Introduction

Glaciers are one of the coldest of Earth’s biomes, characterized by low temperature, strong seasonality, and truncated food webs with unique organisms adapted to harsh environmental conditions (Anesio and Laybourn-Parry 2012; Hodson et al. 2008; Zawierucha et al. 2021). Due to their relatively high biological activity, as well as physical and chemical weathering of minerals, glacier surfaces are important for nutrient cycling in polar and mountain areas since they can serve as both sources and sinks of essential nutrients and compounds, particularly those containing carbon and nitrogen (Bagshaw et al. 2013; Stibal et al. 2012; Telling et al. 2011). For example, supraglacial (on the glacier surface) primary producers can have comparable production rates to those from polar lakes (Callieri and Bertoni 1999; Hood et al. 2015), and input of supraglacial organic matter transported from glaciers by meltwater can have a significant effect on downstream terrestrial and aquatic ecosystems (Bagshaw et al. 2013; Hood et al. 2015; Stibal et al. 2012). Yet, the importance of glacier ecosystems is often overlooked by broader ecological community (Stibal et al. 2020), as highlighted by Cauvy-Fraunié and Dangles (2020) largely because these ice masses are often perceived as lifeless, and their functioning remains enigmatic (e.g., Buda et al. 2021).

The surface of glaciers is a heterogeneous and dynamic landscape with variable environmental conditions which forms distinct habitats (Anesio and Laybourn-Parry 2012). The key element of supraglacial biological activity is cryoconite (Rozwalak et al. 2022). Cryoconite is a dark sediment composed of a consortium of mineral particles, bacteria, algae, and other organic matter (Langford et al. 2010; Takeuchi et al. 2010; Rozwalak et al. 2022). Cryoconite occurs on the glacier surface and through the reduction of albedo (reflection of solar radiation) which leads to greater ice melt, drives the formation of small depressions called cryoconite holes (Takeuchi et al. 2002). Cryoconite holes are formed in the ablation zone of the glacier (area with the ice loss exceeding its gain) where the surface glacier ice is exposed during the summer season, and it is snow and debris free (e.g., MacDonell and Fitzsimons 2008).

Cryoconite holes are water-filled reservoirs which serve as important stores of organic matter and are hot-spots of supraglacial biodiversity with a myriad of inhabitants from microbes to micro-invertebrates, such as tardigrades and rotifers, which dominate cryoconite holes worldwide (Pittino et al. 2018; Sommers et al. 2019; Zawierucha et al. 2021). In some regions of the world, cryoconite holes are inhabited even by macro-fauna like Plecoptera in South America or Diptera in Himalaya (Kohshima 1985; Zawierucha et al. 2022).

Apart from cryoconite holes, some parts of glacier surface are covered by a layer of gravel and stones called supraglacial debris (e.g., Azzoni et al. 2016). Due to glacier melting, supraglacial debris is a dynamic semi-aquatic environment which, depending on region and glacier, is inhabited by organisms, such as nematodes, springtails, and other minute and macro-arthropods (Buda et al. 2020; Valle et al. 2022; Zawierucha et al. 2021). The surface of the ablation zone mostly consists of weathering crust (highly porous surface ice formed by solar heating of the glacier surface) which, due to microscopic water veins, can be compared to a shallow-perched aquifer with enhanced microbial activity on the glacier surface (Christner et al. 2018; Cook et al. 2016; Patterson 1994). Surface ice of the weathering crust can be perceived as another supraglacial habitat and a transitional zone or ecotone between cryoconite holes and supraglacial debris.

Invertebrate consumers are important components in the functioning of supraglacial habitats due to their feeding on bacteria, fungi, and algae, and their ability to shape the structure of primary producer communities (Vonnahme et al. 2016; Zawierucha et al. 2022). Nevertheless, whether different taxa of glacier consumers utilize the same or different food, how they are spatially distributed on the ice surface, and what is their contribution to secondary productivity (quantifiable by estimating the biomass of heterotrophs, equivalent of net primary production by autotrophs) is still far from understood.

Considering that alpine glaciers are one of the fastest disappearing glacier systems worldwide (Gobbi et al. 2021; Lencioni et al. 2021; Zawierucha and Shain 2019), such gaps in the knowledge on supraglacial consumers and their habitats prevent the understanding of how supraglacial ecosystems may transition over time since many supraglacial habitats can change from one to another (e.g., surface ice of the weathering crust into cryoconite holes and vice versa).

