Ecosystem development of streams lengthened by rapid glacial recession

Glaciers worldwide are receding rapidly, thereby causing the lengthening of stream channels below glaciers. In this study, we recorded habitat properties and macroinvertebrates colonizing newly formed channels below 8 different glaciers in the Swiss Alps. Six glacial streams sampled in 1998 were re-sampled in 2009 and 2011, with sites formerly at the glacial snout also sampled in later years. Glacial recession resulted in streams being lengthened by 160 to 480 m between 1998 and 2009, and by 50 to 1305 m between 2009 and 2011. Newly formed streams near glacial snouts also were up to 100+ m higher in elevation than those in 1998. Sites sampled at the glacial snout in 1998 had 2-4 degrees C higher summer water temperatures in 2009 and 2011. Sites in 2009 and 2011 typically had higher pH, phosphorus constituents (PP, SRP), and turbidity values than in 1998, whereas nitrate levels were lower. The biomass (AFDM) of periphyton and benthic organic matter at most sites remained similar among years, although periphyton was highest at Morteratsch sites in 2011. In contrast, seston levels were up to ten-fold higher in later years than in 1998, likely reflecting the increased turbidity from glacial meltwaters (e.g., scouring). Most sites had similar average macroinvertebrate densities and taxon richness regardless of the year sampled, indicating newly formed channels were quickly colonized. The most common macroinvertebrate was Chironomidae (mostly Diamesinae, although chironomid relative abundances decreased by half at most sites between 1998 (50-90% of the composition) and 2011 (20-50% of the composition). Other common taxa in later years included various stoneflies (Leuctra spp., Protonenmra spp., Nemoura spp.), mayflies (Baetis alpinus, Rhithrogena loyolaea), and Oligochaeta. An NMDS analysis of the macroinvertebrate assemblages revealed that sites within a glacier catchment differed both within the year sampled as well as between years. This study documents that recently lengthened glacial streams are quickly colonized by periphyton and macroinvertebrates and that succession patterns (spatial and temporal) may be important in these systems. Further research is needed to determine the mechanistic basis underlying colonization dynamics and succession patterns in glacial streams as glaciers continue to recede.