July Air Temperature Research Articles

The influence of climatic and anthropogenic factors on the productivity of forest stands in the ecotone of the northern treeline on the Yamal peninsula

On 12 sample plots established in the ecotone of the northern forest boundary in the Khadyta-Yakha River valley on the Yamal Peninsula, time of emergence and death, as well as productivity of about 1000 Siberian larch (Larix sibirica Ledeb.) and Siberian spruce (Picea obovata Ledeb.) trees were analyzed. Over the last 140 years, there has been a sharp increase in the stocks and annual growth of stem phytomass of woody plants within the studied ecotone. Since the 1970s, the average annual growth of phytomass has been increased by 1.2–39 times, which was facilitated by a 0.9оC increase in average June–July air temperature. However, this effect is insignificant in areas of mixed open forests subjected to periodic logging.

Holocene ecosystem and temperature development inferred from invertebrate remains in Zminje Jezero (Dinaric Alps, Montenegro)

AbstractLake Zminje Jezero (1535 m a.s.l.) in Montenegro was studied for chironomid and other aquatic invertebrate remains in a sediment sequence dating back to 12,000 calibrated 14C years before present (cal yr BP), providing, to our knowledge, the first lake-sediment record studied for chironomids and other associated chitinous aquatic invertebrate remains in the Dinaric Alps. Changes in chironomid and other invertebrate remains along the record make it possible to constrain changes of relevant environmental variables for aquatic invertebrates in the lake including temperature, oxygen availability, trophic status and water depth. The results suggest moderate changes in trophic conditions with chironomid assemblages indicating meso- to eutrophic conditions during the analysed interval. Invertebrate assemblages were typical for lakes with relatively high oxygen availability with a minor trend during the middle to late Holocene to conditions observed in lakes that are more hypoxic. A change in temperature is suggested in the earlier part of the record when the share of chironomid taxa adapted to warmer conditions increased, at the Younger Dryas to Holocene transition, whereas later no particularly pronounced shifts in temperature-sensitive taxa were observed. July air temperatures were estimated based on chironomid assemblages using a transfer function based on calibration data consisting of 117 lakes sampled in the Swiss Alps and northern Switzerland. The results suggest that temperatures rapidly increased by 5 °C at the onset of the Holocene (11,500 cal yr BP) leading to a relatively warm early to mid-Holocene and thereafter slightly decreased during the late Holocene. Reconstructed temperatures are discussed together with a previously published pollen record for our study site and are broadly consistent with other chironomid-based temperature reconstructions from Central, Eastern and Southern Europe. From ca. 3000 cal yr BP sedimentation rates increased and from ca. 500 cal yr BP onwards pollen data suggest that the vegetation and lake catchment were affected by human activities, possibly influencing chironomid and other invertebrate assemblages in the lake and thereby also reconstructed temperatures. Overall, our results show that combining analyses of chironomid and other invertebrate assemblages can provide valuable insights into long-term environmental changes and can provide temperature reconstructions for small mountain lakes in the Balkans, such as Zminje Jezero.

Temperature and palaeolake evolution during a Middle Pleistocene interglacial–glacial transition at the Palaeolithic locality of Schöningen, Germany