We focused our study on dominant groups of consumers in three key types of supraglacial habitats: cryoconite holes, supraglacial debris, and surface ice of the weathering crust on the alpine glacier Forni (Fig. 1). We combined field observations on the distribution of consumers, estimations of their biomass, stable isotopic analyses of carbon (δ13C) and nitrogen (δ15N), analyses of OM, analyses of carbon and nitrogen contents, and the estimation of mineral composition in cryoconite and debris. We primarily aimed to: (i) provide an initial observation and description of spatial variation in consumer communities on an alpine glacier since most recent studies from alpine glaciers focus on microbial communities; and by analyzing organic matter contents, mineral composition, and δ13C and δ15N, (ii) point out possible drivers of distinctiveness in their habitats.

Fig. 1
figure 1

A Ablation zone of the Forni Glacier; B, C Cryoconite holes on the Forni Glacier; D Surface ice of the weathering crust not covered by dust or algae; E Surface ice of the weathering crust covered by cryoconite and ice algae; F Supraglacial debris—a main habitat for springtails on the Forni Glacier

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Materials and methods

Study site and sampling

Fieldwork was conducted in August 2020 on the ablation zone of the Forni Glacier in the Alps (approximately 2500 m a. s. l., 46° 24ʹ N 10° 35ʹ E, glacier surface shown in Fig. 1A). The detailed description of the Forni Glacier is provided in Azzoni et al. (2016), Citterio et al. (2007), and Senese et al. (2020). We collected metazoans and cryoconite from cryoconite holes (Fig. 1B, C), supraglacial debris (Fig. 1F), and surface ice of the weathering crust (Fig. 1D, E) at randomly chosen sites along the ablation zone of the glacier to reflect the heterogeneity and biotic diversity of the glacier surface. Sampling sites were located in the upper part of the ablation zone (area in front of the icefall, approx. 300 m from the glacier snout, where the glacier slope is mild, and the surface starts to be flat).

Cryoconite holes were a few cm deep and up to approx. 30 cm in diameter. The amount of cryoconite in the bottom of the holes did not exceed 1 cm in depth. Cryoconite granules on the Forni Glacier were approx. 0.5 mm in size (Rozwalak et al. 2022), surrounded by fine organic matter and mineral particles with and average size up to approx. 1 mm (Fig. S1C).

Supraglacial debris occured in a band composed from sediment to boulders which occurred along the entire axis of the ablation zone in front of the icefall (approx. 600 m in length). Fine sediment and smaller fraction of gravel (e.g., granules, pebbles) mostly form a thin layer which is wet or even submerged in some places. Pebbles and cobbles are dry, but the border between their surface and ice forms a wet layer inhabited by springtails (Fig. S1A). Sample of supraglacial debris consisted of fine organic matter and mineral particles with size < 1 mm up to rocks approx. 1.5 cm long (Fig. S1B). This material consists mostly of gravel fraction without cryoconite. Supraglacial debris originates most likely from both moraines and surrounding mountain slopes.

Surface ice of the weathering crust differed between sample types, clean ice was mostly solid while ice covered by cryoconite and, most likely, ice algae was characterized by relatively big crumbling crystals. The thickness of crystals was approx. 7 cm.

In total, one pooled sample of cryoconite (approx. 1 L of sediment from more than 15 cryoconite holes), one pooled sample of supraglacial debris (approx. 1 kg in total collected from many sites in the middle and upper part of supraglacial debris band), and 12 independent samples of surface ice of the weathering crust from which 6 were clean ice and 6 were ice covered by algae and cryoconite (potential food for animals) were collected for the characterization of invertebrate communities and analyses of their carbon and nitrogen contents, δ13C and δ15N, and the organic matter content.

Additionally, cryoconite for the estimation of biomass of tardigrades was collected from 8 randomly chosen cryoconite holes (in the upper part of the ablation zone, see the description above). For this purpose, independent cryoconite samples were collected from the bottom of holes by Pasteur pipette. We collected approx. 1 cm2 of sediment from the bottom of the holes (then the biomass of tardigrades in cryoconite per 1 cm2 for each sample was estimated). Sampled cryoconite holes for the biomass estimation were located in the same area of the ablation zone where cryoconite for isotopic analyses was sampled. Sampled cryoconite holes were of diverse shapes (from irregular to oval), and in size range of approx. 10 cm to 30 cm. Such holes reflected the heterogeneity of hole shapes on the Forni Glacier.

