Long-term changes and seasonal variability in the stream macrofauna of a Himalayan river system

Interpreting long-term variability in mountain stream ecosystems is challenging where potential causes such as climate change and altered land use act simultaneously. The scarcity of historical data also limits research opportunity, especially at low latitudes. Here we assess the macroinvertebrate fauna of 12 Himalayan streams in Nepal covering an altitudinal gradient from 1500 to 3900 m a.s.l. using data from 1992, 2022, and 2023. We aimed to (1) compare long-term change in richness and community composition over 3 decades to short-term seasonal variation, and (2) analyse long-term changes in relation to catchment properties. Rarefied richness decreased equally with altitude in 1992 and 2022 but was greater at most sites in 2022, with several new families detected. By contrast, no clear seasonal pattern was found. The 2022:1992 ratio in rarefied richness was positively correlated with catchment area and weakly with maximum catchment altitude, whereas the 2 highest and glaciated sites gained relatively more taxa than other sites. Non-metric multidimensional scaling (NMDS) ordinations from rarefied Bray-Curtis dissimilarities showed considerable overlap in communities among the 3 datasets. Pairwise rarefied dissimilarities among all sites increased significantly with altitudinal distance between sites in 1992, but no such relationship was detected in 2022. Dissimilarities between 1992 and 2022 samples showed a near-significant correlation with increased catchment vegetation cover, as shown by the normalized difference vegetation index (NDVI). Our study is the first to document long-term variations in stream macroinvertebrate communities in Himalayan mountain streams, illustrating how some altitudinal patterns have remained stable through time while others seem to have weakened with links to global change.