Effects of cadmium and resource quality on freshwater detritus processing chains: a microcosm approach with two insect species
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Detritus processing is vital for freshwater ecosystems that depend on the leaf litter from riparian vegetation and is mediated by microorganisms and aquatic invertebrates. Shredder invertebrates transform coarse particulate organic matter into fine particulate organic matter used as food by collector species. Direct and indirect effects of contaminants can impair detritus processing and thus affect the functioning of these ecosystems. Here, we assessed the combined effects of a toxic metal (cadmium) and resource quality (leaf species) on detritus processing and shredder-collector interactions. We considered two types of leaves, alder and eucalyptus that were microbially conditioned under different Cd concentrations in the laboratory. The microbial communities present on leaves were analyzed by Denaturing Gradient Gel Electrophoresis (DGGE), and we also measured microbial respiration rates. Sericostoma vittatum (a caddisfly shredder) and Chironomus riparius (a midge collector) were also exposed to Cd and allowed to consume the corresponding alder or eucalyptus leaves. We evaluated C. riparius growth and leaf mass loss in multispecies microcosms. Cadmium exposure affected leaf conditioning and fungal diversity on both leaf species, as assessed by DGGE. Cadmium exposure also affected the mass loss of alder leaves by reductions in detritivore feeding, and impaired C. riparius growth. Chironomus riparius consumed alder leaf discs in the absence of shredders, but S. vittatum appear to promote C. riparius growth in treatments containing eucalyptus. These results show that indirect effects of contaminants along detritus-processing chains can occur through effects on shredder-collector interactions such as facilitation but they also depend on the nutritional quality of detritus and on sensitivity and feeding plasticity of detritivore species.
KeywordsShredder-collector interactions Facilitation Indirect effects Leaf decomposition Chironomus
We are grateful to two anonymous reviewers for their comments and suggestions that greatly improved this manuscript. Financial support for this work was provided by the projects “MIDGE - MIcroevolutionary Dynamics and Genetic Erosion in pollution-affected Chironomus populations” ref: FCOMP-01-0124-FEDER-008954 (Ref. FCT PTDC/BIA-BEC/104125/2008) and project “IDEAL—Insecticides, DEtritivores and ALiens: Combined effects of invasive species and insecticides along detritus based stream food webs”, ref: FCOMP-01-0124-FEDER-019380 (Ref. FCT: PTDC/AAC-AMB/119433/2010) both supported by the COMPETE program (Programa Operacional Fatores de Competitividade,) (FEDER component) and by the Fundação para a Ciência e Tecnologia (FCT). FCT also financed a post-doctoral research grant to JLT Pestana (SFRH/BPD/45342/2008). AMVM Soares is “Bolsista CAPES/BRASIL”, Project NºA058/2013. This work was supported by European Funds through COMPETE and by National Funds through the Portuguese Science Foundation (FCT) within project PEst-C/MAR/LA0017/2013.
Conflict of interest
The authors declare that they have no conflict of interest.
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