All samples were collected in sterile falcon tubes or plastic bags using aseptic spoons, clean plastic pipettes, and nitrile gloves. Cryoconite was collected by a pipette, debris was collected with a spoon, and surface ice of the weathering crust was collected by scratching a few centimeters of the surface ice (area of 10 cm × 10 cm) by chisel. Springtails were collected into a falcon tube by air flow made by a big pipette from the stone surface under randomly chosen supraglacial stones. Immediately after collection, all samples were frozen and transported to Adam Mickiewicz University in Poznań and kept at –20 °C until further analyses.

Stable isotopic analyses

Preparation of invertebrate consumers

For analyses of tardigrades, approx. 200 cm3 of cryoconite from the pooled sample was slowly melted and tardigrades were collected by glass Pasteur pipettes under a light microscope. During the collection, tardigrades were constantly cooled by a cooling pad. Individuals were cleaned from superficial mineral and organic particles in distilled water and transferred into a clean Eppendorf tube. Thereafter, the sample was stored at –20 °C until further processing started. Springtails were already separated from supraglacial debris during the field sampling and immediately frozen in a conical tubes. Thus, no preparation before lyophilization was needed. After all samples were prepared, tardigrades and springtails were melted, and individuals were cleaned at least two times in a drop of distilled water under a light microscope (Leica DM750). Overall 546 individuals of tardigrades and 193 individuals of springtails were cleaned, and transferred into pre-weighed tin capsules (Elemental Microanalysis, 8 × 5 mm, D1013) using a glass Pasteur pipette for tardigrades and stainless-steel lab spatula for springtails. The total number of individuals was divided into two subsamples for tardigrades and three subsamples for springtails. Afterward, all samples were stored at –20 °C and at –72 °C half an hour before the lyophilization. The duration of lyophilization was 3.5 h. Thereafter, samples were weighed (Mettler Toledo Excellence Plus XP6; linearity = 0.0004 mg) and tin capsules were closed, wrapped, and stored in a desiccator at 22 °C with 32% humidity for approx. 1 h before, they were analyzed. The average dry weight of subsamples was ⁓ 0.109 mg for tardigrades and ⁓ 1.727 mg for springtails. During the lyophilization, a few individuals of tardigrades (5−10) were scattered around the tin capsule, probably because of their low dry weight.

Preparation of cryoconite and supraglacial debris

For analyses of cryoconite and supraglacial debris, all animals were removed from the material and both samples were frozen in –20 °C before further processing started. For analyses of cryoconite, approx. 6 ml of material from pooled sample was melted and homogenized using an agate pestle and mortar and dried on a Petri dish at 45 °C for 19 h. For analyses of supraglacial debris, approx. 30 ml of material from pooled sample was melted and divided into 3 size fractions using: (i) plastic Pasteur pipette with a 1 mm diameter of the syringe; (ii) plastic Pasteur pipette with a 2 mm diameter of the syringe; (iii) plastic spoon for debris with an average size of 5 mm. Thereafter, all three fractions were separately homogenized using an agate pestle and mortar and dried on Petri dishes at 45 °C for approximately 18 h. We partially covered all Petri dishes by an aluminum film during the drying.

For analyses of δ15N in OM, dry cryoconite and supraglacial debris were transferred without any other preparation into pre-weighed tin capsules (Elemental Microanalysis, 8 × 5 mm, D1013) and weighed. For all samples, three subsamples (from one pooled sample) on average 30.7 mg were prepared. For analyses of δ13C in the OM, on average 11.7 mg of material was transferred into pre-weighed silver capsules (Elemental Microanalysis, 8 × 5 mm, D2008) and carbonates were dissolved using 10% HCl moistened with diH2O. The acid was pipetted into the capsules followed by additions of 20, 30, 50 or 10, 10, 30, 50 and 100 μL with drying after each addition according to Brodie et al. (2011) with the modification after Vindušková et al. (2019). After the last acid addition, samples were left to dry at 50 °C for 17 h. After drying, silver capsules were inserted into tin capsules and put into a desiccator for 22 d.

