Umeå University's logo

An important factor for controlling the chemical signature of surface water is the interactions with soil particles and groundwater. In permafrost landscapes, ground ice restricts groundwater flow, which implies a limited influence of processes such as weathering on the chemical signature of the runoff. The aim of this study was to examine how freeze-thaw processes, hydrology and water age interact to shape the chemical and stable hydrogen and oxygen isotopic signature of surface water in a catchment in West Greenland. Measuring runoff in remote catchments is challenging, and therefore we used a validated hydrological model to estimate daily runoff over multiple years. We also applied a particle tracking simulation to determine groundwater ages and used data on stable isotopic and chemical composition from various water types-including surface water, groundwater, lake water and precipitation-spanning from early snowmelt to the end of the thawed season. Our results show that groundwater age generally is less than one year and rarely exceeds four years, total runoff is dominated by groundwater, and overland flow is restricted to the snowmelt period and after heavy rain events. Monitoring of thaw rates in the active layer indicates a rapid thawing in connection with running water, and meltwater from ground ice quickly becomes an important fraction of the runoff. Taken together, our data suggest that even in continuous permafrost landscapes with thin active layers and an absence of truly old and mobile groundwater, soil processes exert a strong influence on the chemical and stable isotopic signature of runoff, i.e., similar to what has been observed in other climatic settings.

Organic carbon (OC) burial rates in northern lakes are estimated to have increased by 2–3 fold over the past 150 years. However, assessing OC burial efficiency is challenging because (a) long-term (decadal) process are difficult to study in situ, and (b) sediment organic matter (OM) consists of thousands of different compounds from both terrestrial and aquatic sources, which are subject to different degrees of degradation, transformation, or preservation. Here, we used pyrolysis–gas chromatography/mass spectrometry to track changes in the organic molecular composition of individual varve years in a series of sediment freeze cores collected during 1979–2010, allowing us to assess diagenetic changes over ≤31 years (or 12.5 cm depth). As predicted from previous work, the greatest losses over time/depth (18–19 years; 8.5 cm) are for compounds indicative of fresh OM, both terrestrial (e.g., levosugars with 58%–77% lost) and particularly aquatic origin (e.g., phytadiene and phytene amongst chlorophylls with 40%–82% lost). This high variability in degradation of specific compounds has implications for interpreting past changes in C and N. Although OM composition changes only slightly beyond 20 years (8.5 cm), the chlorophyll:lignin ratio (fresh vs. degraded compounds) continues to decline to 31 years (12.5 cm) and is predicted to continue up to 100 years (37 cm depth). In most northern lakes, indications of OM degradation to these depths correspond to sediment ages of 50 to >150 years, suggesting that much of the recent increase in OC burial in northern lakes does not represent permanent sequestration of C.

The climate in southern Patagonia is heavily influenced by the Southern Westerly Winds (SWW). Therefore, climate reconstructions from this region are valuable for our understanding of the temporal dynamics in the atmospheric circulation of the Southern Hemisphere. In this study a sedimentary record from the shallow lake Laguna Amalia – located in the semiarid Fuegian steppe of Tierra del Fuego (53°S) – was used to study how the hydroclimate in this region has changed over the last ~7100 years. Our interpretations rely on a combination of pollen, diatoms, total organic carbon, carbonates, lithology and sediment accumulation rate, together with geomorphological features of the landscape. We conclude that before 6700 cal BP Laguna Amalia was a permanent water body. Around 6700 cal BP the climate becomes drier in response to stronger SWW, and from around 6000 cal BP the lake entered a semi-permanent state with variable salinity and prolonged drier periods. Between 3000 and 600 cal BP Laguna Amalia becomes a more permanent freshwater lake with only shorter periods with desiccation. For the last 600 years the climate has again become drier, and the lake is currently experiencing seasonal desiccation. These hydroclimatic changes can largely be attributed to the variable influence of the SWW, with weaker SWW allowing the advection of moist air masses from the east into northern Tierra del Fuego.

