Water chemistry,landscape, and spatial controls of δ13C and δ15N of zooplankton taxa in boreal lakes: One size does not fit all

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Correlates of elemental‐isotopic composition of stream fishes: the importance of land‐use,species identity and body size

The isotopic (δ¹³C and δ¹⁵N) and stoichiometric (C:N:P) compositions of four fish species (Family Centrarchidae: Lepomis auritus, Lepomis cyanellus; Family Cyprinidae: Nocomis leptocephalus, Semotilus atromaculatus) were examined across four North Carolina Piedmont streams arrayed along an urbanization gradient. Both isotopic and stoichiometric composition of fishes appeared to track changes occurring in basal resource availability. Values of δ¹³C of basal resources and consumers were more enriched at the most urbanized streams. Similarly, basal resources and consumers were δ¹⁵N–enriched at more urbanized streams. Basal resource stoichiometry varied across streams, with periphyton being the most variable. Primary consumers stoichiometry also differed across streams. Intraspecific variation in fish stoichiometry correlated with the degree of urbanization, as the two cyprinids had higher N content and L. cyanellus had higher P content in more urbanized streams, probably due to enrichment of basal resources. Intrinsic factors, specifically species identity and body size also affected stoichiometric variation. Phosphorus (P) content increased significantly with body size in centrarchids, but not in cyprinids. These results suggest that although species identity and body size are important predictors of elemental stoichiometry, the complex nature of altered urban streams may yield imbalances in the elemental composition of consumers via their food resources.     

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a ¹³C‐depleted stable carbon isotopic composition (δ¹³C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ¹³C of other abundant chironomid groups and the relationship between chironomid δ¹³C and methane production in lakes. 2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ¹³C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini). 3. Remains of Orthocladiinae in the seven study lakes had the highest δ¹³C values (?31.3 to ?27.0‰), most likely reflecting δ¹³C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ¹³C values (?33.2 to ?27.6‰ and ?33.6 to ?28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ¹³C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ¹³C of Chironomini (r = ?0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05). 4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ¹³C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ¹³C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ¹³C. More detailed studies with a higher number of analysed samples and detailed measurement of δ¹³C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ¹³C of chironomid remains.     

Factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lake mesocosms with contrasting nutrient loadings and temperatures

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The carbon cycle of Quebec boreal reservoirs investigated by elemental compositions and isotopic values

Dissolved and particulate organic matter (POM) of three Quebec boreal reservoirs of different ages (Laforge-1, 7 years; Robert-Bourassa, 25 years and Cabonga, 70 years at the time of sampling) and sets of lakes from the same watersheds was analyzed using organic carbon concentrations, C/N and C/P elemental composition, δ¹³C and δ¹⁵N isotopic values. The reservoirs are characterized by lower dissolved organic carbon concentrations with lower C/N ratios and by lower δ¹³C and higher δ¹⁵N in POM. They contain more autochthonous dissolved organic matter and less terrigenous organic matter than the lakes. Some of those characteristics are more pronounced in the younger than in the older reservoirs. The differences can be attributed to two causes: (1) more extended degradation of terrigenous organic matter, caused by an increase in residence time; and (2) differences in food web structure resulting from the phenomenon known as trophic upsurge, in newly flooded reservoirs. The results indicate that some effects of reservoir creation on the carbon cycle are short term perturbations, others however long term features of those reservoirs. The implications of these findings for CO₂ emissions from reservoirs are discussed.     

Widespread natural intraspecific variation in tissue stoichiometry of two freshwater molluscs: Effect of nutrient enrichment

