Diatom‐produced allelochemicals trigger trophic cascades in the planktonic food web

Diatoms produce a series of cytotoxic secondary metabolites such as polyunsaturated aldehydes (PUA) in response to cell injury or stress. However, little information exists on the PUA effects on the pelagic food web. A set of experiments was conducted in the Chesapeake Bay and the coastal Atlantic waters using dissolved PUA (2E,4E-octadienal and 2E,4E-heptadienal), natural assemblages of phytoplankton and microzooplankton, and the copepod Acartia tonsa. The results demonstrate that PUA primarily acts as deterrent for microzooplankton herbivory on diatoms, while enhancing herbivory on picophytoplankton. This switch should favor PUA-producing diatoms by simultaneously reducing direct grazing losses and competition. Additionally, PUA stimulated copepod predation on microzooplankton, particularly on ciliates. Combined, these effects have the potential to disrupt growth-grazing equilibrium in the pelagic food web and create an opportunity for bloom development.