Headwaters’ Isotopic Signature as a Tracer of Stream Origins and Climatic Anomalies: Evidence from the Italian Alps in Summer 2018

Abstract

Glaciers are shrinking due to global warming, resulting in a diminishing contribution of ice and snowmelt to headwaters and subsequent consequences to freshwater ecosystems. Within this context, we tested whether water-stable isotopes are spatio-temporal tracers of (i) water in high altitude periglacial environments, being the isotopic signature of surface water inherited from the snow/icemelt, groundwater, and rainfall; and (ii) regional (year-specific) meteorological conditions, being the isotopic signature of precipitations affected by air temperature, humidity and aqueous vapour origin, ascribing stable isotopes to the list of "essential climate variables" (ECVs). To this end, we investigated the ionic and isotopic composition (δ 18 O and δ 2 H) of six high-altitude streams and one pond in the Italian Alps (Noce and Sarca basins) during the ablation season in 2018. Differences between habitat types (pond, kryal, rhithral, krenal) were detected. More negative values of δ 18 O and δ 2 H were recorded in the kryal and glacio-rhithral sites, dominated by ice and snowmelt, in early summer. Less negative values were recorded in these sites in late summer, as well as in the krenal sites, which were dominated by groundwater and rainfall inputs. The isotopic results also show that the complex alpine orography influences air masses and moisture, ultimately resulting in isotopic differences in the precipitations of neighboring but distinct catchments (Sarca and Noce basins). On average, less negative values were recorded in the Sarca basin, characterized by a higher contribution of precipitation of Mediterranean origin. In general, isotopic results of the entire water population appeared to be strongly influenced by the regional climatic anomaly of 2018, which was anomalously warm. Therefore, the study will provide additional information for the climate change debate, proposing water isotopes as ECVs for assessing change in a warmer future.Water 2020, 12, 390 2 of 18 future, mean annual runoff is expected to progressively decrease in glacier-fed streams, and overall changes in the hydrological behavior of small-and large-scale catchments are expected [9,10].Glacier shrinking and its consequences on glacier-fed streams induce variation in the physical, chemical and biological features of freshwater ecosystems [11,12]. This profoundly affects the ecosystem services that glacier-fed rivers provide to humans, particularly the provision of water for agriculture, hydropower, and consumption [11,13,14]. This is crucial for mountain settlements in glacialized regions, where the snow and ice melt contribution, especially in spring and summer, is the most relevant water source for several uses [13][14][15]. In particular, streams draining in Alpine areas are generally identified as some of the environments most sensitive to climate change [16,17]. This is particularly true for glacier-fed streams and rivers [18]. Within this scenario, runoff changes also impact on the biological community structure [13,19]. In fact, alpine stream...

How do macroinvertebrate communities respond to declining glacial influence in the Southern Alps?

“…CR2 was the only site selected downstream of the confluence of a non-glacial tributary. The proglacial pond (AG_pond) is located at a distance of 23 m from the Agola glacier snout, has an area of 227 m 2 and a depth < 1.5 m. It is fed by ice meltwater (mainly in early summer), groundwater, and rainfall (Marchina et al, 2020). The pond undergoes water level fluctuation (from early to late summer we recorded a decrease of 20 cm in water level) and we can presume that it freezes solid during winter.…”

“…2), we made a priori classifications to our study sites. The ratio between Si and SO 4 2is generally used to discriminate stream types based on the dominant water contribution (Brown et al, 2003(Brown et al, , 2006Marchina et al, 2020). In streams fed by ice melt (both kryal and glacio-rhithral), the sulphate and silicium concentrations are generally low in early summer ('quick flow' dilutes rapidly routed meltwater sensu Brown et al, 2003).…”

The Isotopic (δ18O, δ 2H, δ13C, δ15N, δ34S, 87Sr/86Sr, δ11B) Composition of Adige River Water Records Natural and Anthropogenic Processes

“…The oxygen and hydrogen stable isotopes of Adige river water are reported in Table 1. These parameters are sensitive indicators of temperature and humidity at the regional scale, and their monitoring is essential to the understanding of climatic changes [11][12][13][14][15][16]49,50]. For the samples considered in this study, the values refer to distinct sampling periods, plotted as δ 18 O vs. the distance from the source, as shown in a diagram that displays sub-parallel trends (Figure 2A).…”

“…The decrease in river discharge appears to be caused by regional decreases in precipitation and increases in temperature linked to more general climate changes. This critical issue is particularly relevant for riverine systems fed by Alpine glaciers and the associated ecosystems [10,11].…”

Source Water Apportionment of a River Network: Comparing Field Isotopes to Hydrodynamically Modeled Tracers

“…Other studies have used isotope signatures to identify which tributaries have dominant effects on geochemical properties of the mainstem [18], to investigate snowmelt versus rainfall contribution [19], and to identify water origin [20]. Our study aimed to use two methods to find the SWF at each location within the study area over time; one method used is similar to that of Halder et al [21] and Marchina et al [20], which evaluates stable hydrogen ( 2 H) and oxygen ( 18 O) isotopes at sample locations and compares those to the tributary source waters of interest.. Another direct application of using physical properties of tributary water to find the distribution of source waters in a river network was presented by Peter et al [22] in a physically modeled hypothetical watershed. In the study, high-resolution mass spectrometry of organic contaminants was used to confirm the source hydrology of a hypothetical river system with known flows, and thus known mixtures.…”