth, grazing, and cell volume of Strombidinopsis multiauris, a large (~100 μm) coastal planktonic ciliate, is affected by food concentration and temperature. Using growth and grazing data, we modelled small-scale bloom dynamics between the ciliate and its prey. Growth experiments were conducted at 13°C on S. multiauris fed the 10 μm dinoflagellate Gymnodinium simplex; changes in cell numbers and cell volume were monitored. Ingestion rate was measured by 3 methods (uptake of fluorescently labelled latex beads; heat-killed, fluorescently labelled G. simplex; and ¹⁴C-labelled G. simplex). Growth rate versus food concentration followed a rectangular hyperbolic response, with a maximum of μ ~= 0.6 d^-1 above 10⁴ prey ml^-1 (480 ng C ml^-1); below 1.3x10³ ml^-1 (62 ng C ml^-1), mortality occurred. Cell volume followed a rectangular hyperbolic response to food concentration, and showed a doubling in size between zero and maximum prey levels. Grazing rate initially increased with food concentration and was then inhibited at levels >10⁴ prey ml^-1. The ciliate ingested ¹⁴C-labelled live prey at higher rates than either dead or artificial prey at subsaturating concentrations; above saturating concentrations, ingestion rates were similar for the 3 prey types. The maximum observed grazing rate was 35 prey ciliate^-1 h^-1. Growth rate and cell volume were measured under steady-state conditions at 9 temperatures between 3.5 and 22°C: ciliates died at 3.5 and 5°C, growth rate increased linearly to a maximum of μ ~= 0.9 d^-1 at 15°C, did not change between 15 and 20°C, and decreased at 22°C. Cell volume increased between 5 and 10°C and decreased between 10 and 22°C. The population dynamics model revealed that the ciliate was able to control the dinoflagellate population. Over the 20 d model simulation, virtually no predator-prey cycle occurred when prey growth rates were μ < 0.2 d^-1. As prey growth rate was increased bloom dynamics became apparent, with a minimum duration of ~10 d for a bloom to begin and end at a prey growth rate of μ = 0.65 d^-1. During these simulated blooms ciliates reached maximum levels of 35 cells ml^-1, and prey reached levels of 1.7 x 10⁴ cells ml^-1, similar to numbers found in a typical coastal bloom. Our data and model suggest that ciliates and their prey produce episodic, short-term blooms, and we recommend that these events be evaluated more carefully in the field and be incorporated into models.

KEY WORDS: Blooms · Cell volume · Grazing rate · Growth rate · Microzooplankton · Mortality rate · Oligotrich ciliate · Plankton · Temperature response