Structural and functional changes in the macroinvertebrate community in Alpine stream networks fed by shrinking glaciers

Abstract

The status of glaciers is alarming globally with still unknown effects on freshwater ecosystems. Thegeneral aim of this study was to investigate the structural and functional changes in the macroinvertebrate communityin stream networks fed by shrinking glaciers in relation to environmental variables. Feeding glaciers haddifferent surface areas and retreating rates. We selected 10 study sites in the Italian Alps, spanning five kryal, twoglacio-rhithral, two krenal and one proglacial pond, sampled twice in summer 2018. Eight of these sites weresampled previously between 1996 and 2014. In all, in 2018, > 15,000 individuals (73 taxa) were collected, of which82 % were chironomids (Diptera Chironomidae) (33 taxa). Diamesa zernyi gr. (Chironomidae Diamesinae) wasthe most frequent and abundant taxon, followed by Oligochaeta and Chironomidae Orthocladiinae. Taxonomical(Shannon index) and functional (based on functional feeding groups) diversity both increased with decreasingglacial influence (estimated as glacial index, GI, based on distance from the glacier snout of each site and glacierarea), from kryal to glacio-rhithral and krenal habitats. Taxa distribution was explained mainly by GI, maximumwater temperature, substrate stability, silica, epilithic chlorophyll-a, and benthic particulate organic matter. Thesame variables explained temporal differences in the community structure for the eight sites re-sampled in the lasttwo decades. Among the taxa best associated with high GI was the chironomid Diamesa steinboecki, that in 2018was exclusive of the kryal sites with GCC (% glacier cover in the catchment, expressed in a range from 0 %–100 %)> 50 % and maximum temperature < 5 °C. This species was absent only in the kryal site C0 (GCC = 33 %), whereit was dominant in 1996 –1997. This site was still fed by ice melt in 2018, but resembled a glacio-rhithral site inhabitat features (e.g. maximum temperature > 6 °C) and biota (e.g. % Diamesa spp. < 30 %). In C0, it was evidentthat in the last 22 years, the macroinvertebrate community changed remarkably. This change was due to upstreammigration of generalist insect species to sites once exclusive for kryal species with consequent changes in food webstructure and loss of strictly kryal species, first D. steinboecki that we propose as the “flagship” species of the kryalin the Alps. The site C0 represents a “tipping point”, showing us the effects of climate change on alpine biodiversityin a relatively short period; unfortunately, there are many such sites in the Italian Alps.