Trophic cascades in marine planktonic communities: Omnivorous copepods as a key functional group

A recent review by Shuring et al. (Ecology Letters 5: 785-791, 2002) concluded that predator effects on algae are weak in marine plankton. Through a series of mesocosm experiment we demonstrate that this conclusion is based on a misinterpretation of the structure and functioning of marine plankton communities. Omnivorous copepods often dominate the mesozooplantkon in coastal areas, and these organisms may prey strongly and selectively on ciliates and therefore approach tropich position three in the food web. Our experiments show that in a marine plankton community consisting of carnivores (jellyfish), herbivores (copepods and ciliates) and algae, jellyfish predation consistently reduced copepod biomass but produced two distinct, opposite responses of algal biomass. In this way copepods act as a switch between alternative tropic cascades along food chains of different length and with counteracting effects on algal biomass.