Analytical approach

For stable isotopic analyses, the method according to Novotná Jaroměřská et al. (2021) was used and the carbon and nitrogen contents, δ13C, and δ15N in all samples were analyzed using a Flash 2000 elemental analyser (ThermoFisher Scientific, Bremen, Germany) and isotope-ratio mass spectrometer Delta V Advantage (ThermoFisher Scientific, Bremen, Germany), respectively. Released gasses (NOx, CO2) separated in a GC (gas chromatography) column were transferred to an isotope-ratio mass spectrometer through a capillary by Continuous Flow IV system (ThermoFisher Scientific, Bremen, Germany). The stable isotopic values were expressed in standard delta notation (δ) with samples measured relative to Pee Dee Belemnite for carbon isotopes and atmospheric N2 for nitrogen isotopes. All values were normalized to a regression curve based on international standards IAEA-CH-6, IAEA-CH-3, IAEA 600 for carbon and IAEA-N-2, IAEA-N-1, IAEA-NO-3 (International Atomic Energy Agency, Vienna) for nitrogen. The regression curve of the total gas for analyses of cryoconite was based on the international standard Soil Standard Clay OAS (Elemental Microanalysis, UK). Analytical precision as a long reproducibility for standards was within ± 0.03 ‰ for δ13C and ± 0.02 ‰ for δ15N.

For cryoconite and supraglacial debris, the δ13C of decarbonized material and δ15N of material without any treatment were used as a reference to the isotopic composition of potential food source for tardigrades and springtails. Carbon content in decarbonized samples and nitrogen content in samples without treatment were used as a reference to OC (organic carbon) and ON (organic nitrogen).

X-ray diffraction

For analyses of mineral composition of cryoconite and supraglacial debris, homogenized sediment from remaining samples used for isotopic analyses was analyzed by an X-ray diffraction. Analyses were performed on the Panalytical X’Pert Pro (PW3040/60) with an X’Celerator detector. The measurements were conducted under the following conditions: radiation – CuKα, 40 kV, 30 mA; angular range − 3 to 70° 2θ; step 0.02°/200 s. The results were evaluated using an X’Pert HighScore Plus 1.0d software program with a JCPDS PDF-2 (ICDD 2002) database.

Organic matter measurements

The amount of OM in cryoconite and debris was measured as a percentage weight loss through the combustion at 550 °C for 3 h following drying at 50 °C for 24 h (Wang et al. 2011). Supraglacial debris was divided into two fractions, the small below 2 mm as a reference to the majority of consumable OM in the debris and the bigger one which contained debris without any size selection.

Observation of animals, their identification, and biomass estimation

Animals collected from three sampled habitats were investigated under stereo microscopes in the laboratory. Tardigrades and springtails were identified by comparisons with the original literature (Buda et al. 2020; Dastych et al. 2003; Fjellberg 2010; Zawierucha et al. 2019a).

For the biomass estimation of tardigrades, body length and width of fresh (wet) individuals were measured from photographs (Fig. S2A, B) taken by Quick PHOTO Camera 3.0 software (Promicra, Prague, Czech Republic) under an Olympus BX53. Animals were measured using ImageJ 1.53 software (Schneider et al. 2012). Photographs were turned into grayscale and binary defined (Fig. S2C). Thereafter, the “analyze particle command” (this command counts and measures objects in binary or thresholded images) was used to fit an ellipse onto each organism (Fig. S2D). Primary and secondary axis of each ellipse marked body length and body width of tardigrades. All these steps were run as separate macros (a simple program that automates a series of ImageJ commands) to provide a better control on the whole analysis. Biomass (wet weight, W) of each specimen was calculated based on the formula of Hallas and Yates (1972): if body length (L) and width (D) were 4:1; W = L3 × 0.051 × 10−6, or 5:1; W = L3 × 0.033 × 10−6. The wet weight was converted to dry weight based on De Bovée and Labat (1993).

