Spatio-temporal shifts of macroinvertebrate drift and benthos in headwaters of a retreating glacier

Recent alpine stream studies have aimed to predict biotic responses to changing environmental conditions, hydrological regime and climate, but there is little information on macroinvertebrate drift shifts in this context and most research was largely restricted to snow-free period. We compared seasonal drift-benthos patterns in proglacial streams of Val Roseg (Switzerland) between 2008/2009 and 1997/1998. Samples were collected monthly (including winter) in a glacial lake outlet stream, a proglacial stream, and their post-confluence stream (main channel) in both periods. Measured physical and chemical parameters indicated the largest glacial meltwater influence in the proglacial stream and its lower inflow in 2008/2009 compared to 1997/1998. Drift largely coincided with benthic structure at each site, although drift-benthos patterns were at times asynchronous, depending on site and flow conditions. Our results indicate that the initial stages of hydrological freeze-melt events and the period of snow cover are most favorable for macroinvertebrates in glacial streams. Drift density was significantly lower in 2008/2009 than in 1997/1998. In 2008/2009, we observed lower chironomid, but higher ephemeropteran and plecopteran drift densities. The drift-benthos changes between periods likely reflect behavioral and life-history responses of proglacial stream macroinvertebrates to seasonal shifts in environmental conditions in response to glacial retreat.