Diatom biodiversity response to shrinking glaciers in the Peruvian Andes

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 ecosystems 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 the Peruvian Andes (Cordilleras Blanca and Vilcanota), we demonstrate that diatoms, abundant primary producers in glacier-fed rivers, show regionally consistent increased α-diversity and density responses with deglaciation. However, β-diversity 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%, which is considerably higher than numbers recorded in European alpine rivers. 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 glacier retreat as temperate alpine systems, but the large number of taxa found only at mid-high glacial influence sites suggests tropical diatom assemblages may be especially vulnerable to climate change.