Data on the biomass of springtails from the supraglacial debris of the Forni Glacier were taken from Buda et al. (2020). Dry biomass of springtails was calculated based on a linear equation with coefficients proposed by Tanaka (1970). Detailed description of sampling and calculations for the estimation of biomass of springtails in the supraglacial debris can be found in Buda et al. (2020). Weight of tardigrades is expressed as dry biomass per approx. 1 cm2, and weight of springtails is expressed as dry biomass per 1 cm2 of stone surface in a band of supraglacial debris.

Statistical analyses and graphical illustration of results

The graphic visualizing of stable isotopic values was created using the R version 3.5.3 (R Development Core Team 2018) package ggplot2 (Wickham 2009).

Results

Cryoconite holes on the Forni Glacier were inhabited by a single morpho-species of tardigrade (Cryobiotus klebelsbergi) (Fig. 2A). We found only few individual rotifers among hundreds of tardigrades. In supraglacial debris, particularly between stones and ice surface, only springtails belonging to the family Isotomidae (Fig. 2B) occurred and no other invertebrates were found. Occasionally, springtails were found on the surface water of cryoconite holes, but their densities were low compared to those in the supraglacial debris. Samples of surface ice of the weathering crust were devoid of active invertebrates, and only a few carcasses of tardigrades and springtails were found. Thus, surface ice of the weathering crust was not further investigated. The biomass of tardigrades in cryoconite holes ranged from 0 to 75 μg/cm2, with an average of 12.28 μg/cm2 (Fig. 2C). The biomass of springtails was 0–151.82 μg/cm2 of stones, with an average of 12.67 μg/cm2 (Fig. 2D) (Buda et al. 2020).

Fig. 2
figure 2

A Tardigrades (Tardigrada); B Springtails from the family Isotomidae (Collembola); C Dry biomass of tardigrades from cryoconite holes; D Dry biomass of springtails from the supraglacial debris (data on springtails are handled from Buda et al. (2020))

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Results of stable isotopic analyses showed differences between both groups of invertebrate consumers as well as between organic matter in different fractions of supraglacial debris and cryoconite. The δ13C and δ15N of tardigrades and springtails was considerably lower compared to OM in cryoconite and fractions of supraglacial debris (Fig. 3). The differences in mean isotopic values revealed by stable isotopic analysis were 1.72 ‰ for the δ13C and 0.92 ‰ for the δ15N between tardigrades and springtails with higher (more enriched in heavier isotope) values for springtails (Fig. 3). Cryoconite revealed higher δ13C compared to all size fractions of supraglacial debris. Also, each size fraction of debris revealed different isotopic values. Smaller fractions had lower δ15N and higher δ13C values compared to bigger fractions. The biggest size fraction (approx. 5 mm) had highest δ15N and lowest δ13C from all size fractions of debris.

Fig. 3
figure 3

The map of carbon (δ13C) and nitrogen (δ15N) stable isotopic values of tardigrades (Tar), springtails (Coll), cryoconite (Cry), and size fractions of supraglacial debris (Deb) with mean and error bars for their subsamples. The size fractions of debris are Deb_1 < 1 mm, Deb_2 < 2 mm, Deb_3 ≤ 5 mm

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The organic matter content differed between cryoconite and supraglacial debris. The OM content in cryoconite varied from 11 to 12.5%. The OM content in debris was 2.05% in the fraction below 2 mm and 0.62% in all size fractions together. Cryoconite had higher amount of OC and ON, and lower OC:ON (mol/mol) ratio compared to all fractions of supraglacial debris. In supraglacial debris, OC and ON contents decreased with increasing size of fraction (Fig. 4). The detailed results of stable isotopic ratios, carbon and nitrogen contents, and OC:ON ratios in consumers (tardigrades and springtails), organic matter in each size fraction of supraglacial debris (Deb 1 < 1 mm, Deb 2 < 2 mm, Deb 3 ≤ 5 mm), cryoconite, and their subsamples are shown in Table 1.

