Predicting river ecosystem response to glacial meltwater dynamics: a case study of quantitative water sourcing and glaciality index approaches

Glacier retreat and thinning are occurring in many regions of the world, leading to significant changes in river discharge, water quality and aquatic ecosystems. Therefore, it is increasingly necessary for environmental scientists and managers to develop and adopt methods that link changes in meltwater dynamics and biotic response to better understand and predict future ecosystem change in glacially influenced rivers. To this end, two approaches have been developed recently: (1) the ARISE quantitative meltwater contribution approach of Brown et al. (Freshw Biol 54:1357–1369, 2009b) and (2) the glaciality index (GI) of Ilg and Castella (Freshw Biol 51:840–853, 2006), which provides a measure of glacial runoff influence on stream ecosystems based on four environmental variables (water temperature, channel stability, electrical conductivity and suspended sediment concentration). However, the relative performance and potential complementarities of these two approaches have yet to be evaluated. We conducted a methodological comparison using detailed hydrological, water quality and biological datasets collected over two summers in the French Pyrénées. Analyses revealed strong and significant correlation between ARISE meltwater contributions and the GI. However, the ARISE approach performed better when differentiating glacial influence between streams and over time. Both approaches were significant predictors of macroinvertebrate taxonomic richness, beta diversity, the number of EPT genera and total abundance, although regression models were typically stronger for the ARISE meltwater contribution approach. At the species level, ARISE performed better for predicting the abundance of 13 of the 20 most common taxa. We propose both approaches are valuable for assessing the effects of decreasing meltwater contributions on river ecosystems at the community level but this case study suggests ARISE was better able to identify subtle differences in hydrological change, community response and the abundance of individual taxa. Comparative studies from other catchments are required to further evaluate the two methods.