Bottom-up processes control benthic macroinvertebrate communities and food web structure of fishless artificial wetlands

In freshwater environments, the relative contributions of top-down and bottom-up effects on invertebrate communities in relation to productivity are largely ecosystem dependent. Artificial wetlands are increasingly developed to compensate for the loss of natural wetlands; however, their trophic processes remain poorly studied. The present study aimed to evaluate the respective contributions of bottom-up and top-down processes in structuring benthic food webs of three artificial wetlands with varying levels of benthic primary productivity. We found that phototrophic-based food webs in our artificial wetlands were controlled from the bottom-up by primary productivity and algal biomass developing at the water-sediment interface. No significant top-down control of herbivore species by invertebrate predators was detected even in the wetland with the highest productivity. Increased richness of invertebrate grazers and scrapers with benthic primary productivity and algal biomass might have dampened the trophic cascade from predators to primary producers. In contrast with the phototrophic-based food web, analyses performed on the detritus-based food web showed that deposit-feeder invertebrate abundance was not correlated with the quantity of organic matter in sediments, suggesting no bottom-up effect of sedimentary organic matter content on deposit-feeders. More surprisingly, deposit-feeders, especially aquatic oligochaetes, seemed to influence the detritus-based food webs by stimulating organic matter processing and bacterial growth through bioturbation. The present study highlights the occurrence of contrasting trophic processes between phototrophic-based and detritus-based food webs which can have implications on ecosystem functions, such as nutrient cycling and energy fluxes;