A central premise of ecological stoichiometry is that consumers maintain relatively fixed elemental composition in their bodies, a process known as elemental homoeostasis. Although nutrient enrichment is a ubiquitous problem facing many freshwater lakes around the world, intraspecific variation in elemental composition of freshwater invertebrates and its relationship with nutrient loading have not been well addressed. Here, we examined carbon:nitrogen:phosphorus (C:N:P) stoichiometry of two widely distributed molluscs, Corbicula fluminea and Bellamya aeruginosa, from several subtropical shallow lakes across a nutrient gradient. Our results showed that these two species exhibited substantial natural intraspecific variation in tissue stoichiometry which can reach or even exceed the values among different freshwater taxa investigated before. Our results suggest that tissue P content presents the greatest variations, followed by N content, and lowest in C content. Tissue P content ranged about three-fold (0.56%–1.65%) and five-fold (0.41%–2.28%) for B. aeruginosa and C. fluminea, respectively. Correspondingly, N content ranged from 5.16% to 12.06% and from 6.47 to 11.36%, respectively. Tissue %P, C:P and N:P ratios were strongly correlated with PO₄³⁻-P and/or chlorophyll-a in the water column. Tissue N and P contents of B. aeruginosa and P content of C. fluminea increased with increasing lake trophic levels (mesotrophic to eutrophic to hypertrophic). These results suggest that the two molluscs can adjust their tissue stoichiometry in relation to nutrient enrichment. Relaxing the assumption of strict homeostasis may help them cope with potential stoichiometric constraints. The results provide additional clues to why these two species are successful invaders and widely distributed.     

Influences of fish on food web structure and function in mountain lakes

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Does rapid glacial recession affect feeding habits of alpine stream insects?

1. Glacial retreat, accompanied by shifts in riparian vegetation and glacier meltwater inputs, alters the energy supply and trophic structure of alpine stream food webs. Our goal in this study was to enhance understanding of dietary niches of macroinvertebrates inhabiting different alpine streams with contrasting glacial and non‐glacial (groundwater, precipitation, snowmelt) water inputs in conjunction with seasonal and habitat‐specific variation in basal resource availability.
2. We measured a range of stream physico‐chemical attributes as well as carbon and nitrogen isotopes (δ¹³C, δ¹⁵N) of macroinvertebrates and primary food sources at seven sites across seasons within a Swiss glaciated catchment (Val Roseg) undergoing rapid glacial retreat (1–2 km between 1997 and 2014). Sampling sites corresponded to streams used in a previous (1997/1998) study within the same alpine catchment.
3. Physico‐chemical attributes showed wide variation in environmental conditions across streams and seasons. Significant correlation among physico‐chemical proxies of glacier meltwater (phosphate‐P, total inorganic carbon, conductivity, turbidity) and macroinvertebrate δ¹³C, δ¹⁵N, and size‐corrected standard ellipse area (a proxy for feeding niche width) values showed that the extent of glacial water input shapes the energy base among alpine streams. Feeding niche differences among common alpine stream insect taxa (Chironomidae, Baetidae, Heptageniidae) were not significant, indicating that these organisms probably are plastic in feeding behaviour, opportunistically relying on food resources available in a particular stream and season.
4. Seasonal trends in macroinvertebrate δ¹³C largely followed patterns in periphyton δ¹³C values, indicating that autochthonous resources were the main consumer energy source within the stream network, as shown previously. The overall range in macroinvertebrate δ¹³C (?33.5 to ?18.4‰) and δ¹⁵N (?6.9 to 6.7‰) values also corresponded to values measured in the previous study, suggesting that macroinvertebrates altered diets in line with changes in environmental conditions and food resources during a period of rapid glacial retreat. Our results suggest that environmental changes brought on by rapid glacial retreat have not yet caused a profound change in the trophic structure within these fluvial networks.

     

The “isohydric trap”: A proposed feedback between water shortage,stomatal regulation,and nutrient acquisition drives differential growth and survival of European pines under climatic dryness

Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO₂ uptake and transpiration. Given that transpiration‐driven water flow is required for nutrient uptake, climatic stress‐induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an “isohydric trap,” a dryness‐induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ¹³C, δ¹⁸O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ¹⁸O and higher δ¹³C values than the other species, indicating higher time‐integrated transpiration and water‐use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry‐biomass (up to 63‐fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ¹⁸O and lower δ¹³C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.     

Order from disorder: do soil organic matter composition and turnover co-vary with iron phase crystallinity?

