Long-term ecological patterns of alpine streams: An imprint of glacial legacies

We examined for a glacial fingerprint on the ecology of two postglacial streams within the Swiss National Park over a 14 yr period (1999-2013). One stream (Aqua) emerges from a small rock glacier (0.2% of the catchment area), whereas the other stream (Fuorn) flows from a remnant ice field (0.02% of the catchment area). In general, abiotic and biotic properties of each stream showed a typical glacial-nival seasonality and strong between-year synchrony. Benthic organic matter and periphyton biomass peaked during so-called windows of opportunity in early spring and late autumn as in other glacial streams. Importantly, the streams contrasted regarding the glacial signature in which Fuorn had highest average turbidity, total inorganic carbon (TIC), particulate organic carbon and phosphorus levels, whereas Aqua had highest average nitrogen levels and lowest temperature and electrical conductivity. The difference in glacial signature also was evident in macroinvertebrate density (fourfold higher in Aqua) and diversity (lower in Aqua) differences between streams. In contrast to most proglacial streams, macroinvertebrate assemblages were relatively diverse and dominated by typical alpine stream species, stoneflies in particular. Both streams experienced increases in phosphorus and TIC following 2006 in response to external climate drivers with concomitant increases in simuliids and Leuctra stoneflies and decreases in heptageniid mayflies in Fuorn and Protonemura stoneflies in Aqua. Our results suggest that a glacial legacy is an inherent feature of most postglacial alpine streams, this being especially important as glacial streams transform to postglacial status in response to ongoing transformation of alpine glacial landscapes globally.