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Trophic cascades in the marine pelagic food web, especially in the microbial food web in which copepods are the top controllers, are difficult to quantify and are generally overlooked. In this study, we simulated a simple pelagic food web in laboratory feeding experiments to demonstrate the trophic interactions among copepods, heterotrophic dinoflagellates and chain-forming diatoms and to quantify the trophic cascade effect and the true feeding rate of copepods. Results from the study showed that heterotrophic dinoflagellates play a central role in the lower trophic level of the marine food web by consuming diatoms and by serving as a quality food source for copepods, especially during the period of diatom blooms. Copepod omnivory was mediated by the concentration of diatoms, with the highest ingestion of diatoms occurring at intermediate diatom abundances and the selectivity toward dinoflagellates increasing as the diatom concentration increased. The trophic cascade was surprisingly low and even negative in treatments with low diatom concentrations, suggesting a competition effect when there was not enough diatom food. In this situation, the omnivorous copepods together with heterotrophic dinoflagellates exhibited strong herbivory and were able to control diatoms. In contrast, a high trophic cascade effect occurred at high diatom concentrations in which copepods had a high ingestion of heterotrophic dinoflagellates. We conclude that the role of copepods in the food web structure is mediated by the concentrations of diatoms, but one of the most important biological factors that determine the strength of trophic cascades is the ingestion rates of both copepods and heterotrophic dinoflagellates.

Although grazer inclusion incubation experiments are commonly used to evaluate mesozooplankton grazing mortality of phytoplankton, few studies have employed molecular methods to examine the changes in the composition of the microbial community across different grazing treatments. We used denaturing gradient gel electrophoresis (DGGE) to study the impact of mesozooplankton-mediated trophic cascades on microbial food web components at two subtropical coastal sites with contrasting trophic conditions. Reciprocal changes in microbial food web components resulting from trophic cascades were observed, with decreases in > 3 µm eukaryotes, increases in