Soil organic matter (SOM) often increases with the abundance of short-range-ordered iron (SRO Fe) mineral phases at local to global scales, implying a protective role for SRO Fe. However, less is known about how Fe phase composition and crystal order relate to SOM composition and turnover, which could be linked to redox alteration of Fe phases. We tested the hypothesis that the composition and turnover of mineral-associated SOM co-varied with Fe phase crystallinity and abundance across a well-characterized catena in the Luquillo Experimental Forest, Puerto Rico, using dense fractions from 30 A and B horizon soil samples. The δ¹³C and δ¹⁵N values of dense fractions were strongly and positively correlated (R²?=?0.75), indicating microbial transformation of plant residues with lower δ¹³C and δ¹⁵N values. However, comparisons of dense fraction isotope ratios with roots and particulate matter suggested a greater contribution of plant versus microbial biomass to dense fraction SOM in valleys than ridges. Similarly, diffuse reflectance infrared Fourier transform spectroscopy indicated that SOM functional groups varied significantly along the catena. These trends in dense fraction SOM composition, as well as ?¹⁴C values indicative of turnover rates, were significantly related to Fe phase crystallinity and abundance quantified with selective extractions. Mössbauer spectroscopy conducted on independent bulk soil samples indicated that nanoscale ordered Fe oxyhydroxide phases (nano-goethite, ferrihydrite, and/or very-SRO Fe with high substitutions) dominated (66–94%) total Fe at all positions and depths, with minor additional contributions from hematite, silicate and adsorbed Fe^II, and ilmenite. An additional phase that could represent organic-Fe^III complexes or aluminosilicate-bearing Fe^III was most abundant in valley soils (17–26% of total Fe). Overall, dense fraction samples with increasingly disordered Fe phases were significantly associated with increasingly plant-derived and faster-cycling SOM, while samples with relatively more-crystalline Fe phases tended towards slower-cycling SOM with a greater microbial component. Our data suggest that counter to prevailing thought, increased SRO Fe phase abundance in dynamic redox environments could facilitate transient accumulation of litter derivatives while not necessarily promoting long-term C stabilization.     

Effects of increased temperature and nutrient enrichment on the stoichiometry of primary producers and consumers in temperate shallow lakes

1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ¹³C) of the different primary producers to elucidate differences in the degree of CO₂ limitation. 2. Epiphytes were the only primary producer with significantly higher δ¹³C in the enriched mesocosms. No temperature effects were observed in δ¹³C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ¹³C signatures indicative of differences in CO₂ limitation. Seston had signatures indicating negligible or low CO₂ limitation, followed by epiphytes and sediment biofilm, with moderate CO₂ limitation, while macrophytes showed the strongest CO₂ limitation. CO₂ together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ¹³C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes. 3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes. 4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment. 5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency. 6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.     

IMPACT OF TAXONOMY,GEOGRAPHY, AND DEPTH ON δ13C AND δ15N VARIATION IN A LARGE COLLECTION OF MACROALGAE1

The natural abundance of carbon stable isotopes (δ¹³C) of marine macrophytes has been measured in previous studies and used to analyze differences in C_i assimilation among the three macroalgal phyla, Chlorophyta, Ochrophyta, and Rhodophyta, and seagrasses, distinguishing diffusive CO₂ entry from the operation of a CO₂‐concentrating mechanisms (CCM). The work reported here further resolves the patterns of δ¹³C variation in aquatic macrophytes related to their taxonomy, geographic location (and consequently climatic conditions), and vertical zonation. Analyses of δ¹³C for 87 species are reported, including eight that have not been previously examined, belonging to taxa in the three macroalgal phyla, plus two species of seagrasses, collected at different latitudes. For one species of each phylum, analyses were also conducted through a vertical depth gradient. Representative species were used in a pH drift experiment, in order to compare the mechanism of C_i acquisition for photosynthesis with the δ¹³C subsequently determined on the same specimen. Our results suggest that the δ¹³C values were mostly determined by taxonomy. Depth effects on C stable isotope composition differed among taxa. The parallel measurements of δ¹⁵N are more difficult to interpret mechanistically; there are no robust phylogenetic and large‐scale biogeographic correlations; local factors of natural (e.g., upwellings) and anthropogenic (e.g., sewage outfall) inputs predominate in determining the macrophyte δ¹⁵N.     

Extending the growth rate hypothesis to species development: Can stoichiometric traits help to explain the composition of macroinvertebrate communities?

