Combined Effects of Fish, Light and Allochthonous Dissolved Organic Carbon on Interactions Between Green and Brown Food Webs: An Aquatic Mesocosm Experiment

The classical concepts of top‐down and bottom‐up cascading effects in ecosystems are challenged by the interactions between green and brown food webs. The green food web relies on photosynthesis and the brown food web relies on the mineralisation of dead organic matter. Mutualistic interactions between these two food webs are crucial for ecosystem functioning because a major fraction of organic carbon is produced by the green food web, while mineral nutrients are mainly recycled through the brown one. However, green and brown food webs can compete for mineral nutrients if decomposers are nutrient limited. Thus, bottom‐up or top‐down effects on one food web can have major cascading effects on the other one. In our freshwater mesocosm experiment, we tested cascading bottom‐up effects targeting the green food web through sunlight filtering and the brown food web through organic carbon addition; cascading top‐down effects mediated by fish presence. Our main finding is a positive effect of fish on phytoplankton density in mesocosms where daylight was reduced. Fish also had a positive effect on the brown food web with increased abundances of heterotrophic prokaryotes and microbial eukaryotes. However, they had no significant effect on zooplankton biomass. We did not observe any cascading effect of light treatment on the brown food web and of organic carbon treatment on the green food web. Our organic carbon treatment had nearly no effect on the brown food web. The most likely explanation for the observed fish × light interaction is a bottom‐up effect mediated by nutrient cycling embodied by the increased sediment production and DOC accumulation in the presence of fish. The lack of effect of our organic carbon treatment may be due to the moderate quantity of added carbon and of the low bioavailability of some of the added molecules. Our experiment suggests that fish can strongly stimulate phytoplankton through bottom‐up effects due to nutrient cycling, and not only through the classic trophic cascade due to the top‐down control on large herbivorous zooplankton.