The Middle Pleistocene Reinsdorf sequence at the Lower Palaeolithic sites of Schöningen offers the opportunity to reconstruct a rarely well‐preserved post‐Holsteinian environmental transition from an interglacial into a glacial phase along with its highly dynamic interjacent climatic oscillations. Combining biological proxies, element composition and stable isotope ratios of two lakeshore sequences at excavation site 13 II, we demonstrate repeated variations in climate, hydrology and catchment vegetation cover. New ostracod‐based quantitative mean summer and winter air temperature reconstructions with the Mutual Ostracod Temperature Range (MOTR) method provide the first detailed information about the temperature evolution. The interglacial temperature maximum, probably corresponding to Marine Isotope Stage 9e, is followed by a first dry phase and, during the younger part of the Reinsdorf sequence, by a second dry period. Both were marked by lower precipitation/evaporation ratios, reduced vegetation cover in the catchment and increased surface inflows from springs. Temperature reconstructions of these two steppe (open woodland) phases yield very narrow ranges for mean January (−4–0 °C) and July (+17–19 or +17–21 °C) air temperatures, demonstrating that, while summers were similar to those of today, winters were at least 1 °C colder, hinting at a more pronounced continental climate. Precise temperature estimates for the interjacent woodland and steppe (woodland) phase are hindered by generally wider ranges produced by the MOTR method (January mean −4–3 °C, July mean +15–21 °C). The development of a more extensive vegetation cover, reducing surface runoff and erosion in favour of increased river and groundwater discharge, as indicated by a shift in microfossil and stable isotope records, suggests generally more humid climates with higher precipitation/evaporation ratios as well as reduced seasonal temperature variations.

Summer temperatures from the Middle Pleistocene site Schöningen 13 II, northern Germany, determined from subfossil chironomid assemblages

We present the first climatic and environmental reconstruction based on subfossil chironomid head capsules from the Middle Pleistocene Reinsdorf sequence, Schöningen, northern Germany, corresponding to Marine Isotope Stage 9e‐a. The sequence is characterized by interglacial forest successions followed by alternating woodland and steppe phases. Higher levels of runoff formed lacustrine habitats during post‐interglacial, cool steppe (woodland) phases. These were characterized by diverse chironomid assemblages with up to 27 chironomid morphotypes occurring simultaneously. Warmer forest phases were mostly void of chironomids when the site Schöningen 13 II fell dry owing to higher vegetational coverage and therefore lower runoff. Transitional periods between woodland and steppe phases show higher abundances of profundal, bottom‐dwelling chironomid taxa, suggesting oligo‐mesotrophic aquatic conditions, while steppe phases are dominated by shallow‐lake taxa with higher tolerance to increasing productivity. We applied temperature inference models to the chironomid assemblages based on a Swiss–Norwegian and a Swiss–Norwegian–Polish chironomid–temperature calibration data set to reconstruct mean July air temperatures for the Reinsdorf sequence. The Swiss–Norwegian–Polish training set (TS) seems better suited owing to a longer temperature gradient and the presence of the dominant taxon, Propsilocerus lacustris‐type, which is missing from the Swiss–Norwegian TS. In sections of the record with low taxon richness (Shannon index <2) and a dominance of P. lacustris‐type, indicating increased nutrient impact, summer temperatures may have been overestimated by the Swiss–Norwegian–Polish TS. In the other sections, the chironomid‐based reconstructions based on the Swiss–Norwegian–Polish TS were in line with ostracod and plant remains‐based temperature reconstructions, suggesting summer temperatures of the post‐interglacial Reinsdorf oscillations between 16.5 and 22 °C. Our results show that summer air temperatures were lower during warmer, wetter transitional zones (−0.5–0.2 °C colder/warmer than today) and increasing during cooler, dry steppe phases (1 °C warmer than today), most likely caused by higher continentality.

Climate change-associated declines in water clarity impair feeding by common loons.

Climate change has myriad impacts on ecosystems, but the mechanisms by which it affects individual species can be difficult to pinpoint. One strategy to discover such mechanisms is to identify a specific ecological factor related to survival or reproduction and determine how that factor is affected by climate. Here we used Landsat imagery to calculate water clarity for 127 lakes in northern Wisconsin from 1995 to 2021 and thus investigate the effect of clarity on the body condition of an aquatic visual predator, the common loon (Gavia immer). In addition, we examined rainfall and temperature as potential predictors of water clarity. Body mass tracked July water clarity strongly in loon chicks, which grow chiefly in that month, but weakly in adult males and females. Long-term mean water clarity was negatively related to chick mass but positively related to adult male mass, suggesting that loons foraging in generally clear lakes enjoy good foraging conditions in the long run but might be sensitive to perturbations in clarity during chick-rearing. Finally, chick mass was positively related to the density of docks, perhaps because angling removes large fishes and thus boosts the abundance of the small fishes on which chicks depend. Water clarity itself declined strongly from 1995 to 2021, was negatively related to July rainfall, and was positively related to July air temperature. Our findings identified both long-term and short-term water clarity as strong predictors of loon foraging efficiency, and suggest that climate change, through water clarity, impacts freshwater ecosystems profoundly. Moreover, our results identified the recent decrease in water clarity as a likely cause of population decline in common loons.

