Diversification of Nitrogen Sources in Various Tundra Vegetation Types in the High Arctic

Low nitrogen availability in the high Arctic represents a major constraint for plant growth, which limits the tundra capacity for carbon retention and determines tundra vegetation types. The limited terrestrial nitrogen (N) pool in the tundra is augmented significantly by nesting seabirds, such as the planktivorous Little Auk (Alle alle). Therefore, N delivered by these birds may significantly influence the N cycling in the tundra locally and the carbon budget more globally. Moreover, should these birds experience substantial negative environmental pressure associated with climate change, this will adversely influence the tundra N-budget. Hence, assessment of bird-originated N-input to the tundra is important for understanding biological cycles in polar regions. This study analyzed the stable nitrogen composition of the three main N-sources in the High Arctic and in numerous plants that access different N-pools in ten tundra vegetation types in an experimental catchment in Hornsund (Svalbard). The percentage of the total tundra N-pool provided by birds, ranged from 0–21% in Patterned-ground tundra to 100% in Ornithocoprophilous tundra. The total N-pool utilized by tundra plants in the studied catchment was built in 36% by birds, 38% by atmospheric deposition, and 26% by atmospheric N₂-fixation. The stable nitrogen isotope mixing mass balance, in contrast to direct methods that measure actual deposition, indicates the ratio between the actual N-loads acquired by plants from different N-sources. Our results enhance our understanding of the importance of different N-sources in the Arctic tundra and the used methodological approach can be applied elsewhere.     

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of ¹⁴C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of ¹⁴C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.     

Distributional patterns of fishes in an Alaskan Arctic Lagoon

Summary In summer, the fish community of Simpson Lagoon and adjacent coastal waters of the Beaufort Sea was dominated by two marine species (Arctic cod, fourhorn sculpin) and three anadromous species (Arctic and least cisco, Arctic char). The anadromous species remained in the relatively warm and brackish waters near shore and demonstrated an affinity for shoreline edges, particularly the mainland shoreline where species occurrence and catch per unit effort (CPUE) were highest. Spatial segregation was low, presumably reflecting the migratory nature of these species. Marine species were less restricted to nearshore waters in summer and were typically the only species present in winter because anadromous species return to rivers, lakes and deltas to spawn or overwinter. Winter CPUE was low and consisted primarily of Arctic cod and fourhorn sculpin.     

Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth

We compared on eight dates during the ice-free period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow ( < 2.5 m), intermediate in depth (2.5 m <  < 4.5 m), and deep ( > 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always >70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO₃ ⁻–N + NH₄ ⁺–N) were consistently below the detection limit (0.05 μmol l⁻¹) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 μmol l⁻¹), total-P (0.29 μmol l⁻¹), and total-N (33 μmol l⁻¹), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 μg l⁻¹ and 0.7–3.8 μmol C l⁻¹ day⁻¹. Overall, however, increased resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m⁻² and 3.2–5.8 mmol C m⁻² day⁻¹. Average values of epipelic chl a ranged from 131 to 549 mg m⁻² for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m⁻² day⁻¹) than in intermediate and deep lakes (3.4 and 2.4 mmol C m⁻² day⁻¹). Total primary production (6.7–15.4 mmol C m⁻² day⁻¹) and percent contribution of the epipelon (31–66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes. Handling editor: L. Naselli-Flores     

Scaling an Instantaneous Model of Tundra NEE to the Arctic Landscape

We scale a model of net ecosystem CO₂ exchange (NEE) for tundra ecosystems and assess model performance using eddy covariance measurements at three tundra sites. The model, initially developed using instantaneous (seconds–minutes) chamber flux (~m²) observations, independently represents ecosystem respiration (ER) and gross primary production (GPP), and requires only temperature (T), photosynthetic photon flux density (I ₀), and leaf area index (L) as inputs. We used a synthetic data set to parameterize the model so that available in situ observations could be used to assess the model. The model was then scaled temporally to daily resolution and spatially to about 1 km² resolution, and predicted values of NEE, and associated input variables, were compared to observations obtained from eddy covariance measurements at three flux tower sites over several growing seasons. We compared observations to modeled NEE calculated using T and I ₀ measured at the towers, and L derived from MODIS data. Cumulative NEE estimates were within 17 and 11% of instrumentation period and growing season observations, respectively. Predictions improved when one site-year experiencing anomalously dry conditions was excluded, indicating the potential importance of stomatal control on GPP and/or soil moisture on ER. Notable differences in model performance resulted from ER model formulations and differences in how L was estimated. Additional work is needed to gain better predictive ability in terms of ER and L. However, our results demonstrate the potential of this model to permit landscape scale estimates of NEE using relatively few and simple driving variables that are easily obtained via satellite remote sensing.     

