Population dynamics of the marine planktonic ciliate strombidinopsis multiauris: Its potential to control phytoplankton blooms

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 1.3 × 103 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 × 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.