Inhibition of growth rate and swimming speed of the harmful dinoflagellate Cochlodinium polykrikoides by diatoms: Implications for red tide formation

• S. costatum, C. danicus, and T. decipiens reduced swimming speed of C. polykrikoides.
• Filtrates from cultures of these 3 diatoms also lowered swimming speeds.
• S. costatum and C. danicus caused negative growth rates of C. polykrikoides above certain diatom concentrations.
• Diatoms may inhibit vertical migration of C. polykrikoides and its red tides.

The harmful dinoflagellate Cochlodinium polykrikoides is responsible for red tides that cause large fish kills and extensive economic losses for the fishing industry. Diatoms, another component of planktonic communities, may play critical roles in the red tide dynamics of C. polykrikoides. Possible inhibition of C. polykrikoides growth rate and swimming speed by the common diatoms Chaetoceros danicus, Skeletonema costatum, and Thalassiosira decipiens through physical and chemical mechanisms was explored. S. costatum, C. danicus, and T. decipiens reduced the swimming speed of C. polykrikoides at diatom concentrations of >5000, 25,000, and 1000 cells ml−1, respectively. Filtrates from cultures of S. costatum, C. danicus, and T. decipiens also lowered swimming speeds of C. polykrikoides, at diatom concentrations of >250,000, 50,000, and 1000 cells ml−1, respectively. S. costatum caused negative growth rates of C. polykrikoides at concentrations of >∼130,000 cells ml−1, while C. danicus caused negative growth rates at concentrations of >∼1200 cells ml−1. Simple models parameterized using the experimental data reproduced the changes in C. polykrikoides cell concentrations driven by the presence of diatoms. Thus common diatoms may inhibit growth rate and swimming speed of C. polykrikoides; reduce the depths reached by C. polykrikoides through vertical migration; and, in turn, delay or prevent the outbreak of C. polykrikoides red tides.