The diatom-produced polyunsaturated aldehydes can induce trophic cascades in the planktonic food web in productive coastal waters.

Allelopathy is wide spread among marine phytoplankton, including diatoms that produce cytotoxic secondary metabolites such as polyunsaturated aldehydes (PUA). Most published PUA studies focused on the reproduction and development of specific marine invertebrates under laboratory conditions. In this study, we examined the effect of PUA on the trophic interactions between the copepod Acartia tonsa and natural microplankton collected from the Chesapeake Bay and the Virginia coastal waters. A set of bottle incubation experiments was conducted using the environmentally realistic concentrations of dissolved 2E, 4E-heptadienal and 2E, 4E-octadienal. Although PUA did not change phytoplankton growth, microzooplankton growth was affected at the species-specific level and their community herbivory rates declined. At the same time, the rates of copepod herbivory and predation on ciliates increased in the PUA treatments. These preliminary results suggest that production of cytotoxic compounds by diatoms may be a defense mechanism primarily against microzooplankton. The cascading effects induced by PUA can alter the composition and dynamics of microbial plankton communities, which in turn could have strong implication for the carbon cycling in productive coastal ecosystems.