Long Term Invertebrate Monitoring in Glacier‐Fed Streams Reveals Decrease of Abundance‐Based Metrics

ABSTRACT Climate change in the Alps is affecting glaciers, altering stream flow and biological communities. Aquatic macroinvertebrates are useful indicators for describing biological temporal trends in glacial ecosystems, but comprehensive long‐term data are rarely available. Here, we studied 18 Alpine glacially influenced streams in northwestern Italy over a 15‐year period (2010–2024), focussing on multiple taxonomic and functional metrics. The stream sites were classified into four groups according to the percentage of glacial coverage within the basin. In parallel, to contextualize climate warming in the area, we analysed daily air temperature data recorded by 54 weather monitoring stations in the same period. Macroinvertebrate‐based metrics were then assessed using linear mixed‐effects models within a Bayesian framework to investigate temporal trends. The temperature increase was 0.084°C per year, and winter was the season most affected. Functional and taxonomic richness showed a heterogeneous temporal pattern, with an increasing trend in spring. Community‐level weighted means based on temperature‐preference traits revealed a reduction in summer for hot stenotherms and an increase in spring for eurytherms. Interestingly, crustaceans have been found to be distributed in a way that is inversely related to glacial influence, and they are currently absent from rivers with higher glacial influence. Our results revealed a complex picture of changes in freshwater macroinvertebrates, which overall showed some level of resilience across multiple descriptors, but also reflected uneven signs of climate change. However, abundance‐based metrics showed a clear declining trend for both insects and the total macroinvertebrate community. Findings also suggested that in the analysed period glacial retreat has not yet caused a profound change in multiple macroinvertebrate‐based metrics commonly used when biomonitoring. These results highlight the need for closer attention to existing metrics and for the development of additional ones tailored to fluvial networks in the context of global change. Alpine streams are likely facing a crucial transitional phase and this long‐term monitoring revealed mixed signals of climate change. To disentangle these dynamic glacial environments, continued monitoring is essential to better track trends and tendencies in their communities. Expanding datasets on an annual basis while accounting for seasonal and sub‐seasonal variability will help clarify invertebrate patterns and provide a solid foundation for future ecological research.