Chironomidae‐based inference model for mean July air temperature reconstructions in the eastern Baltic area

Here we present a new eastern Baltic Chironomidae training set (TS) containing 35 sites that was collected and merged with neighbouring published Finnish (82 lakes) and northern part of the Polish (nine lakes) TSs. Chironomidae, non‐biting midges, are known to be strongly responsive to the July air temperature and are widely used to infer palaeotemperature. Several modern analogue‐based TSs necessary for calibrating the relationships between mean July air temperature (MJAT) and chironomids are available for Europe. However, none of these is representative of the transitional climate typical for eastern Baltic (Estonia, Latvia, Lithuania). The Finno–Baltic–Polish TS contains 121 sites and covers a geographically continuous 70–50°N latitudinal and 7 °C (12.1–19.2 °C) MJAT gradient. Canonical correspondence analysis revealed that, among the tested environmental variables (pH, water depth, dissolved oxygen, MJAT), the MJAT explains the highest amount of variation, both for the eastern Baltic separately and the Finno–Baltic–Polish TSs. The weighted averaging–partial least squares‐based cross‐validation test reveals that the Finno–Baltic–Polish TS has a low root mean square error of prediction (0.7 °C) confirming the high reliability of the TS. The temperature optima of the taxa included in the new Finno–Baltic–Polish TS and widely used Swiss–Norwegian TS were examined. The observed dissimilarities can be attributed to the differences in the temperature ranges represented by the TS, the taxonomic identification level, the general cosmopolitan taxa distribution patterns and the influence of TS‐specific geographic position, climatic or environmental conditions. The new Finno–Baltic–Polish TS adds to the knowledge on the modern distribution of Chironomidae taxa and widens the geographical area of reliable Chironomid‐based MJAT reconstructions into the eastern European lowland.

Two chironomid-inferred mean July air temperature reconstructions in the South Carpathian Mountains over the last 2000 years

We present chironomid-based reconstructions of mean July air temperature changes over the last 2000 years from Lake Latoriței (1530 m a.s.l.) in the Southern Carpathians. A multi-proxy analysis was performed along a 58 cm long sediment core and two training sets were used for quantitative July air temperature reconstructions: the Eastern-European (EE, 212 lakes) and the Finnish-Polish-Carpathian (FPC, 273 lakes). The transfer functions had a coefficient of determination ( r2) 0.88 and 0.91 with a root mean squared error of prediction (RMSEP) 0.88°C and 1.02°C. Despite possible biases resulting from methodological problems and the ecological complexity of the chironomid response to both climatic and environmental changes, the agreement of the temperature reconstruction of Lake Latoriței with other alpine records suggests that the transfer function successfully reconstructed past summer temperatures between 750 and 1830 CE. Biases in the temperature reconstruction in the period before 750 and after 1830 CE were likely caused by increased abundance of rheophilic and semi-terrestrial chironomid species related to increased inflow activity before 750 CE and local land use changes after 1830 CE, which was also indicated by increasing deforestation and increasing lake productivity in the pollen and diatom records. Our results suggest that the region experienced a warm period between 750 and 1360 CE, and a cold period between 1360 and 1600 CE followed by fluctuating summer temperatures until 1830 CE. These events were associated with the so-called ‘Mediaeval Warm Period’ (MWP) and the ‘Little Ice Age’ (LIA), respectively. The inference models reconstructed a decrease in July air temperatures by 0.7°C–1.1°C during the LIA relative to the warmer MWP. We also demonstrated that the FPC training set gives better results, supporting that local/continental training sets are efficient to detect weak amplitude summer temperature changes in the Late-Holocene.

