Neonicotinoids influence on trophic relations and ecosystem functioning in freshwater ecosystems (NITRO)
Final Report Abstract
Freshwaters provide crucial ecosystem services to human societies. They rank as one of the most impacted ecosystems worldwide with environmental alterations leading to declines in abundances and extinctions of species and hence degradation of related ecosystem processes. In streams, the decomposition of leaf litter of terrestrial origin is an important ecosystem process because it provides nutrients and energy to local and downstream communities and it is mainly driven by consumer microorganisms and invertebrates. The diversity of leaf species has been shown to affect the decomposition process with the species identity, particularly functional identity, being recognized as a more important driver than the number of species per se. Additionally, the decomposition process in streams might also be affected by the presence of pesticides. Neonicotinoid insecticides are a class of pesticides of particular success but also concern. Among other characteristics, their ubiquity in the treated plant and high affinity for the insect nervous receptors has made them the major insecticide group in the market for the last two decades. However, these characteristics can also translate into potential contamination as well as irreversibility of the effects of contamination on non-target insects. The presence of neonicotinoid insecticides in water bodies has been confirmed worldwide and sub-lethal and lethal effects from waterborne contamination on non-target aquatic insects have been observed at relatively low concentrations. However, the ingestion of contaminated food source by aquatic insects is a contamination pathway that has been largely overlooked. Therefore, the main goals of this project were to investigate (i) potential cumulative effects of a neonicotinoid insecticide on freshwater detritivores, (ii) the relevance of resource diversity for effects of imidacloprid on leaf litter decomposition, and (iii) the impacts of a neonicotinoid in tri-trophic systems. We conducted several experiments in microcosms (beakers and aquaria) and mesocosms (artificial streams) to study the effects of neonicotinoids in the food chain. We selected imidacloprid (commercial formulation Confidor WG70 (Bayer)) as model neonicotinoid insecticide for this project that was used to contaminate leafs. This was done by treating alder trees with imidacloprid and collecting the leaf material. Imidacloprid was detected in leafs and in water samples from microcosms containing contaminated leaf material, whereas it was below the detection limit in mesocosms. Overall, contamination of leaf litter with imidacloprid led to lower decomposition rates. In microsoms, this was concurrent with an accumulation of imidacloprid in water and a decrease in growth and feeding activity of invertebrates compared to controls. In mesocosms, the consumption of contaminated leaf litter reduced invertebrate growth but increased their feeding activity. When contaminated and non-contaminated alder leafs were mixed with beech and hazel leafs, the increased resource diversity led to an homogenization of the leaf decomposition rate, which was reduced when contaminated leaf litter was present. Furthermore, net diversity effects changed from positive to negative with increased leaf species richness. In the presence of contaminated leaf species net diversity levels were negative for all levels of leaf species richness. In food chain experiments, predators that were fed with prey feeding on contaminated leaf material exhibited a lower growth. Overall, our results show that the contamination of non-target aquatic invertebrates through the consumption of contaminated food source can be a relevant contamination pathway and propagate along the aquatic food chains. The consumption of contaminated leaf litter by aquatic invertebrates had a detrimental effect on an ecosystem process and at the individual level. Ecologically, this might translate in short- and long-term alterations in aquatic food webs and associated ecosystem processes.
