Organic matter decomposition in Neotropical streams: a comparison of paired catchments with different stressors

Functional indicators are valuable tools to assess how human activities affect stream ecosystems. In Neotropical regions, however, our understanding of how anthropogenic stressors influence ecosystem processes such as organic matter (OM) decomposition is still limited. Here, we explored human impacts on OM decomposition in Neotropical streams by comparing a reference and an impacted subcatchment within the same watershed in southeastern Brazil. We used the cotton-strip assay as a standardized proxy for OM decomposition and incubated strips for 41 days under in situ conditions, measuring tensile strength loss and microbial respiration throughout the experiment. Environmental conditions differed markedly between streams: the impacted stream exhibited higher nutrient concentrations (e.g., ammonium, soluble reactive phosphorus, and total phosphorus), higher mean pH (6.52 vs. 5.85), and greater diel temperature amplitude (4.39 vs. 0.89 °C). Decomposition rates (Kd) were significantly higher in the impacted stream (0.0796 d–1) than in the reference stream (0.0176 d–1), with the largest differences occurring in the first half of the experiment. Respiration rates were similar in the impacted stream and the reference stream, and not statistically different. Our high-resolution temporal sampling (every 2–3 d) allowed us to capture dynamic changes in decomposition over time. These results suggest that human activities alter stream ecosystem functioning through multiple, co-occurring stressors, including temperature, nutrients, and other physicochemical changes, and that high-frequency functional indicators like cotton-strip decomposition can provide valuable insights into the ecological impacts of human pressures in Neotropical freshwater systems.