After the peak water: the increasing influence of rock glaciers on alpine river systems

Abstract

Human‐accelerated climate change is quickly leading to glacier‐free mountains, with consequences for the ecology and hydrology of alpine river systems. Water origin (i.e., glacier, snowmelt, precipitation, and groundwater) is a key control on multiple facets of alpine stream ecosystems, because it drives the physico‐chemical template of the habitat in which ecological communities reside and interact and ecosystem processes occur. Accordingly, distinct alpine stream types and associated communities have been identified. However, unlike streams fed by glaciers (i.e., kryal), groundwater (i.e., krenal), and snowmelt/precipitation (i.e., rhithral), those fed by rock glaciers are still poorly documented. We characterized the physical and chemical features of these streams and investigated the influence of rock glaciers on the habitat template of alpine river networks. We analysed two subcatchments in a deglaciating area of the Central European Alps, where rock glacier‐fed, groundwater‐fed, and glacier‐fed streams are all present. We monitored the spatial, seasonal, and diel variability of physical conditions (i.e., water temperature, turbidity, channel stability, and discharge) and chemical variables (electrical conductivity, major ions, and trace element concentrations) during the snowmelt, glacier ablation, and flow recession periods of two consecutive years. We observed distinct physical and chemical conditions and seasonal responses for the different stream types. Rock glacial streams were characterized by very low and constant water temperatures, stable channels, clear waters, and high concentrations of ions and trace elements that increased as summer progressed. Furthermore, one rock glacier strongly influenced the habitat template of downstream waters due to high solute export, especially in late summer under increased permafrost thaw. Given their unique set of environmental conditions, we suggest that streams fed by thawing rock glaciers are distinct river habitats that differ from those normally classified for alpine streams. Rock glaciers may become increasingly important in shaping the hydroecology of alpine river systems under continued deglaciation.

Rock glaciers and related cold rocky landforms: overlooked climate refugia for mountain biodiversity

“…However, unlike nearby streams and an emphasis on temperature, the chemical composition of high mountain lakes has received considerable attention. High concentrations of ions (including nitrates, calcium, magnesium, and sulphates) and heavy metals, often exceeding drinking water limits, are common in rock glacier outflows (Williams et al 2007;Colombo et al 2018;Brighenti et al 2019b). High metal concentrations promote sublethal effects on lake biodiversity, as shown by a high prevalence of mouth deformities in the midge Pseudodiamesa nivosa in a rock glacial lake of the Italian Alps (Ilyashuk et al 2014).…”

“…Current knowledge is also based almost solely on active rock glaciers, with little known for other CRLs, whose outflow water chemistry appears to be less harsh than rock glaciers (Brighenti et al 2019b).…”

“…Quantifying thermal regimes, and biological and chemical settings of CRLs requires field-based surveys, ideally paired with longer term monitoring. Monitoring of water temperature may be a particularly inexpensive strategy for identifying CRL-based refugia, especially when combined with satellite imagery showing a lack of visible ice or snow upstream (Hotaling et al 2019;Brighenti et al 2019b). When considering the long-term viability of CRL-influenced climate refugia, the distribution and type of CRL is important.…”

Rock glaciers and paraglacial features influence stream invertebrates in a deglaciating Alpine area

“…Outflows from rock glaciers, i.e. creeping rocky debris containing permafrost ice (Jones et al., 2019), have been recently proposed as a particular habitat (Brighenti, Tolotti, Bruno, Engel, et al., 2019; Hotaling et al., 2019), given the unique environmental conditions that include clear (<5 NTU) and cold (<1.5°C) waters, stable channels, and high concentrations of major ions and trace elements (Colombo et al., 2018). Despite contributing to the hydrological diversity of alpine river networks, rock glacial streams have hitherto received little attention from ecological research on benthic invertebrates, when compared to glacier‐fed (kryal) and groundwater‐fed (krenal) systems (Brighenti, Tolotti, Bruno, Wharton, et al., 2019; Tronstad et al, 2020).…”

“…Finn et al., 2010; Hieber et al., 2005; Milner et al., 2001; Milner & Petts, 1994), little is known on the ecological relevance of other paraglacial features (i.e. occurring following glacier retreat) such as moraines and taluses, although these landforms have been shown to decrease the glacial influence along the glacier‐fed streams (Brighenti, Tolotti, Bruno, Engel, et al., 2019). The pattern of glacial influence is also mediated by local conditions such as mesohabitat diversity and riffle‐pool prevalence, which influences the invertebrate communities and their functional traits over small spatial scales (Herbst et al., 2018).…”