The effects of declining glacier cover on river ecosystems across broad biogeographic scales

Climate change is driving the retreat of glaciers globally. Declining glacier meltwater input to rivers alters their flow regime, water chemistry and geomorphology. In turn, biotic communities face substantial habitat modification which can reshape biodiversity and ecosystem functioning. As glaciers feed many of the world’s largest river systems, these changes may impact ecosystem services for a significant proportion of the human population. However, the influence of glacier retreat on river ecosystems across wide geographic scales has received limited attention. To investigate the biodiversity responses of river ecosystems to glacier retreat, this thesis utilised space-for-time and temporal gradients of declining glacier cover across 15 regions within the Andes, Himalayas, New Zealand Southern Alps, Pyrenees, European Alps, Alaska, Norway and the Arctic. In Peru, diatoms identified along a declining glacier cover gradient provided the first record of increased α-diversity and decreased β-diversity in this region and identified the importance of geology in structuring community responses. At a continental scale, 25-years of glacier retreat in Europe increased invertebrate richness locally, but communities became more similar to groundwater-fed rivers. Inter-regional heterogeneities, such as proglacial lakes, induced considerable variation in the strength of glacier retreat effects on biodiversity, which was also recorded in global-scale evaluations of algal and bacterial communities. Whilst algal and bacterial assemblages were highly unique, partly driven by geographic isolation, glacier ice loss substantially altered community structure, leading to major shifts in taxa responsible for important ecosystem functions such as metabolism and energy provision. Across regions, tens to thousands of taxa were associated exclusively with high glacier ice cover, underscoring the potential threat by glacier retreat to biodiversity. With the strong connections identified between glacier ice loss and river biodiversity from regional through to global scales, sustained glacier retreat is expected to drive major changes to Arctic and alpine river ecosystems worldwide.