Fig. 4
figure 4

The proportion (%) of carbon (C) and nitrogen (N) in organic matter of cryoconite and size fractions of supraglacial debris (Deb) with mean and error bars (standard deviation) for their subsamples. The size fractions of debris are Deb_1 < 1 mm, Deb_2 < 2 mm, Deb_3 ≤ 5 mm

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Table 1 Stable isotopic values (δ13C, δ15N), carbon and nitrogen contents, and mean OC:ON molar ratio of tardigrades, springtails, cryoconite (Cry), and three different size fractions of supraglacial debris (Deb_1 < 1 mm, Deb_2 < 2 mm, Deb_3 ≤ 5 mm). OC:ON molar ratio is averaged across all subsamples
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Results of X-ray diffraction showed that all samples contained quartz, plagioclase, muscovite or illite, chlorite and orthoclase. The composition of mineral phases was the same in samples, but every sample differed in the proportion of each phase. The proportion of plagioclase (albite) increased with decreasing size of the debris and the proportion of quartz decreased with increasing size of the debris. Cryoconite was rich in organic matter which increased the background of the X-ray recording (Fig. S3).

Discussion

Results of this study revealed differences in communities of dominant supraglacial consumers among three supraglacial habitats on the alpine Forni Glacier. Cryoconite holes were inhabited by tardigrades while the band of supraglacial debris was inhabited by springtails. The dominance of tardigrades in cryoconite holes corroborates results of Zawierucha et al. (2019a, 2021) who described this phenomenon over four ablation seasons since 2012. Such stability in spatial distribution of consumers indicates their high adaptability to dynamic conditions, and stability of their niches, given that the surface of a small valley glacier is a dynamic environment, mostly controlled by physical forces (radiation, rain, wind) which often cause unexpected mixing, redistribution and transport of cryoconite, OM produced in situ, and windblown OM over the whole supraglacial zone (author’s personal observation; Azzoni et al. 2016; Hodson et al. 2008; Stibal et al. 2006; Takeuchi et al. 2010, 2018; Vonnahme et al. 2016; Zawierucha et al. 2019b).

The absence of tardigrades in supraglacial debris can be triggered by low survival rate in high temperatures (Zawierucha et al. 2019a). Although tardigrades were observed in sparse, fine debris mixed with cryoconite in area of the formation of cryoconite holes on the Forni Glacier, their abundance was significantly lower compared to cryoconite holes (Zawierucha et al. 2019a). Moreover, they were still absent in the band of supraglacial debris where springtails occur. Whereas most supraglacial habitats fluctuate around 0 °C, supraglacial debris can reach much higher temperature with strong diurnal fluctuations (Conway and Rasmussen 2000). Springtails are not known to survive in fully aquatic conditions. Thus, their absence at the bottom of cryoconite holes is not surprising. Springtails found in cryoconite holes were likely transported by wind or meltwater from supraglacial debris. The absence of active animals from algae and cryoconite covered samples of surface ice of the weathering crust was unexpected since active invertebrates were previously found in glacier surface ice (Shain et al. 2016, 2021). Thus, other environmental factors, such as high doses of UV radiation (e.g., Morgan-Kiss et al. 2006), short-term stability (e.g., Zawierucha et al. 2019a, b), or absence of preferred food, could prevent alpine species of supraglacial consumers from expanding their distribution to the surface ice of the weathering crust on the Forni Glacier.

The mineral composition of cryoconite and supraglacial debris was similar suggesting similar source of material in both habitats. However, supraglacial debris was approximately ten times lower in organic matter content compared to cryoconite. The lower amount of OC and ON was noticeable, likewise from OC:ON ratio. Overall, OC:ON ratios of cryoconite and debris were lower than those previously reported from arctic cryoconite holes (Stibal et al. 2008, 2010) which may be related to the different composition of OM, lower input of nutrient-rich OM, or lower atmospheric nitrogen deposition (precipitation) in the Arctic compared to the Alps.

Cryoconite was higher in δ13C and lower in δ15N compared to all fractions of OM in supraglacial debris likely indicating different composition of carbon and nitrogen sources and their utilization by microbes and consumers. The high δ13C of cryoconite could be related to the high proportion and photosynthetic activity of primary producers (Musilova et al. 2015). Lower δ13C of OM in supraglacial debris could indicate a higher input of allochthonous OC originated from terrestrial plants (Musilova et al. 2015; O’Leary 1981). The higher OC:ON in debris supports this assumption since terrestrial biomass often invests more into structural compounds with higher requirements for carbon (e.g., Cleveland and Liptzin 2007). The differences in δ15N between debris and cryoconite are likely a result of their different origin and composition (e.g., Michener and Lajtha 2008). However, the decreasing δ13C and increasing δ15N with the size of the debris fraction may also be the result of a higher abundance of cyanobacterial biofilms on the rocks analyzed as the biggest fraction (e.g., Kohler et al. 2018) or a decline in recent organic matter and its replacement by an old organic matter in pores of minerals, such as chlorite and illite (Hayatdavoudi and Ghalambor 1996; McKirdy and Powell 1974).