Ecological stoichiometry seeks to understand the ecological consequences of elemental imbalances between consumers and their resources. Therein, the well-accepted growth rate hypothesis (GRH) states that organisms exhibiting rapid growth have higher phosphorus (P) demand – and thus lower C:P and N:P ratios – than slow growing ones, due to a higher allocation to P-rich rRNA. However, GRH has rarely been extended to other biological traits than growth, especially at the community level. In this study, we investigated whether macroinvertebrate stoichiometric traits (e.g. C:P and N:P ratios) can be linked to their development traits, and whether these stoichiometric traits are related to macroinvertebrate community assemblage under different nutrient conditions. We allocated more than 400 European taxa to different groups, defined using available information about three development-related traits: ‘life span', ‘voltinism' and ‘number of reproductive cycles per individual'. We sampled 18 invertebrate taxa in six streams exhibiting different levels of nutrient concentration and measured their stoichiometric traits. Further, we quantified invertebrate taxon abundances in these streams during an annual survey. Based on these data, we tested whether community composition regarding the developmental groups differs, depending on nutrient concentration. We found significant differences in the proportions of the developmental groups along a gradient of water N:P, in relation to their stoichiometric traits. Taxa with low C:P and N:P ratios were generally associated with faster development groups, and these taxa tended to occur at higher proportions in streams exhibiting low dissolved N:P ratios. In contrast, communities from P-poor, high dissolved N:P streams, were dominated by slowly developing taxa with high N:P ratios. Our results highlight that extending the GRH to species development rate might give some insights about the mechanisms by which nutrient concentrations in ecosystems influence consumers' community composition.     

Minor stable carbon isotope fractionation between respired carbon dioxide and bulk soil organic matter during laboratory incubation of topsoil

A common assumption in paleoenvironmental reconstructions using soils is that the carbon isotope composition of soil-respired CO₂ is equivalent to the carbon isotope composition of bulk soil organic matter (SOM). However, the occurrence of a non-zero per mil carbon isotope enrichment factor between CO₂ and SOM (\(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\)) during soil respiration is the most widely accepted explanation for the down-profile increase in SOM δ¹³C values commonly observed in well-drained soils. In order to shed light on this apparent discrepancy, we incubated soil samples collected from the top 2 cm of soils with pure C₃ vegetation and compared the δ¹³C values of soil-respired CO₂ to the δ¹³C values of bulk SOM. Our results show near-zero \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values (?0.3 to 0.4 ‰), supporting the use of paleosol organic matter as a proxy for paleo soil-respired CO₂. Significantly more negative \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values are required to explain the typical δ¹³C profiles of SOM in well-drained soils. Therefore our results also suggest that typical SOM δ¹³C profiles result from either (1) a process other than carbon isotope fractionation between CO₂ and SOM during soil respiration or (2) \(\varepsilon_{{{\text{CO}}_{ 2} - {\text{SOM}}}}\) values that become increasingly negative as SOM matures.     

Seston quality drives feeding,stoichiometry and excretion of zebra mussels

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Temporal variation in body composition (C : N) helps explain seasonal patterns of zooplankton δ13C

1. The stable carbon isotope ratio δ¹³C is a useful tracer of energy flow in lake food webs, and the zooplankton signature is commonly used to establish a baseline for the pelagic habitat. However, sources of temporal variability in the δ¹³C of different zooplankton taxa are rarely considered. 2. Here, we investigate to what extent temporal variation in the δ¹³C of particulate organic matter (POM) (<41 μm) and the C : N of zooplankton can explain the temporal variability in δ¹³C of freshwater zooplankton. We compare temporal patterns of δ¹³C and C : N for Daphnia, Hesperodiaptomus franciscanus and Leptodiaptomus tyrelli over a 6‐month period at four sites in two oligotrophic lakes. 3. In all three taxa, seasonal variation in zooplankton C : N explained more of the variation in zooplankton δ¹³C than did the δ¹³C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of δ¹³C in zooplankton. 4. Using these data, we evaluate procedures that estimate the δ¹³C of only the non‐lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or ‘normalising’δ¹³C based on C : N will exclude a major dietary source, and therefore may be inappropriate. 5. We conclude that temporal variation in body composition (C : N) of zooplankton can significantly influence the temporal variation of zooplankton δ¹³C signatures.     

