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Freshwater organisms are often sensitive to pesticides, but their sensitivity varies across different taxa and with pesticide type and action mode, as shown by multiple acute toxicity tests. Such variability hampers predictions about how freshwater ecosystems may be altered by pesticide toxicity, which is especially critical for understudied areas of the world such as the tropics. Furthermore, there is little information about the sensitivity of some organisms that are key components of stream food webs; this is the case of litter-feeding detritivorous invertebrates, which contribute to the fundamental process of litter decomposition. Here, we examined the sensitivity of three common detritivores [Anchytarsus sp. (Coleoptera: Ptilodactylidae), Hyalella sp. (Amphipoda: Hyalellidae) and Lepidostoma sp. (Trichoptera: Lepidostomatidae)] to three pesticides commonly used (the insecticides bifenthrin and chlorpyrifos and the fungicide chlorothalonil) using acute (48 or 96h) toxicity tests. Our study demonstrates that common-use pesticides provoke the mortality of half their populations at concentrations of 0.04-2.7mugL-1. We found that all species were sensitive to the three pesticides, with the highest sensitivity found for chlorpyrifos. Additionally, we used the approach of species sensitivity distributions (SSD) to compare our study species with Daphnia magna and other temperate and tropical invertebrates. We found that the study species were among the most sensitive species to chlorpyrifos and chlorothalonil. Our results suggest that tropical detritivores merit special attention in ecological risk assessment of pesticides and highlight the need for accurate ecotoxicological information from ecologically relevant species in The Tropics. This work was supported by the National Secretariat for Science, Technology and Innovation of Panama (SENACYT; project APY-GC-2018B-052; contract no. 259-2018) and the Ministry of Economy and Finance of Panama (MEF; project 019910.001). AC was supported by a fellowship from SENACYT (contract no. 001-2015) and by the National Research System of Panama (SNI; Ph.D. category; contract no. 186-2018). GC was supported by a fellowship from IFARHU-SENACYT (contract no. 270-2018-1011).

The expansion of agriculture is particularly worrying in tropical regions of the world, where native forests are being replaced by crops at alarming rates, with severe consequences for biodiversity and ecosystems. However, there is little information about the potential effects of agriculture on the functioning of tropical streams, which is essential if we are to assess the condition and ecological integrity of these ecosystems. We conducted a litter decomposition experiment in streams within a tropical catchment, which were subjected to different degrees of agricultural influence: low (protected area, PA), medium (buffer area, BA) and high (agricultural area, AA). We quantified decomposition rates of litter enclosed within coarse-mesh and fine-mesh bags, which allowed the distinction of microbial and detritivore-mediated decomposition pathways. We used litter of three riparian species representing a gradient in litter quality (Alnus acuminate > Ficus irtsipida > Querns burnelioides), and examined detritivore assemblages through the contents of litterbags and benthic samples. We found that the increasing agricultural influence promoted microbial decomposition, probably due to nutrient-mediated stimulation; and inhibited detritivore-mediated and total decomposition because of reduced detritivore numbers, most likely caused by pesticides and sedimentation. Effects were evident for Alnus and Ficus, but not for Querns, which was barely decomposed across the gradient. Our study provides key evidence about the impact of agriculture on topical stream ecosystem functioning, which is associated to changes in stream assemblages and may have far-reaching repercussions for global biochemical cycles. (C) 2020 The Authors. Published by Elsevier B.V. Thisworkwas supported by the National Secretariat for Science, Technology and Innovation (SENACYT; project APY-GC-2018B-052; contract no. 259-2018) and the Ministry of Economy and Finance (MEF; project 019910.001). AC was supported by a fellowship from SENACYT (contract no. 001-2015) and by the National Research System of Panama (SNI; PhD category; contract no. 186-2018). GC was supported by a fellowship from IFARHU-SENACYT (contract no. 270-2018-1011).