The Quantitative Importance of the Benthic Microfauna of an Arctic Tundra Pond

The microfauna (protozoa and micrometazoa) and bacteria and microalgae of the sediment of an arctic tundrapond at Barrow, Alaska were quantified through the summer. Very small protozoans (i.e. zooflagellates) and burrowing micrometazoans were found to be the most important components of the microfauna, whereas ciliates play a smaller role. This composition of microfauna is attributed to the mechanical property of the detrital sediment.Through a combination of laboratory experiments and field observations the grazing rates of protozoa for bacteria and microalgae in the field were estimated; protozoa were found to be important as bacterial grazers, but they consume only a modest fraction of the microalgae. An estimate of the grazing activity of the micrometazoans, based on indirect considerations, is also offered. It is of the same magnitude as that of protozoans but microalgae play a relatively larger role for the former. Based on previous studies on the pond it is estimated that microfaunal grazing constitutes about 30% of that of the detritus feeding macrofauna and that all categories of grazers together do not consume the total microbial production. The reasons for this results and the fate of this surplus production of algae and bacteria are discussed.     

Nutrient Addition Prompts Rapid Destabilization of Organic Matter in an Arctic Tundra Ecosystem

Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate.     

长白山苔原植被变化对土壤呼吸影响研究

气候变化导致长白山苔原由灌木苔原向灌草苔原演化,对土壤呼吸及碳循环造成了重要影响。为了明确植被变化对苔原土壤呼吸的影响,该研究选取了长白山苔原典型的群落,测定分析了不同草本植物盖度下的土壤呼吸的季节动态变化及差异。结果表明：(1)在生长季,3个群落下不同变化阶段样地的土壤呼吸速率均有明显的动态变化,均呈单峰型变化特征;草本植物盖度增加没有改变土壤呼吸的季节动态变化趋势。(2)草本植物盖度增加对土壤呼吸速率有显著影响,随着草本植物盖度的增加,土壤呼吸速率也逐渐增大。(3)不同植物群落下土壤呼吸不同,在草本植物盖度相同的条件下土壤呼吸速率依次为：牛皮杜鹃 小叶章群落>牛皮杜鹃 地榆群落>笃斯越桔 苔草群落。(4)不同群落草本植物盖度增加对土壤呼吸的增速效应不同,牛皮杜鹃 小叶章群落的土壤呼吸增速最快,笃斯越桔 苔草群落的次之,牛皮杜鹃 地榆群落最小;草本植物盖度的增加也使3个群落之间土壤呼吸的差值出现明显的变化。     

Vegetation-Associated Impacts on Arctic Tundra Bacterial and Microeukaryotic Communities

The Arctic is experiencing rapid vegetation changes, such as shrub and tree line expansion, due to climate warming, as well as increased wetland variability due to hydrological changes associated with permafrost thawing. These changes are of global concern because changes in vegetation may increase tundra soil biogeochemical processes that would significantly enhance atmospheric CO₂ concentrations. Predicting the latter will at least partly depend on knowing the structure, functional activities, and distributions of soil microbes among the vegetation types across Arctic landscapes. Here we investigated the bacterial and microeukaryotic community structures in soils from the four principal low Arctic tundra vegetation types: wet sedge, birch hummock, tall birch, and dry heath. Sequencing of rRNA gene fragments indicated that the wet sedge and tall birch communities differed significantly from each other and from those associated with the other two dominant vegetation types. Distinct microbial communities were associated with soil pH, ammonium concentration, carbon/nitrogen (C/N) ratio, and moisture content. In soils with similar moisture contents and pHs (excluding wet sedge), bacterial, fungal, and total eukaryotic communities were correlated with the ammonium concentration, dissolved organic nitrogen (DON) content, and C/N ratio. Operational taxonomic unit (OTU) richness, Faith''s phylogenetic diversity, and the Shannon species-level index (H′) were generally lower in the tall birch soil than in soil from the other vegetation types, with pH being strongly correlated with bacterial richness and Faith''s phylogenetic diversity. Together, these results suggest that Arctic soil feedback responses to climate change will be vegetation specific not just because of distinctive substrates and environmental characteristics but also, potentially, because of inherent differences in microbial community structure.     