Global estimates of methane (CH4) emissions from lakes to the atmosphere rely on understanding CH4 processes at the sediment-water interface (SWI). However, in the Arctic, the variability, magnitude, and environmental drivers of CH4 production and flux across the SWI are poorly understood. Here, we estimate CH4 diffusive fluxes from the sediment into the water column in 10 lakes in Arctic Scandinavia and Svalbard using porewater modeling and mass transfer estimates, which we then compare with 60 published estimates from the Arctic to the tropics. Diffusion of CH4 in the sampled lake sediments ranged from −0.46 to 3.1 mmol m−2 day−1, which is consistent with previous reports for Arctic and boreal lakes, and lower than for temperate and tropical biomes. Methane production occurs primarily within the top ∼10 cm of sediment, indicating a biogenic origin. Random forest predictive modeling of the sampled lakes revealed that conditions promoting production and deposition of autochthonous organic carbon in Arctic lakes drive CH4 diffusion into the water column by fueling sediment CH4 production. For small lakes across biomes, determinants of the estimated CH4 flux were also best captured by climate predictors, with warmer and wetter conditions favoring ecosystem productivity and enhancing flux but also lake morphometry resulting in important regional variability in estimates. Our study emphasizes the importance of quantifying diffusive CH4 fluxes from sediments in diverse lake types to account for differences in the controls on primary production and the preservation of organic carbon across and within different biomes, to refine CH4 emission estimates in a warming climate.

Ecosystems are continuously responding to both natural and anthropogenic environmental change. Lake sediments preserve local and global evidence of these ecological transitions through time. This archived information can yield crucial insights through the reconstruction of past changes over hundreds to many thousands of years. This chapter provides an overview on what lake sedimentary DNA (sedDNA) is, which biological groups can be detected with this novel paleoecological proxy, and the workflow and analytical techniques currently employed in sedDNA research. Finally, the implications of lake sedDNA studies are illustrated through five topics, illustrating how sedDNA can reconstruct lake response to environmental change.

There is an increased awareness that the biogeochemical cycling at high latitudes will be affected by a changing climate. However, because biogeochemical studies most often focus on a limited number of elements (i.e., C, P and N) we lack baseline conditions for many elements. In this work, we present a 42-element mass-balance budget for lake dominated catchment in West Greenland. By combining site specific concentration data from various catchment compartments (precipitation, active layer soils, groundwater, permafrost, lake water, lake sediments and biota) with catchment geometries and hydrological fluxes from a distributed hydrological model we have assessed present-day mobilization, transport and accumulation of a whole suite of elements with different biogeochemical behavior. Our study shows that, under the cold and dry conditions that prevails close to the inland ice-sheet: i) eolian processes are important for the transport of elements associated with mineral particles (e.g., Al, Ti, Si), and that these elements tend to accumulate in the lake sediment, ii) that even if weathering rates are slowed down by the dry and cold climate, weathering in terrestrial soils is an important source for many elements (e.g., lanthanides), iii) that the cold and dry conditions results in an accumulation of elements supplied by wet deposition (e.g., halogens) in both terrestrial soils and the lake-water column, and iv) that lead and sulfur from legacy pollution are currently being released from the terrestrial system. All these processes are affected by the climate, and we can therefore expect that the cycling of the majority of the 42 studied elements will change in the future. However, it is not always possible to predict the direction of this change, which shows that more multi-element biogeochemical studies are needed to increase our understanding of the consequences of a changing climate for the Arctic environment.

The use of lake sedimentary DNA to track the long-term changes in both terrestrial and aquatic biota is a rapidly advancing field in paleoecological research. Although largely applied nowadays, knowledge gaps remain in this field and there is therefore still research to be conducted to ensure the reliability of the sedimentary DNA signal. Building on the most recent literature and seven original case studies, we synthesize the state-of-the-art analytical procedures for effective sampling, extraction, amplification, quantification and/or generation of DNA inventories from sedimentary ancient DNA (sedaDNA) via high-throughput sequencing technologies. We provide recommendations based on current knowledge and best practises.

