Effect of turbulence on sedimentation and net population growth of the dinoflagellate Ceratium tripos and interactions with its predator, Fragilidium subglobosum

We investigated the effect of natural levels of turbulence on the vertical distribution and net population growth of the dinoflagellate Ceratium tripos and on the interaction with its predator, the mixotrophic dinoflagellate Fragilidium subglobosum. Unialgal cultures of each species were exposed to four kinetic energy dissipation rates, epsilon: 0.0001, 0.01, 0.05, and 1 cm(2) S-3, generated by vertically oscillating grids in 2-liter cylindric containers. Autotrophic growth of F. subglobosum was not affected by any level of turbulence tested. In contrast, at epsilon >= 0.05 cm(2) S-3 (this value is generated in the upper 10 m of the ocean by a moderate gale or at 0.5 In in depth by a gentle breeze), C. tripos decreased its net population growth, and the vertical distribution of the cells was affected. At the highest turbulence level, C. tripos stopped swimming, settled, and accumulated at the bottom. Mixotrophic growth of F. subglobosum, when fed C tripos at high densities (i.e.,>> 10 prey cells mL(-1)) was not affected by turbulence. However, at low prey cell densities (i.e., 5 to 8 C. tripos cells mL(-1)), growth and ingestion rates of F. subglobosum were significantly higher at the highest turbulence level compared to the rates at other turbulence levels and were close to the rates measured at high prey cell densities, presumably because sedimentation of C. tripos cells resulted in patches where the cell densities were not food-limiting for F. subglobosum.