Two mechanisms of aquatic and terrestrial habitat change along an Alaskan Arctic coastline

Arctic habitats at the interface between land and sea are particularly vulnerable to climate change. The northern Teshekpuk Lake Special Area (N-TLSA), a coastal plain ecosystem along the Beaufort Sea in northern Alaska, provides habitat for migratory waterbirds, caribou, and potentially, denning polar bears. The 60-km coastline of N-TLSA is experiencing increasing rates of coastline erosion and storm surge flooding far inland resulting in lake drainage and conversion of freshwater lakes to estuaries. These physical mechanisms are affecting upland tundra as well. To better understand how these processes are affecting habitat, we analyzed long-term observational records coupled with recent short-term monitoring. Nearly the entire coastline has accelerating rates of erosion ranging from 6 m/year from 1955 to 1979 and most recently peaking at 17 m/year from 2007 to 2009, yet an intensive monitoring site along a higher bluff (3–6 masl) suggested high interannual variability. The frequency and magnitude of storm events appears to be increasing along this coastline and these patterns correspond to a greater number of lake tapping and flooding events since 2000. For the entire N-TLSA, we estimate that 6% of the landscape consists of salt-burned tundra, while 41% is prone to storm surge flooding. This offset may indicate the relative frequency of low-magnitude flood events along the coastal fringe. Monitoring of coastline lakes confirms that moderate westerly storms create extensive flooding, while easterly storms have negligible effects on lakes and low-lying tundra. This study of two interacting physical mechanisms, coastal erosion and storm surge flooding, provides an important example of the complexities and data needs for predicting habitat change and biological responses along Arctic land–ocean interfaces.     

Plant Species Composition and Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra

Climate warming is expected to have a large impact on plant species composition and productivity in northern latitude ecosystems. Warming can affect vegetation communities directly through temperature effects on plant growth and indirectly through alteration of soil nutrient availability. In addition, warming can cause permafrost to thaw and thermokarst (ground subsidence) to develop, which can alter the structure of the ecosystem by altering hydrological patterns within a site. These multiple direct and indirect effects of permafrost thawing are difficult to simulate in experimental approaches that often manipulate only one or two factors. Here, we used a natural gradient approach with three sites to represent stages in the process of permafrost thawing and thermokarst. We found that vascular plant biomass shifted from graminoid-dominated tundra in the least disturbed site to shrub-dominated tundra at the oldest, most subsided site, whereas the intermediate site was co-dominated by both plant functional groups. Vascular plant productivity patterns followed the changes in biomass, whereas nonvascular moss productivity was especially important in the oldest, most subsided site. The coefficient of variation for soil moisture was higher in the oldest, most subsided site suggesting that in addition to more wet microsites, there were other microsites that were drier. Across all sites, graminoids preferred the cold, dry microsites whereas the moss and shrubs were associated with the warm, moist microsites. Total nitrogen contained in green plant biomass differed across sites, suggesting that there were increases in soil nitrogen availability where permafrost had thawed.     

Tundra be dammed: Beaver colonization of the Arctic

Increasing air temperatures are changing the arctic tundra biome. Permafrost is thawing, snow duration is decreasing, shrub vegetation is proliferating, and boreal wildlife is encroaching. Here we present evidence of the recent range expansion of North American beaver (Castor canadensis) into the Arctic, and consider how this ecosystem engineer might reshape the landscape, biodiversity, and ecosystem processes. We developed a remote sensing approach that maps formation and disappearance of ponds associated with beaver activity. Since 1999, 56 new beaver pond complexes were identified, indicating that beavers are colonizing a predominantly tundra region (18,293 km²) of northwest Alaska. It is unclear how improved tundra stream habitat, population rebound following overtrapping for furs, or other factors are contributing to beaver range expansion. We discuss rates and likely routes of tundra beaver colonization, as well as effects on permafrost, stream ice regimes, and freshwater and riparian habitat. Beaver ponds and associated hydrologic changes are thawing permafrost. Pond formation increases winter water temperatures in the pond and downstream, likely creating new and more varied aquatic habitat, but specific biological implications are unknown. Beavers create dynamic wetlands and are agents of disturbance that may enhance ecosystem responses to warming in the Arctic.     

