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Comparative Sensitivity Of Decomposers And Decomposition To Chemical Stress

Decomposition of detritus is a vital ecosystem process driven by both microorganisms and invertebrate detritivores. Many ecosystems are, however, under continuous toxic pressure, directly affecting decomposer organisms and therewith indirectly essential ecosystem processes. However, it remains largely unknown to which extent toxic effects cascade towards disordered ecosystem processes.

Since it has been recognized that a large part (more than 50%) of the detritus becomes trapped in subsurface sediments, the link between bioturbation by invertebrates and organic matter processing gained increased interest. Decomposition has indeed been demonstrated to be directly related to the functional composition of invertebrate communities in a number of studies. However, toxic pressure may alter the locomotion and behavior of aquatic invertebrates, potentially affecting these links between invertebrate community structure and ecosystem functioning.

A recent study evaluated how toxicants in sediments affect the functional links between invertebrate bioturbation and ecosystem functioning. To this purpose, the effects of different concentrations of the model toxicant copper on functionally distinct invertebrates (Asellus aquaticus and Tubifex spp.), detritus processing and microbial activity and metabolic diversity were determined in 5–day microcosm experiments. The effect of altered locomotion and activity and reduced bioturbation were assessed with spatio-temporal redox potential (Eh) profiles in the upper sediment layer. Concentration-response curves of the functional parameters decomposition and the faunal mediated microbial activity and metabolic diversity in the presence of Tubifex spp. depended on Tubifex spp. survival, i.e. similar EC50 values for both endpoints. In contrast, functional parameters in the presence of A. aquaticus were more sensitive than survival. Sediment Eh-profiles showed that reduced decomposition was caused by reduced sediment reworking by A. aquaticus at sub-lethal copper concentrations. The observed differences between Tubifex spp. and A. aquaticus were likely caused by differences in locomotion and feeding behavior, i.e. Tubifex spp. lives within the sediment, while A. aquaticus is crawling on top of the sediment and therefore able to avoid direct exposure to toxicants. This outcome suggests that toxicants can affect the functional links between invertebrate bioturbation and ecosystem functioning, highlighting the importance of species specific locomotion for the effects of chemical stressors on ecosystem functioning. These observations hint at a decoupling of invertebrate community structure and ecosystem functioning upon stress, suggesting that functional parameters (e.g. decomposition) may serve as more sensitive and reliable parameters for assessing ecological water quality and ecosystem functioning.

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