Declining glacier cover threatens the biodiversity of alpine river diatom assemblages

Climate change poses a considerable threat to the biodiversity of high altitude ecosystems worldwide, including cold‐water river systems that are responding rapidly to a shrinking cryosphere. Most recent research has demonstrated the severe vulnerability of river invertebrates to glacier retreat but effects upon other aquatic groups remain poorly quantified. Using new datasets from the European Alps, we show significant responses to declining glacier cover for diatoms, which play a critical functional role as freshwater primary producers. Specifically, diatom α‐diversity and density in rivers presently fed by glaciers will increase with future deglaciation, yet β‐diversity within and between‐sites will reduce because declining glacier influence will lower the spatiotemporal variability of glacier cover and its associated habitat heterogeneity. Changes in diatom assemblage composition as glacier cover declined were associated strongly with increasing river‐bed stability and water temperature. At the species level, diatoms showed a gradation of responses; for example, Eunotia trinacria, found exclusively at river sites with high (≥ 52%) catchment glacier cover, may be affected negatively by ice loss. Conversely, seven taxa confined to sites with no glacier cover, including Gomphonema calcareum, stand to benefit. Nineteen (22%) taxa were noted as threatened, endangered, rare or decreasing on the Red List of Algae for Germany, with most at sites ≤ 26% glacier cover, meaning further ice loss may benefit these diatoms. However, six taxa found only in rivers ≥ 28% glacier cover may require reclassification of their Red List conservation status, as this habitat is threatened by deglaciation. Our identification of clear links between decreasing glacier cover and river diatom biodiversity suggests there could be significant reorganisation of river ecosystems with deglaciation, for example through alterations to primary production, biogeochemical cycles and the shifting resource base of alpine freshwater food webs which lack significant allochthonous energy inputs.