Glacier–groundwater stress gradients control alpine river biodiversity

In alpine river networks, water source (i.e. snow, ice or groundwater) plays a major role in influencing flow regimes, benthic habitat and macro-invertebrate community structure. Across these systems, a natural stress gradient can be conceptualized, from rivers fed exclusively by meltwater (harsh habitat) to those with no melt input (relatively benign, groundwater fed). However, despite the current context of rapid glacier retreat, our understanding of linkages between meltwater contributions, physico-chemical habitat and biodiversity remains limited. To address this research gap, habitat characteristics and macro-invertebrate community structure were studied at 26 sites (five river basins) in the French Pyrénées across a meltwater gradient from 0% to 99%. A combination of generalized regression models and multivariate analyses showed that the stress gradient was associated with the following: (i) linear responses of key physico-chemical habitat variables, in particular bed stability, (ii) unimodal responses at the community level (e.g. richness and total density peaks at 40–60% meltwater contribution), and (iii) both unimodal and monotonic responses at the level of individual taxa. Sites characterized by high contributions of meltwater, although species poor, were important for beta diversity because of their specialist endemic fauna. Our findings suggest that continued glacier and snowpack retreat due to expected future climate change are likely to lead to more homogeneous alpine river habitats (i.e. reduced meltwater–groundwater stress gradient breadth). As a result, increased alpha diversity is expected as previously harsh habitats become more favourable; however, an associated decrease in beta diversity is likely as glacial stream specialists become replaced by generalists.