The history of lake Antyukh-Lambina (Kandalaksha coast of the White Sea) based on the study of bottom sediments

We performed a comprehensive analysis of the sediment cores from lake Antyukh-Lambina. Studied area is situated on the Kandalaksha coast of the White Sea. The lake is connected to lake Kolvitskoye by a narrow strait. We have identified several stages in the lake development from brackish to freshwater conditions, since the early Holocene. The desalination and increased organic matter in the sediment have led to an increase in diversity chironomid and cladocera species. Using chironomid analysis, we performed quantitative reconstructions of mean July air temperatures and water depths. The results showed that both parameters have increased gradually since the early Holocene.

Holocene summer temperature reconstruction based on a chironomid record from Sierra Nevada, southern Spain

Obtaining accurate temperature reconstructions from the past is crucial in understanding the consequences of changes in external climate forcings, such as orbital-scale insolation or multidecadal to centennial-scale variability on the climate system and the environment. In addition, these reconstructions help in comprehending the amplitude of natural temperature changes in the past, which can assist in evaluating the amplitude and rate of recent anthropogenic global warming. Here we present the first detailed Holocene mean July air temperature reconstruction based on chironomid assemblages from sediments retrieved from Laguna de Río Seco, an alpine lake in Sierra Nevada, southern Spain. Coldest climate conditions are recorded during the last glacial maximum and the last deglaciation. Warming occurred in the Early Holocene and warmest summer temperature conditions and the Holocene thermal maximum (HTM) occurred in the interval roughly between 9000 and 7200 cal yr BP, concurrent with summer insolation maxima. Rapid cooling of ∼1.5 °C occurred after the warmest maximum and between ∼7200 and 6500 cal yr BP, and temperatures stabilized between ∼6500 and 3000 cal yr BP. A further cooling began ∼3000 cal yr BP and culminated with coldest summer conditions during the Dark Ages (DA) and Little Ice Age (LIA) at ∼1550 cal yr BP (∼400 CE) and ∼200 cal yr BP (∼1750 CE), respectively. This cooling temperature trend was interrupted by warmer conditions during the Iberian-Roman Humid Period (IRHP) ∼2000 cal yr BP and during the Medieval Climate Anomaly (MCA) at ∼1000 cal yr BP. Our reconstruction shows a greater than two-degree cooling during the Middle and Late Holocene, agreeing with global mean surface temperature (GMST) reconstructions. Modern climate warming (MCW) during summer exceeds the two-degree Celsius forecasted for the future due to anthropogenic greenhouse gases, suggesting that recent warming is amplified at high elevations. Alpine environments and the biodiversity contained there are thus in danger if the observed temperature trend continues in the next decades.

Arctic Plant Responses to Summer Climates and Flooding Events: A Study of Carbon and Nitrogen‐Related Larch Growth and Ecosystem Parameters in Northeastern Siberia

AbstractBuilt upon a 5‐year field investigation and a 13‐year satellite data set, this study examines the intricate interrelationships among ecophysiological parameters of Larix gmelinii trees and the prevailing ecosystem, climatic, and environmental factors present in the Indigirka lowlands of northeastern Siberia. It identified spatial‐temporal patterns in July needle nitrogen (N) content as an indicator of N availability from 2009 to 2013. Needle N content (%) revealed distinct yearly values: 2012 (1.31 ± 0.24), 2013 (1.67 ± 0.39), 2009 (1.72 ± 0.15), 2011 (1.84 ± 0.34), and 2010 (2.08 ± 0.25). Positive correlations were found between ecosystem and larch parameters, as well as between September temperature or February/May precipitation and subsequent July ecosystem productivity. Soil moisture (SM) primarily influences N availability across sites, with higher SM levels reducing N availability. However, July air temperature (AT) is the primary driver of interannual N availability changes, with higher temperatures enhancing N availability. Larch photosynthesis is mainly influenced by solar radiation (SR), temperature, N availability, and SM. Annual fluctuations in SR positively impact larch photosynthesis, while high temperatures or wetting events impose limitations on photosynthesis, even if N availability has increased. Consequently, a moderate correlation exists between N availability and photosynthesis across various sites and years (r = 0.422, P = 0.133, n = 14). In summary, this research provides valuable insights into climatic and environmental impacts on larch trees and ecosystems, emphasizing the significance of SM, AT, and SR for predicting future growth patterns of larch.