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with ¹³C and ¹⁵N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the ¹³C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter ¹³C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m⁻² y⁻¹ of SOM accumulation. Recovery of litter ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of ¹⁵N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha⁻¹ y⁻¹) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.     

Water geochemistry and sedimentary pigments in northern Victoria Land lakes,Antarctica

Total concentrations of algal pigments, organic C, C, N, P and S were determined in surface sediments from the littoral zone of 21 lakes in ice-free areas of northern Victoria Land (Antarctica) with different climatic and environmental conditions. Concentrations of major ions and nutrients were also determined in water samples from the same lakes. The latter samples had extremely variable chemical compositions; however, all the lakes resulted oligotrophic. Pigment concentrations in surface sediments were comparable to those reported for other Antarctic lakes and lower than those in oligotrophic lakes at lower latitudes. Cyanophyta, Chlorophyta and Bacillariophyta were the main taxa identified. These taxa correspond to those reported in previous microscopy-based studies on Antarctic phytoplankton and phytobenthos. Discriminant Function Analysis and Canonical Correspondence Analysis of data indicate that the distribution of pigments in these Victoria Land lakes depends mainly on their geographical location (particularly the distance from the sea) and nutrient status.     

Preimaginal stages of Diptera (excluding Chironomidae and Simuliidae) of mountain lakes,their inlets and outlets in the Tatra Mountains (Slovakia)

The occurrence of preimaginal stages of Diptera (excluding Chironomidae and Simuliidae) in the Tatra Mountain lakes, their inlets and outlets, and the influence of acidification on the presence of these taxa were assessed. At 21 sites, 18 Diptera taxa were recorded in 2000 and 2001. The most frequent were Dicranota sp. (73%), Pedicia (C.)rivosa (50%) and Wiedemannia sp. (14%). Diptera were ordinated by RDA; relationships between taxa and environmental variables were significantly determined by temperature and habitat characteristics (altitude, habitat, alkalinity, Ca²⁺, pH, temperature, conductivity). The RDA diagram confirmed the existence of four Diptera assemblage groups corresponding with four habitat types. Group 1 — alpine and subalpine oligotrophic lakes at higher altitudes threatened by acidification, with the acid tolerant taxa Tipula rufina, T. alpium, Dicranota sp. and Rhypholophus sp. Group 2 — subalpine oligotrophic lakes not threatened by acidification with the acid sensitive taxa Wiedemannia sp., Hemerodromia sp., Tipula (S.) benesignata, Orimarga sp., Chionea sp., Pedicia (C.) rivosa and Bazarella subneglecta which prefer higher temperature. Group 3 — subalpine oligotrophic lakes, with the acid sensitive species Berdeniella illiesi, which prefers cold water. Group 4 — outlets of submontane oligotrophic lakes with characteristic taxa Tricyphona sp. and Tipula goriziensis. Acidified alpine and subalpine Tatra lakes are situated mostly at higher altitudes (1705–2145 m. a.s.l.) and have specific taxonomic composition.     

Bimodality in stable isotope composition facilitates the tracing of carbon transfer from macrophytes to higher trophic levels

Even though the suitability of macrophytes to act as a carbon source to food webs has been questioned by some studies, some others indicate that macrophyte-derived carbon may play an important role in the trophic transfer of organic matter in the food web of shallow lakes. To evaluate the importance of macrophytes to food webs, we collected primary producers—macrophytes and periphyton—and consumers from 19 South American shallow lakes and analyzed their carbon stable isotopes composition (δ¹³C). Despite the diversity of inorganic carbon sources available in our study lakes, the macrophytes’ δ¹³C signatures showed a clear bimodal distribution: ¹³C-depleted and ¹³C-enriched, averaging at ?27.2 and ?13.5‰, respectively. We argue that the use of either CO₂ or HCO₃ ^? by the macrophytes largely caused the bimodal pattern in δ¹³C signals. The contribution of carbon from macrophytes to the lake’s food webs was not straightforward in most of the lakes because the macrophytes’ isotopic composition was quite similar to the isotopic composition of periphyton, phytoplankton, and terrestrial carbon. However, in some lakes where the macrophytes had a distinct isotopic signature, our data suggest that macrophytes can represent an important carbon source to shallow lake food webs.