Effect of Freeze-Thaw Cycles on Bacterial Communities of Arctic Tundra Soil

The effect of freeze-thaw (FT) cycles on Arctic tundra soil bacterial community was studied in laboratory microcosms. FT-induced changes to the bacterial community were followed over a 60-day period by terminal restriction fragment length polymorphism (T-RFLP) profiles of amplified 16S rRNA genes and reverse transcribed 16S rRNA. The main phylotypes of the active, RNA-derived bacterial community were identified using clone analysis. Non-metric multidimensional scaling ordination of the T-RFLP profiles indicated some shifts in the bacterial communities after three to five FT cycles at −2, −5, and −10°C as analyzed both from the DNA and rRNA. The dominating T-RFLP peaks remained the same, however, and only slight variation was generally detected in the relative abundance of the main T-RF sizes of either DNA or rRNA. T-RFLP analysis coupled to clone analysis of reverse transcribed 16S rRNA indicated that the initial soil was dominated by members of Bacteroidetes, Acidobacteria, Alpha-, Beta-, and Gammaproteobacteria. The most notable change in the rRNA-derived bacterial community was a decrease in the relative abundance of a Betaproteobacteria-related phylotype after the FT cycles. This phylotype decreased, however, also in the control soil incubated at constant +5°C suggesting that the decrease was not directly related to FT sensitivity. The results indicate that FT caused only minor changes in the bacterial community structure.     

Methanotrophs of the Psychrophilic Microbial Community of the Russian Arctic Tundra

In tundra, at a low temperature, there exists a slowly developing methanotrophic community. Methane-oxidizing bacteria are associated with plants growing at high humidity, such as sedge and sphagnum; no methanotrophs were found in polytrichous and aulacomnious mosses and lichens, typical of more arid areas. The methanotrophic bacterial community inhabits definite soil horizons, from moss dust to peat formed from it. The potential ability of the methanotrophic community to oxidize methane at 5°C enhances with the depth of the soil profile in spite of the decreasing soil temperature. The methanotrophic community was found to gradually adapt to various temperatures due to the presence of different methane-oxidizing bacteria in its composition. Depending on the temperature and pH, different methanotrophs occupy different econiches. Within a temperature range from 5 to 15°C, three morphologically distinct groups of methanotrophs could be distinguished. At pH 5–7 and 5–15°C, forms morphologically similar to Methylobacter psychrophilus predominated, whereas at the acidic pH 4–6 and 10–15°C, bipolar cells typical of Methylocella palustris were mostly found. The third group of methanotrophic bacteria growing at pH 5–7 and 5–10°C was represented by a novel methanotroph whose large coccoid cells had a thick mucous capsule.     

Multivariate Relationships between Snowmelt and Plant Distributions in the High Arctic Tundra

We investigated multivariate relationships among snowmelt, soil physicochemical properties and the distribution patterns of Arctic tundra vegetation. Seven dominant species were placed in three groups (Veg-1, 2, 3) based on niche overlap (Pianka’s Index) and ordination method, and a partial least squares path model was applied to estimate complex multivariate relationships of four latent variables on the abundance and richness of plant species. The abundance of Veg-1 (Luzula confusa and Salix polaris) was positively correlated with early snowmelt time, high soil nutrients and dense moss cover, but the abundance of Veg-2 (Saxifraga oppositifolia, Bistorta vivipara and Silene acaulis) was negatively correlated with these three variables. Plant richness was positively associated with early snowmelt and hydrological properties. Our results indicate that the duration of the snowpack can directly influence soil chemical properties and plant distribution. Furthermore, plant species richness was significantly affected by snow melt time in addition to soil moisture and moss cover. We predict that L. confusa and S. polaris may increase in abundance in response to early snowmelt and increased soil moisture-nutrient availability, which may be facilitated by climate change. Other forb species in dry and infertile soil may decrease in abundance in response to climate change, due to increasingly unfavourable environmental conditions and competition with mosses.     