Subarctic Vegetation under the Mixed Warming and Air Pollution Influence

In the Siberian Arctic, worldwide largest forest mortality was caused by chronical (since the 1940s) influence of SO2 emissions on the larch-dominant communities. We hypothesized that warming might mitigate SO2 influence by increasing trees’ vigor and growth. We studied trees (larch, Larix sibirica; spruce, Picea obovate; birch, Betula pendula) and bushes (willow, Salix sp., alder, Duschekia fruticosa) growth dependence on SO2, air temperature, soil temperature and moisture, and precipitation. We sampled woods in severely damaged larch and moderately damaged mixed larch, spruce and birch forests. We generated tree ring chronologies and growth indices (GI). We used Terra/MODIS satellite data for mapping trends of vegetation (NDVI) and productivity (GPP, NPP) indexes. In the larch forest, we found a strong decrease in GI and tree mortality, which lasted until the end of 1990s. In the mixed forest, larch and birch were more resistant to SO2 influence compared to spruce. SO2, air and soil temperatures were mediators of all woody species growth. Winter precipitation stimulated trees growth by mitigating spring water stress. Warming onset in the 2000s led to a pronounced increase of all woody species growth. June–July air and soil temperatures, together with a moderate decrease in SO2 emissions, were the primary drivers of that phenomenon. Increasing trends of GPP, NPP, and NDVI were observed within the large part of earlier damaged forests, which was attributed to trees GI increase together with the expansion of SO2-resistant grasses and bushes.

Chironomid-Based Modern Summer Temperature Data Set and Inference Model for the Northwest European Part of Russia

Northwestern Russia remains the only region in Northern Eurasia where no regional chironomid-based inference model for quantitative palaeoclimatic reconstructions has been developed. Using palaeolimnological methods, we investigated the subfossil chironomid remains in surface sediments from a data set of 98 lakes from nine subregions of the European part of Northern Russia. We identified 143 chironomid taxa in the investigated lakes. Multivariate statistical analyses of chironomid and environmental data demonstrated that the mean July air temperature (T July), distance from the tree line, water depth, pH, and altitude explain the most significant variance in chironomid distribution. T July appeared to be the most important environmental variable. We established a chironomid-based inference model for reconstructing T July from subfossil data. The resulting West Russian two-component WA-PLS model includes 96 lakes (two lakes were excluded as outliers), 143 chironomid taxa, r2 = 0.84 (r2 boot = 0.60), RMSEP boot = 1.34 °C, and can be recommended for application in palaeoclimatic studies in the East of Northern Eurasia.

An Early Pleistocene interglacial deposit at Pingorsuit, North‐West Greenland

At the Pingorsuit Glacier in North‐West Greenland, an organic‐rich deposit that had recently emerged from the retreating ice cap was discovered at an elevation of 480 m above sea level. This paper reports on macrofossil analyses of a coarse detritus gyttja and peaty soil, which occurred beneath a thin cover of till and glacifluvial deposits. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The flora includes black spruce, tree birch, boreal shrubs and wetland and aquatic taxa, which shows that mires, lakes and ponds were present in the area. We describe a new extinct waterwort species Elatine odgaardii. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present. The fossil assemblages show strong similarities with others from Greenland that have been assigned an Early Pleistocene age, and we suggest a similar age for the sediments found at the margin of the Pingorsuit Glacier.At the Pingorsuit Glacier in North‐West Greenland, an organic‐rich deposit was discovered at an elevation of 480 m above sea level. The sediments contained remains of vascular plants, mosses, beetles, caddisflies, midges, bryozoans, sponges and other invertebrates. The fossils were deposited in a boreal environment with a mean July air temperature that was at least 9 °C higher than at present.