On the annual and interannual scales, lake microbial communities are known to be heavily influenced by environmental conditions both in the lake and in its terrestrial surroundings. How-ever, the influence of landscape setting and environmental change on shaping these communities over a longer (millennial) timescale is rarely studied. Here, we applied an 18S metabarcoding approach to DNA preserved in Holocene sediment records from two pairs of co‐located Swedish mountain lakes. Our data revealed that the microbial eukaryotic communities were strongly influenced by catchment characteristics rather than location. More precisely, the microbial communities from the two bedrock lakes were largely dominated by unclassified Alveolata, while the peatland lakes showed a more diverse microbial community, with Ciliophora, Chlorophyta and Chytrids among the more predominant groups. Furthermore, for the two bedrock‐dominated lakes—where the oldest DNA samples are dated to only a few hundred years after the lake formation—certain Alveolata, Chlorophytes, Stramenopiles and Rhizaria taxa were found prevalent throughout all the sediment profiles. Our work highlights the importance of species sorting due to landscape setting and the persistence of microbial eukaryotic diversity over millennial timescales in shaping modern lake microbial communities.

Climate change is predicted to have far reaching consequences for the mobility of carbon in arctic landscapes. On a regional scale, carbon cycling is highly dependent on interactions between terrestrial and aquatic parts of a catchment. Despite this, studies that integrate the terrestrial and aquatic systems and study entire catchments using site-specific data are rare. In this work, we use data partly published by Lindborg et al. (2016a) to calculate a whole-catchment carbon mass-balance budget for a periglacial catchment in West Greenland. Our budget shows that terrestrial net primary production is the main input of carbon (99% of input), and that most carbon leaves the system through soil respiration (90% of total export/storage). The largest carbon pools are active layer soils (53% of total carbon stock or 13 kg C m (2)), permafrost soils (30% of total carbon stock or 7.6 kg C m (2)) and lake sediments (13% of total carbon stock or 10 kg C m (2)). Hydrological transport of carbon from the terrestrial to aquatic system is lower than in wetter climates, but the annual input of 4100 kg C yr (1) (or 3.5 g C m (2) yr (1)) that enters the lake via runoff is still three times larger than the eolian input of terrestrial carbon. Due to the dry conditions, the hydrological export of carbon from the catchment is limited (5% of aquatic export/storage or 0.1% of total export/storage). Instead, CO2 evasion from the lake surface and sediment burial accounts for 57% and 38% of aquatic export/storage, respectively (or 0.8% and 0.5% of total export/storage), and Two-Boat Lake acts as a net source of carbon to the atmosphere. The limited export of carbon to downstream water bodies make our study system different from wetter arctic environments, where hydrological transport is an important export pathway for carbon.

Chlorophyll is frequently used as a proxy for autochthonous production in lakes. This use of chlorophyll concentrations in sediments to infer historical changes in lake primary production relies heavily on the assumption that preservation is sufficient to reflect the productivity in a meaningful way. In this study, we use a series of freeze cores from a lake with annually laminated sediments to assess how reliable down-core trends in chlorophyll are, and to what extent chlorophyll is degraded in the sediment. A striking consistency in the down-core chlorophyll trends in four sediment cores collected in different years (1983, 1992, 2002 and 2010) shows that the sediment preserves a consistent chlorophyll signal over longer timescales. However, there are also clear signs that diagenetic processes within the sediment affect the chlorophyll concentration in sediment layers younger than 10-15 years. This implies that care is needed when interpreting chlorophyll trends in recent sediments. Further, our data show that high-performance liquid chromatography (HPLC) and visible reflectance spectroscopy (VRS) detect similar chlorophyll concentrations for recently dried samples. A third analytical technique, pyrolysis-gas chromatography/mass spectrometry, which provides semi-quantitative values for chlorophyll, also produce a temporal trend that is highly correlated with data from the two quantitative techniques. For samples that have been stored dried at room temperature for several years there is, however, a large discrepancy between the two quantitative techniques. The VRS method is more robust with regards to degradation during storage, while HPLC results demonstrate clear storage effects.