Original paper

Factors masking the trophic cascade in shallow eutrophic wetlands: Evidence from a microcosm study

Ortega-Mayagoita, Elizabeth.; Rojo, Carmen; Rodrigo, Marfa A.


The study of the trophic cascade hypothesis on plankton communities has mainly focused on large herbivorous Daphnia, but in many wetlands, large zooplankton is often composed of omnivorous cyclopoid copepods. We made a factorial experiment in microcosms to study the effects of the planktivorous fish Gambusia holbrooki and of sediments, and a second experiment to test for the influence of biomass and composition of zooplankton (excluding the copepod Acanthocyclops robustus and 50 % of total zooplankton biomass) and nutrient dilution, on plankton from a semiarid eutrophic wetland. We monitored autotrophic picoplankton (APP), nanophytoplankton, microphytoplankton, ciliates, heterotrophic flagellates (HF), rotifers, and copepods. The presence of a planktivorous fish had no cascading effects on nanophytoplankton but on metazooplankton, and affected the microphytoplankton negatively, decreasing the total primary producers biomass. In our experiment the sediments increased the metazooplankton abundance (presumably by the hatching of resting stages), probably causing rather a top-down control of APP and microphytoplankton than a bottom-up stimulation by nutrients. On the other hand, there was no net effect of zooplankton biomass or omnivorous copepods on edible algae biomass, possibly due to the trophic role of these copepods: these could be consuming nanophytoplankton at the same time as feeding on rotifers and protozoans (grazers of the same algae fraction), producing a zero net effect on nanophytoplankton standing stock.


Microcosm experimentsplankton regulationtrophic cascadewetlandseutrophicationomnivory.