Rock Glaciers and Paraglacial Features Influence Stream Invertebrates in a Deglaciating Alpine Area

Climate change and progressive glacier loss are leading to rapid ecological shifts in alpine aquatic systems. Rock glaciers and paraglacial features such as proglacial lakes, moraines, and taluses can alter the gradients of glacial influence along alpine river networks, but the ecological responses are still understudied. We investigated benthic invertebrate communities of different stream types (upper kryal, lower kryal, glacio-rhithral, krenal, and rock glacial) within a deglaciating area of the Italian Alps (Solda and Zay sub-catchments). Alpha- (Shannon index and taxa richness) and β-diversity (i.e. nestedness and turnover), production (abundance, biomass), average body mass of invertebrate taxa, and functional feeding groups were used to analyse the influence of habitat conditions on invertebrate communities in different microhabitats (mineral substrates, bryophytes, Hydrurus foetidus). We defined an index of habitat mildness based on water temperature, channel stability, turbidity, and organic detritus, to measure the difference in community metrics over a gradient of habitat amelioration, driven by the mixing of distinct stream types and their interactions with paraglacial features. While diversity proxies showed an asymptotic behaviour along the gradient, the quantitative metrics peaked at intermediate mildness. Peaks of invertebrate biomass/abundance were recorded at the glacio-rhithral station below the talus. Highest average body mass occurred at Zay, specifically at the rock glacial stream, and at the lower kryal stations influenced by the lake/moraine and the rock glacier. The moraine outflow showed unexpected high biomass/abundance values of shredders and grazers, that were supported by a high epilithic production. Bryophytes acted as local scale multipliers of mildness as they stored more detritus, and hosted higher biomass/abundance of invertebrates and, in particular, of gatherer–collectors. Beta-diversity, driven by the turnover component, was positively related to different habitat mildness of the sampling stations. Although influenced by distinct lentic/lotic conditions, rock glacial streams hosted several taxa typical of glacio-rhithral and krenal stations, alongside large numbers of Diamesa (Chironomidae), a genus typical of kryal habitats. Under progressive deglaciation, rock glaciers and paraglacial features influence the invertebrate communities of alpine river networks. As they host a high number of taxa dwelling in non-glacial locations, rock glacial streams may act as stepping stones facilitating invertebrate colonisation following glacier retreat. After glacier loss, rock glacial streams may represent climate refugia for cold adapted taxa and/or kryal specialists, because the slow thawing of their ice might sustain cold water conditions for a longer period of time.