Alpine River Ecosystem Response to Glacier Retreat

Climate change is accelerating glacier retreat across mountain regions globally. Reductions in ice melt inputs are rapidly altering the geomorphology, flow regime and physicochemistry of rivers, modifying their habitats and ecological communities. These changes will influence the biodiversity and ecosystem service provision of alpine rivers. The responses of many taxonomic groups remain undescribed, with implications for freshwater networks and their functions not fully assessed. This thesis used chronosequences of sites representative of reducing catchment glacier cover to investigate the response of alpine river ecosystem structure and functioning to glacier retreat. A new literature-derived conceptual model was developed to synthesise the multitrophic responses of taxonomic groups to declining glacier cover and identify research gaps for this study. Focus was then placed upon benthic diatoms, a less studied group which drive primary production in alpine rivers. Observations in the Austrian Alps provided first evidence of their increased alpha- but reduced beta-diversity as glacier cover was lost. To consider concurrent taxonomic responses, nine new river food webs were constructed using gut contents analysis, for the natural successional gradient imposed by glacier retreat. Significant rewiring of food webs occurred along the chronosequence, with absence of species loss and network collapse indicating robustness to deglaciation. A global-scale field experiment revealed the structure of mountain river ecological communities to be associated significantly with their ecosystem functioning. Ice loss increased the abundance of aquatic fungi and a cellulolytic gene, which were correlated to accelerated decomposition of cellulose, the principal component of riparian vegetation. Findings were used to revise ecological models and propose future research. The identification of clear links between decreasing glacier cover and freshwater biodiversity, food webs and functioning suggests that glacier retreat will drive major alterations in alpine river ecosystems.