Potential influence of the proliferation of sediment-based diatoms on blooms of a harmful dinoflagellate Cochlodinium polykrikoides: a microcosm approach

Typhoons cause significant environmental damage in coastal areas and one of their effects is the suspension of the resting stage cells of diatoms from the surface sediment. We performed microcosm experiments in 10-L containers using natural sediments from three different sites of southern Korean coastal waters (Geoje, Goheung, and Tongyeong) to simulate the effect of suspension of sediment-based diatoms by a typhoon on blooms of the harmful dinoflagellate Cochlodinium polykrikoides. This dinoflagellate grew well under control conditions and exhibited a maximum abundance of 985 cells mL(-1) on day 10, but all treatment groups (Geoje, Goheung, and Tongyeong) had decreased abundances by day 4 and fewer than 50 cells mL(-1) on day 10. As C. polykrikoides declined, two diatoms (Skeletonema spp. and Chaetoceros spp.) dominated in the three treatment groups. In particular, these diatoms increased to 2.9 x 10(4) cells mL(-1) on day 5 and 2.3 x 10(4) cells mL(-1) on day 7 in the Geoje group. A multivariate redundancy analysis indicated a negative correlation between the abundances of C. polykrikoides and Chaetoceros spp., and this corresponded to the sharpest decrease of C. polykrikoides in the Geoje group. There were also changes in the bacterial community associated with changes in phytoplankton. During the early phase, when C. polykrikoides was dominant, Rhodobacterales prevailed (> 50%) in all treatment groups, and the proportion of these bacteria in the Geoje group decreased earlier than in the other groups. At the end of the experiment, there was a high proportion of Verrucomicrobiales, suggesting that sediment addition led to changes in the bacterial community. Overall, our microcosm experiments suggest that the significant environmental changes following the passage of a typhoon, especially the suspension and proliferation of sediment-based diatoms, directly affects the bacterial community and decreases blooms of C. polykrikoides.