We studied trophic interactions in experimental rockpools with three different food web structures: phytoplankton and small-bodied zooplankton; phytoplankton, small-bodied zooplankton and Daphnia; and phytoplankton, small-bodied zooplankton, Daphnia and Notonecta. Nutrients, primary productivity, chlorophyll a and zooplankton species composition and biomass were measured over eight weeks.

2. Daphnia had a negative impact on other zooplankton and reduced the phytoplankton biomass and primary productivity. In the absence of Daphnia, small-bodied zooplankton species were abundant, in particular cyclopoid copepods. Concentrations of dissolved nutrients were lower and the standing crop of primary producers was higher when Daphnia was absent.

3. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i.e. a selective reduction of daphnids followed by an increase of small-bodied zooplankton species and phytoplankton biomass.

4. The study showed that consumer regulation of Daphnia by Notonecta and of algae by Daphnia are important, but also demonstrated that trophic level biomasses were controlled by a combination of predation and resource limitation.