Springtails and tardigrades on the Forni Glacier are herbivorous/detritivorous groups (Guidetti et al. 2012; Hao et al. 2020) which likely utilize algae and detritus (organic matter, microbes), as described in their non-glacier counterparts (Bryndová et al. 2020; Endlweber et al. 2009; Hao et al. 2020; Rusek 1998). Results of carbon and nitrogen isotopic analyses showed that springtails had higher δ13C and δ15N values compared to tardigrades. The δ13C of consumers is often close (approx. ±1 ‰) to δ13C of consumed food, while its δ15N usually increases by approximately 3 ‰ with trophic level (e.g., Peterson and Fry 1987). The differences between tardigrades and springtails were less than one per mil 1 ‰, which indicates that they are not different trophic levels (Peterson and Fry 1987).

The δ13C and δ15N of tardigrades were considerably lower than those of their potential food, which is contrary to the general pattern of isotopic fractionation between consumer and food (Peterson and Fry 1987). A similar distribution of δ13C and δ15N between tardigrades and OM in cryoconite was reported by Novotná Jaroměřská et al. (2021). Based on these facts, we assume that tardigrades likely prefer to select some compounds of cryoconite OM (e.g., specific algae, cyanobacteria, or compounds of detritus) which may be a sufficient source of carbon and nitrogen, but their δ13C and δ15N could be affected by nutrient limitation or specific fractionation due to metabolic processes connected with low temperature.

The δ13C of springtails could indicate the consumption of microbial biofilms or the selective consumption of bigger particles of OM which were not present or dominant in the smallest fraction of debris. Also, compared to the results of studies on species of both tardigrades and springtails and their food from soil, polar microbial mats, or Arctic cryoconite holes (Almela et al. 2019; Novotná Jaroměřská et al. 2021; Potapov et al. 2016; Potapov and Tiunov 2016; Shaw et al. 2018; Velázquez et al. 2017), consumers on the Forni Glacier had lower δ13C and δ15N than their potential food. However, a reliable method for analyses of debris and cryoconite compounds separately has not been established yet (mostly because cryoconite is a consortium of autotrophs, heterotrophs, and mineral particles), and we can only open the discussion on differences in isotopic values of consumers caused by different food, or by other mechanisms such as deviation from the usual fractionation between food and consumer due to enrichment or limitations of nutrients in the food (e.g., Haubert et al. 2005).

The biomass of springtails and tardigrades was similar. Biomass and size of consumers often indicates rates of consumers’ secondary production (e.g., Edgar 1990), and it is a pivotal indicator of response of invertebrates to changes and availability of resources (Reed et al. 1994). Competition for food could also be one of the crucial factors driving the spatial distribution of both groups of invertebrates and formation of different niches on the glacier surface. Considering that debris and cryoconite covered area on the Forni Glacier is approximately 1/3 of the ablation zone (Ambrosini et al. 2019), differences in the distribution of invertebrates and their biological activity may have potential impact to the functioning of the Forni Glacier supraglacial environment.

Conclusions

This study reports the distribution and trophic niches of dominant supraglacial consumers in three microhabitats (supraglacial debris, cryoconite holes, and surface ice of the weathering crust) on the alpine valley glacier Forni. Differences in δ13C, δ15N, OC:ON ratio and carbon and nitrogen contents between cryoconite and fractions of supraglacial debris indicate differences in organic matter source (autochthonous/allochthonous) and its usability as food source for consumers. The δ13C and δ15N of springtails which mainly inhabited supraglacial debris were higher compared to tardigrades which inhabited cryoconite holes likely indicating differences in food sources or fractionation. The biomass of tardigrades in cryoconite and springtails in debris indicates similar secondary production. Our results indicate that studies on differences between supraglacial habitats using invertebrates and stable isotopic analyses can be useful in investigations on supraglacial ecosystem functioning and changes in nutrient pathways during the glacier retreat since one habitat can become another.