Does rapid glacial recession affect feeding habits of alpine stream insects?

Glacial retreat, accompanied by shifts in riparian vegetation and glacier meltwater inputs, alters the energy supply and trophic structure of alpine stream food webs. Our goal in this study was to enhance understanding of dietary niches of macroinvertebrates inhabiting different alpine streams with contrasting glacial and non-glacial (groundwater, precipitation, snowmelt) water inputs in conjunction with seasonal and habitat-specific variation in basal resource availability. We measured a range of stream physico-chemical attributes as well as carbon and nitrogen isotopes (delta C-13, delta N-15) of macroinvertebrates and primary food sources at seven sites across seasons within a Swiss glaciated catchment (Val Roseg) undergoing rapid glacial retreat (1-2 km between 1997 and 2014). Sampling sites corresponded to streams used in a previous (1997/1998) study within the same alpine catchment. Physico-chemical attributes showed wide variation in environmental conditions across streams and seasons. Significant correlation among physico-chemical proxies of glacier meltwater (phosphate-P, total inorganic carbon, conductivity, turbidity) and macroinvertebrate delta C-13, delta N-15, and size-corrected standard ellipse area (a proxy for feeding niche width) values showed that the extent of glacial water input shapes the energy base among alpine streams. Feeding niche differences among common alpine stream insect taxa (Chironomidae, Baetidae, Heptageniidae) were not significant, indicating that these organisms probably are plastic in feeding behaviour, opportunistically relying on food resources available in a particular stream and season. Seasonal trends in macroinvertebrate delta C-13 largely followed patterns in periphyton delta C-13 values, indicating that autochthonous resources were the main consumer energy source within the stream network, as shown previously. The overall range in macroinvertebrate delta C-13 (-33.5 to -18.4 parts per thousand) and delta N-15 (-6.9 to 6.7 parts per thousand) values also corresponded to values measured in the previous study, suggesting that macroinvertebrates altered diets in line with changes in environmental conditions and food resources during a period of rapid glacial retreat. Our results suggest that environmental changes brought on by rapid glacial retreat have not yet caused a profound change in the trophic structure within these fluvial networks.

Automated summary

Glacial retreat in alpine streams affects energy supply and trophic structures, but our study found that the trophic structure within fluvial networks remained relatively stable during a period of rapid glacial retreat. The input of glacier meltwater shapes the energy base among alpine streams, with macroinvertebrates adapting their diets in response to changes in environmental conditions and food resources.