Effects of cadmium on a model stream detritus processing chain

cadmium, indirect effects, exotic species, shredder-collector interactions, DGGE, trophic cascades The ecological integrity of ecosystems is often evaluated in terms of species occurrence and composition of communities and not in terms of trophic relationships between species or functional parameters. The processing of detritus is a vital ecological process for freshwater ecosystems, mediated by microorganisms and aquatic invertebrates. Here, shredders contribute to the nutrient cycle, transforming coarse into fine particulate organic matter which serve as food for collectors. The aim of this work was to evaluate the combined effects of cadmium exposure and changes in resource quality (leaf species) on detritus trophic chain and on shredder–collector interactions. In the laboratory, alder leaves (native species) and eucalyptus (exotic species) were conditioned at different cadmium concentrations (0, 50 and 200 μg/L). The microbial community present in leaves from the different treatments was analyzed by molecular DGGE technique and microbial respiration rates were measured. Sericostoma vittatum (a caddisfly shredder) and Chironomus riparius (a midge collector) were exposed to the same concentrations of cadmium and fed the corresponding leaf discs. C. riparius growth and leaf weight loss were evaluated in these multispecies microcosms. Cadmium exposure affected the leaf conditioning and the reduction in fungal diversity in both leaf species was clear. Cadmium affected the decomposition of alder leaves through reductions in invertebrate feeding and also impaired C. riparius’ growth. C. riparius showed to be a non-exclusive collector and capable of feeding on alder leaf discs in the absence of shredders. However shredders appear to promote collectors growth in treatments with eucalyptus as food source. The observed pattern suggest that shredder-collector facilitation hypothesis can hold true and also that it might be dependent on resource quality and detritivore species present. The results suggest that detritus processing chains can be valuable tools to community ecotoxicology. We advocate that these investigations should continue with the use of different detritivore species and stressors (natural and anthropogenic) to improve our current understanding of indirect effects of contaminants in detritus based food webs under different ecological scenarios. Table of contents Page Chapter I 1 1.General Introduction 3 1.1 Trophic cascades and indirect effects of contaminants 3 1.2 Detritus processing chains 4 1.3 Shredder collector interactions and the facilitation hypothesis 9 1.4 Thesis objectives and Model trophic chain used 9 References 12 Chapter II 17 2.Effects of Cadmium and Changes in resource quality (leaf species) on freshwater detritus based food webs 19 2.1 Abstract 20 2.2 Introduction 21 2.3 Methods 23 2.3. 1 Test Organisms 23 2.3. 2 Leaf Conditioning 23 2.3. 3 Analysis of leaf microbial community 23 2.3. 4 Detritivore trials 25 2.3. 5 Statistical analysis 25 2.4 Results 26 2.4. 1 Effects of cadmium on microbial conditioning 26 2.4. 2. Effects of cadmium on leaf discs weight loss (leaf decomposition) 28 2.4. 3.Chironomus riparius growth 30 2.5 Discussion 32 References 36 Chapter III 39 3.General Discussion 41 References 43 Chapter I: General Introduction