Sufficient conditions for rapid range expansion of a boreal conifer

Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra1, thereby reducing albedo2–4, altering carbon cycling4 and further changing climate1–4, yet the patterns and processes of this biome shift remain unclear5. Climate warming, required for previous boreal advances6–17, is not sufficient by itself for modern range expansion of conifers forming forest–tundra ecotones5,12–15,17–20. No high-latitude population of conifers, the dominant North American Arctic treeline taxon, has previously been documented5 advancing at rates following the last glacial maximum (LGM)6–8. Here we describe a population of white spruce (Picea glauca) advancing at post-LGM rates7 across an Arctic basin distant from established treelines and provide evidence of mechanisms sustaining the advance. The population doubles each decade, with exponential radial growth in the main stems of individual trees correlating positively with July air temperature. Lateral branches in adults and terminal leaders in large juveniles grow almost twice as fast as those at established treelines. We conclude that surpassing temperature thresholds1,6–17, together with winter winds facilitating long-distance dispersal, deeper snowpack and increased soil nutrient availability promoting recruitment and growth, provides sufficient conditions for boreal forest advance. These observations enable forecast modelling with important insights into the environmental conditions converting tundra into forest.

Quantifying the resilience of coldwater lake habitat to climate and land use change to prioritize watershed conservation

AbstractManaging ecological systems for resilience can increase their capacity to maintain key functions even under global change. Oxygenated coldwater (oxythermal) habitat in lakes is an important ecological resource that is threatened by both climate change and eutrophication. Here, we quantify the resilience of oxythermal habitat in over 10,000 glacial lakes in the upper Midwestern United States to climate change and watershed disturbance and classify lakes for conservation prioritization based on their current conditions and resilience. Oxythermal habitat was predicted by lake morphometry, July air temperatures, and watershed land use. Temperatures are projected to increase by mid‐century, and the magnitude of warming, its effect on oxythermal habitat, and the uncertainty surrounding that effect varied among lakes. Under mid‐century climate conditions, the number of lakes containing suitable coldwater habitat was predicted to decline by 67%, while the number of lakes with unsuitable habitat was predicted to increase by over 200%. Lakes varied in the amount of temperature increase that they could sustain without a resultant change in habitat tier (i.e., their climate resilience). Median climate resilience was 4.3°C, with some lakes capable of remaining in their habitat tier even with temperature increases up to 14°C. Changing watershed land use was predicted to influence oxythermal habitat in 24% of lakes (n = 2391). We used the magnitude of increase in watershed development that a lake could sustain while remaining in its current habitat class as a measure of its resilience to watershed disturbance. Conversely, decreased watershed development may improve oxythermal habitat conditions and push a lake into an improved condition, and this value represented a lake's restoration potential. We classified lakes into seven management classes based on their current oxythermal habitat conditions and the resilience of oxythermal habitat to climate and watershed disturbance. To facilitate management on individual lakes, we also assessed the vulnerability and resilience of individual lakes and the uncertainty surrounding these estimates. By quantifying the resilience of lakes and how it is influenced by local action across a multistate region, we can prioritize conservation action across multiple scales to maintain the critical habitat and ecosystem function of glacial lakes.

