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Our results show that Nectopsyche sp. individuals were significantly affected by the addition of pesticides, especially at high concentrations of Chlorpyrifos, and that their response was time-dependent. The decomposition rates of organic matter were not significantly affected by treatment, but showed a significant response to differences in pH between treatments. Although we did not detect strong effects of the pesticides on leaf litter decomposition, the increase in Nectopsyche sp. mortality suggests that chronic pesticide pollution could eventually affect it.
The environmental targets in the Latin American region are focused on biodiversity, habitat degradation, pollution, climate change and human welfare [34]. However, the legislation in Andean countries is lax, with scarce controls to achieve adequate environmental health. For example, the environmental control entities do not have any regular monitoring or quality assessment of water bodies which would aid in understanding the effect of agriculture on Andean streams [35]. The existing information on water quality comes from grey literature, environmental consultancies or research [35,36]. Therefore, the effect of anthropogenic activities on riverine biodiversity and ecosystem functioning could easily be underestimated.Agrochemicals used on crops are transported to aquatic ecosystems through different pathways, and affect non-target organisms, such as some aquatic insects [33,37]. For example, in alignment with our study, Morabowen et al. (2019) found that Nectopsyche decreased along an agricultural land use gradient. It is important to notice that some individuals survived our experimental treatments, even at the highest pesticide concentration. This means that pesticides have the potential to bioaccumulate in Nectopsyche and to be transferred into the river food chain [4,38]. Concordantly, previous studies have demonstrated that long term pesticide effects can be detected in freshwater ecosystems [39,40]. Taking into account that Chlorpyrifos is especially toxic to aquatic life [7,12,24], with some studies showing long term effects on aquatic biodiversity [41], the potential bioaccumulation of Chlorpyrifos in the environment through aquatic insects deserves to be further studied.In our study, Chlorpyrifos caused a significant mortality of the caddisfly Nectopsyche sp. at the highest concentration used (10 μ gL−1), as we hypothesized. However, both Chlorpyrifos and Engeo are classified with the same toxicity in the national legislation [28], and we clearly observed differences in the effects of both on Nectopsyche sp. Thus, it is important to develop locally adapted maximum permissible values for the main pesticides used in Ecuador, in order to adequately protect water quality and conserve aquatic biodiversity.Considering that Nectopsyche is very sensitive to pollution, our results suggest that current legislation could be protecting aquatic biodiversity to some extent. However, at the ecosystem level, it is expected that more sensitive taxa will be affected. Current regulations indicate that a maximum concentration of 10 μ gL−1 for total organophosphates is allowed in water. However, considering that bad practices are common [24,40], this may not be enough to adequately protect water quality [6]. In Latin America (including Ecuador), pesticides are used in agriculture with almost no technical expertise, leading to excessive pesticide application, which causes the pollution of soils, water and sediments [6,24,42]. Additionally, pesticides such as Engeo and Chlorpyrifos are easily accessible due to the lack of regulation and their low cost. Therefore, the maximum permissible pesticide concentrations may often be exceeded [6,15,16]. In addition, there is no information on the concentration of organophosphates in the water, which can affect biodiversity and ecosystem functioning [4,43]. Finally, the accumulation of pesticides in the sediment is not considered in the environmental legislation. This is an important point, because sediment accumulation can affect the biotic and abiotic characteristics in the short and long term [16,39], as well as the legacy effects of pesticides on freshwater ecosystems [41,44]. The lack of legislation regarding pollutants in sediments is an important issue to be addressed in order to effectively protect freshwater ecosystems in Ecuador.According to our data, there were no differences in leaf litter decomposition between treatments, but decomposition rates changed with time. For the first eight days, the decomposition rate was high. This could be related to the shredder’s activity and the availability of food resources in the microcosms. However, we did not measure the effect of pesticides on microbial communities, which can be largely responsible for leaf litter decomposition [31,45]. Since pH can significantly affect the microbial decomposition of organic matter in rivers [46,47], the pH increase throughout the experiment could partly explain the low decomposition rates observed after the eighth day. Nonetheless, the microbial activity in rivers is one of the most important factors in decomposition rates, and is especially influenced by land use changes [46]. However, the agrochemicals input by runoff cause the eutrophication process or water chemical changes, the microbial activity is altered and the decomposition rate could be decreased [31,48]. It is important to denote other factors that intervene in the components of decomposition. The forest quality is highlighted as a factor that determines the importance of decomposers by the Kg/Kf quotient, and in Andean rivers with high forest quality, the shredders are determined to be important to the decomposition rates [48], while the importance of microorganisms increases in impacted rivers [49,50]. This is one of the limitations of our study. We encourage future researchers to study changes in the composition and biomass of microbial communities, as well as how they might affect organic matter processing in Andean streams affected by pesticide pollution. Another limitation of our study is that we focused on a single species, neglecting potential trophic interactions. In this regard, future studies combining mesocosm experiments with field studies could be helpful in assessing how pesticide pollution might affect species interactions and the biological routes of pesticides in riparian and aquatic compartments [51].Our study includes the genus Nectopsyche (Trichoptera: Leptoceridae) as a sentinel organism. According to previous studies in highland Andean rivers, Nectopsyche is among the organisms which are most sensitive to environmental changes [52,53]. Concordantly, our study proves that it this a good model organism for use in laboratory toxicity tests. However, it is important to highlight that the environmental complexity and interactions between the contaminants and physicochemical variables in rivers were not captured by our study. Additionally, the model organisms most likely belong to a new species that needs to be further described and studied. Many insect species and microbial organisms differ in their interaction with chemical or biological pesticides in aquatic or terrestrial habitats [54,55,56], according to the composition of the toxins and mode of actions. Moreover, future steps could include quantifying pesticide concentrations in the rivers along land use gradients, while simultaneously assessing changes in the structure of aquatic communities and organism stress using biomarkers [57]. Another future research line that deserves attention is the importance of riverine forests as buffers against pesticide pollution in streams surrounded by agricultural fields. Finally, we aim to incorporate higher ecological complexity in future studies, as the ecological effects of pesticides in river ecosystems can be affected by multiple factors (e.g., the geology of the catchment, the interaction with other stressors and the composition of the biological communities, among others).
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