Shrinking glaciers reshape diatom biodiversity in tropical high-altitude rivers

Alpine river biodiversity is under threat as climate change is driving extensive glacier retreat worldwide, altering aquatic habitats and restructuring biotic communities. However, with the exception of macroinvertebrates, the responses of aquatic groups to deglaciation have received little attention, particularly in tropical mountain ranges where species may respond differently owing to additional pressures such as distinct wet and dry seasons, limited snowmelt and extreme high altitude and thus lower oxygen availability. Utilising a continuum of glacier cover across 23 rivers in both the Cordillera Blanca and Cordillera Vilcanota ranges in the Peruvian Andes, we demonstrate that diatoms, abundant primary producers in glacier-fed rivers, show regionally consistent increases in α-diversity and density responses with deglaciation. However, community dissimilarity was found to decline, potentially reflecting the reduction of habitat heterogeneity and niche space both within and between alpine rivers as glaciers are lost. High regional biodiversity, with just 23% of taxa found in both regions, was closely linked to the distinct underlying geologies of the Andes which influence water chemistry. Thirty-eight taxa were found exclusively at sites with glacier cover ≥ 25%, placing them at risk of local extinction as glaciers are lost. However, 31 taxa were identified only at non-glacial sites, suggesting deglaciation could open up more habitat for these species. Peruvian rivers overall showed similar responses to decreasing glacier cover as temperate alpine systems, but the large number of taxa found only in rivers with mid-high glacial influence suggests tropical diatom assemblages may be especially vulnerable to climate change.