Towards the Third Millennium Changes in Siberian Triple Tree-Ring Stable Isotopes

Significant air temperature and precipitation changes have occurred since the 2000s in vulnerable Siberian subarctic regions and urged updates of available chronologies towards the third millennium. It is important to better understand recent climatic changes compared to the past decades, centuries and even millennia. In this study, we present the first annually resolved triple tree-ring isotope dataset (δ13C, δ18O and δ2H) for the eastern part of the Taimyr Peninsula (TAY) and northeastern Yakutia (YAK) from 1900 to 2021. We found that the novel and largely unexplored δ2H of larch tree-ring cellulose was linked significantly with δ18O for the YAK site, which was affected by averaged April–June air temperatures and evaporation. Simulated by the Land Surface Processes and Exchanges (LPX-Bern 1.0) model, the water fraction per year for soil depths at 0–20 and 20–30 cm was significantly linked with the new eco-hydrological tree-ring δ2H data. Our results suggest increasing evapotranspiration and response of trees’ water relation to rising thaw water uptake from lower (20–30 cm) soil depth. A positive effect of July air temperature on tree-ring δ18O and a negative impact of July precipitation were found, indicating dry conditions. The δ13C in larch tree-ring cellulose for both sites showed negative correlations with July precipitation and relative humidity, confirming dry environmental conditions towards the third millennium.

Climatic and hydrological variability as a driver of the Lake Gościąż biota during the Younger Dryas

The Younger Dryas (YD) is a roughly 1,100-year cold period marking the end of the last glaciation. Climate modelling for northern Europe indicates high summer temperatures and strong continentality. In eastern Europe, the scale of temperature variation and its influence on ecosystems is weakly recognised. Here, we present a multi-proxy reconstruction of YD conditions from Lake Gościąż (central Poland). The decadal-resolution analysis of its annually varved sediments indicates an initial decrease in Chironomidae-inferred mean July air temperature followed by steady warming. The pollen-inferred winter-to-summer temperature amplitude and annual precipitation is highest at the Allerød/YD transition and the early YD (ca. 12.7–12.4 ky cal BP) and YD/Holocene (11.7–11.4 ka cal BP) transition. Temperature and precipitation were the main reasons for lake level fluctuations as reflected in the planktonic/littoral Cladocera ratio. The lake’s diatom-inferred total phosphorus decreased with increasing summer temperature from about mid YD. Windy conditions in the early YD until ∼12.3 ka cal BP caused water mixing and a short-lived/temporary increase in nutrient availability for phytoplankton. The Chironomidae-inferred summer temperature and pollen inferred summer temperature, winter temperature and annual precipitation herein are one of only a few in eastern Europe conducted with such high resolution.

Summer temperatures and environmental dynamics during the Middle Würmian (MIS 3) in the Eastern Alps: Multi-proxy records from the Unterangerberg palaeolake, Austria

Several millennial-scale warm phases perturbed the glacial climate during Marine Isotope Stage 3 (MIS 3, ca. 57–29 ka BP). Little is known about the impact of these climatic changes on Alpine ecosystems due to the sparsity of undisturbed sediment records in the Alps and their foreland. In this study, multiple sediment-archived proxies (sediment geochemistry, stable oxygen and carbon isotopes of autochthonous carbonate, and subfossil remains of macrophytes and aquatic invertebrates) were examined in five drill cores from an ancient inner-Alpine lake at Unterangerberg (Eastern Alps) to reconstruct the palaeolake environment and to estimate summer temperature changes for the first half of MIS 3. The lacustrine sedimentation in the basin began ca. 54.6 ka, tentatively correlated with the start of Greenland Interstadial (GI) 14. We identified three distinct phases in the development of the lake. (1) A cold, oligotrophic water body influenced by snow/glacier meltwater ca. 54.6–52.2 cal ka BP. (2) A clear-water, macrophyte-dominated, productive lake ca. 52.2–44.9 cal ka BP. Submerged macrophytes were dominated by the charophyte alga Chara hispida and chironomid assemblages – by Corynocera ambigua, which is absent from the present-day fauna of the Alpine region. (3) A turbid-water, less productive lake ca. 44.9–41.5 cal ka BP. This shift towards a turbid-water state, as evidenced by the drastic reduction in the abundance of submerged macrophytes and associated invertebrates, likely occurred due to increased input of meltwater. The regime shift is tentatively correlated with the start of GI 11, for which the highest temperatures of the studied MIS 3 interval are inferred. Chironomid-based reconstructions of mean July air temperatures provide interstadial temperature estimates between ca. 11 and 12.5°C (i.e. ca. 5–6°C below present-day values), which concurs with reconstructions available from the northern Alpine foreland. Cooler July temperatures (ca. 9–10°C) are reconstructed for MIS 3 stadials. The Unterangerberg lacustrine records provide valuable new insights into MIS 3 climate dynamics inside the Eastern Alps and contribute to a better understanding of the effects of climate change on the Alpine environment.