Feeding ecology of phocid seals and some walrus in the Alaskan and Canadian Arctic as determined by stomach contents and stable isotope analysis

Feeding habits of ringed (Phoca hispida), bearded (Erignathus barbatus), spotted (Phoca largha) and ribbon (Phoca fasciata) seals and walrus (Odobenus rosmarus) were studied using stomach contents and stable carbon and nitrogen isotopes. Bearded seals fed benthically, primarily crustaceans and mollusks. Both zooplankton and fish were significant prey for ringed seals, while fish was principal spotted seal prey. Few gastric contents were available from ribbon seals. δ¹⁵N was positively correlated with age in ribbon seals and δ¹³C was positively correlated with age in ringed and ribbon seals. δ¹⁵N was highest in spotted seals, in agreement with their fish-dominated diet. δ¹⁵N was not different between Alaskan-harvested ringed and bearded seals, while δ¹⁵N was lowest in ribbon seals and walrus. Carbon-13 was most enriched in bearded seals and walrus reflecting benthic ecosystem use. Canadian ringed seals were depleted in ¹³C compared to Alaskan pinnipeds, likely because of Beaufort Sea versus Chukchi and Bering seas influence.     

Vegetation Type Dominates the Spatial Variability in CH<Subscript>4</Subscript> Emissions Across Multiple Arctic Tundra Landscapes

Methane (CH₄) emissions from Arctic tundra are an important feedback to global climate. Currently, modelling and predicting CH₄ fluxes at broader scales are limited by the challenge of upscaling plot-scale measurements in spatially heterogeneous landscapes, and by uncertainties regarding key controls of CH₄ emissions. In this study, CH₄ and CO₂ fluxes were measured together with a range of environmental variables and detailed vegetation analysis at four sites spanning 300 km latitude from Barrow to Ivotuk (Alaska). We used multiple regression modelling to identify drivers of CH₄ flux, and to examine relationships between gross primary productivity (GPP), dissolved organic carbon (DOC) and CH₄ fluxes. We found that a highly simplified vegetation classification consisting of just three vegetation types (wet sedge, tussock sedge and other) explained 54% of the variation in CH₄ fluxes across the entire transect, performing almost as well as a more complex model including water table, sedge height and soil moisture (explaining 58% of the variation in CH₄ fluxes). Substantial CH₄ emissions were recorded from tussock sedges in locations even when the water table was lower than 40 cm below the surface, demonstrating the importance of plant-mediated transport. We also found no relationship between instantaneous GPP and CH₄ fluxes, suggesting that models should be cautious in assuming a direct relationship between primary production and CH₄ emissions. Our findings demonstrate the importance of vegetation as an integrator of processes controlling CH₄ emissions in Arctic ecosystems, and provide a simplified framework for upscaling plot scale CH₄ flux measurements from Arctic ecosystems.     

Long-Term Release of Carbon Dioxide from Arctic Tundra Ecosystems in Alaska

Releases of the greenhouse gases carbon dioxide (CO₂) and methane (CH₄) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current net carbon (C) balance of terrestrial arctic ecosystems is unknown. Recent studies suggest that these ecosystems are sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO₂ fluxes over the full annual cycle. Here, we describe a pattern of CO₂ loss based on the longest continuous record of direct measurements of CO₂ fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. We also report on a shorter time series of continuous measurements from a third ecosystem, tussock tundra. The amount of CO₂ loss from both heath and wet sedge ecosystems was related to the timing of freeze-up of the soil active layer in the fall. Wet sedge tundra lost the most CO₂ during the anomalously warm autumn periods of September–December 2013–2015, with CH₄ emissions contributing little to the overall C budget. Losses of C translated to approximately 4.1 and 1.4% of the total soil C stocks in active layer of the wet sedge and heath tundra, respectively, from 2008 to 2015. Increases in air temperature and soil temperatures at all depths may trigger a new trajectory of CO₂ release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.