Identifying the influence of terrestrial–aquatic connectivity on palaeoecological inferences of past climate in Arctic lakes

Increased hydrological connectivity due to permafrost degradation is likely to have substantial implications for shallow aquatic systems common to sub‐arctic landscapes due to changes to overland and subsurface flow of water and transport of sediments and dissolved nutrients. Here, we explore the influence of increased connectivity on aquatic productivity based on multi‐parameter palaeolimnological analysis of two lakes located near Inuvik (Northwest Territories, Canada). We contrast a lake with little evidence of permafrost degradation in the surrounding area (Lake PG03) to one that has multiple connections to the terrestrial landscape through a network of thaw polygons in the lake catchment (Lake PG09). Comparisons of biological indicators (chironomids) and organic carbon and nitrogen elemental and isotope composition reveal recent divergent lake histories. The chironomid assemblage of Lake PG03 followed an expected temperature gradient, with a warming signal evident since ˜1970 CE, whereas the chironomid assemblage of Lake PG09 was found to primarily respond to nutrient availability and changes in habitat, likely as a result of increasing hydrological connectivity to the landscape. Rapid assemblage and habitat change along with a prominent increase in chironomid abundance were observed at Lake PG09 after ˜1960 CE, following a shift to greater inputs from the terrestrial environment as indicated by high C:N ratios (>15) and low δ13Corg (−30‰). Increased aquatic productivity following high allochthonous additions (˜1960–2014 CE) is supported by decreased C:N and rapidly increasing organic matter (Corg, N). These results demonstrate that increased connectivity along the terrestrial–aquatic interface for lakes is likely to foster elevated productivity in the future. Likewise, increased production poses a challenge to chironomid‐inferred July air temperature reconstructions in lakes that are less resilient to secondary gradients, where analogue mismatches can occur due to shifts in dominance of indicators that are orthogonal to the temperature gradient.

Preservation of sedimentary plant DNA is related to lake water chemistry

AbstractLittle is currently known about preservation of plant DNA in lake sediments. Most prior information originates from laboratory experiments while systematic field‐based studies are still lacking. Here, we used the “g” and “h” universal primers for the P6 loop region of the chloroplast trnL (UAA) intron to amplify plant DNA from 219 lake surface sediments from China and Siberia. We introduce (i) the percentage of sequence counts with the best identity ≥95%, (ii) weighted average identity, (iii) weighted average DNA fragment length, and iv) rarefied richness of terrestrial seed plants of plant DNA metabarcoding as proxies for sedimentary DNA preservation and relate them to five environmental variables (lake water conductivity, lake water pH, mean July air temperature, and sampling depth, lake size) using boosted regression tree (BRT) analyses. Our results suggest that lake water chemical characteristics, that is, electrical conductivity and pH, are the most important variables for the preservation of plant DNA in lake sediments. Intermediate water conductivities (100–500 μS cm−1) and neutral to slightly alkaline water pH (7–9) may facilitate plant DNA preservation. Furthermore, deep lakes seem to support plant DNA preservation as indicated by relatively high rarefied richness. We also find high rarefied richness in small lakes compared with large lakes, but this result needs to be assessed by more studies in the future. None of our BRT models shows that mean July air temperature is a key variable to limit plant DNA preservation. To conclude, our results suggest that sedimentary DNA studies can preferentially select deep lakes characterized by intermediate water conductivities and neutral to slightly alkaline pH conditions.