Winter accumulation of methane and its variable timing of release from thermokarst lakes in subarctic peatlands.

Previous studies of thermokarst lakes have drawn attention to the potential for accumulationof CH4under the ice and its subsequent release in spring; however, such observations have not beenavailable for thermokarst waters in carbon‐rich peatlands. Here we undertook a winter profiling of fiveblack‐water lakes located on eroding permafrost peatlands in subarctic Quebec for comparison with summerprofiles and used a 2‐year data set of automated water temperature, conductivity, and oxygenmeasurements to evaluate how the annual mixing dynamics may affect the venting of greenhouse gases tothe atmosphere. All of the sampled lakes contained large amounts of dissolved CH4under their winter icecover. These sub‐ice concentrations were up to 5 orders of magnitude above air equilibrium (i.e., theexpected concentration in lake water equilibrated with the atmosphere), resulting in calculated emissionrates at ice breakup that would be 1–2 orders of magnitude higher than midsummer averages. The amount ofCO2dissolved in the water column was reduced in winter, and the estimated ratio of potential diffusive CO2to CH4emission in spring was half the measured summer ratio, suggesting a seasonal shift inmethanogenesis and bacterial activity. All surface lake ice contained bubbles of CH4and CO2, but thisamounted to <5% of the total amount of the dissolved CH4and CO2in the corresponding lake water column.The continuous logging records suggested that lake morphometry may play a role in controlling the timingand extent of CH4and CO2release from the water column to the atmosphere

Benthic Cyanobacterial Mats in the High Arctic: Multi-Layer Structure and Fluorescence Responses to Osmotic Stress

Cyanobacterial mats are often a major biological component of extreme aquatic ecosystems, and in polar lakes and streams they may account for the dominant fraction of total ecosystem biomass and productivity. In this study we examined the vertical structure and physiology of Arctic microbial mats relative to the question of how these communities may respond to ongoing environmental change. The mats were sampled from Ward Hunt Lake (83°5.297′N, 74°9.985′W) at the northern coast of Arctic Canada, and were composed of three visibly distinct layers. Microsensor profiling showed that there were strong gradients in oxygen within each layer, with an overall decrease from 100% saturation at the mat surface to 0%, at the bottom, accompanied by an increase of 0.6 pH units down the profile. Gene clone libraries (16S rRNA) revealed the presence of Oscillatorian sequences throughout the mat, while Nostoc related species dominated the two upper layers, and Nostocales and Synechococcales sequences were common in the bottom layer. High performance liquid chromatography analyses showed a parallel gradient in pigments, from high concentrations of UV-screening scytonemin in the upper layer to increasing zeaxanthin and myxoxanthin in the bottom layer, and an overall shift from photoprotective to photosynthetic carotenoids down the profile. Climate change is likely to be accompanied by lake level fluctuations and evaporative concentration of salts, and thus increased osmotic stress of the littoral mat communities. To assess the cellular capacity to tolerate increasing osmolarity on physiology and cell membrane integrity, mat sections were exposed to a gradient of increasing salinities, and PAM measurements of in vivo chlorophyll fluorescence were made to assess changes in maximum quantum yield. The results showed that the mats were tolerant of up to a 46-fold increase in salinity. These features imply that cyanobacterial mats are resilient to ongoing climate change, and that in the absence of major biological perturbations, these vertically structured communities will continue to be a prominent feature of polar aquatic ecosystems

Dynamique et modélisation de l’oxygène dissous en rivière

La concentration en oxygène dissous en milieu fluvial varie selon un cycle diurne (24 h) qu’il est essentiel de considérer dans l’évaluation de l’état d’oxygénation d’un cours d’eau. En principe, seules des mesures en continu recueillies au cours de cycles de 24 h permettent d’évaluer correctement l’état d’oxygénation d’une rivière, ce que, en pratique, les contraintes logistiques et budgétaires ne permettent pas de réaliser. Le présent article vise à faire la synthèse des connaissances sur les facteurs de contrôle et la modélisation des variations diurnes de la concentration en oxygène dissous en rivière. Parmi les facteurs biologiques et physico-chimiques, les activités autotrophe et hétérotrophe sont les facteurs dominants responsables des variations diurnes de l’oxygène dissous. Certains modèles de qualité de l’eau permettent de modéliser la teneur en oxygène et, dans certains cas, la variation diurne. Toutefois, ces modèles sont souvent complexes, d’utilisation ardue et impliquent la mesure directe sur des cycles de 24 h des variables qui régissent la concentration en oxygène dans le milieu. Une démarche est proposée pour l’élaboration d’un modèle et a été appliquée dans une étude de cas réalisée dans la rivière Saint-Charles (Québec, Canada). Un modèle simple (une fonction sinusoïdale) dont les paramètres ont été corrélés à la température et à la concentration moyenne en nitrate a permis de générer des valeurs simulées d’oxygène dissous très proches des valeurs observées in situ. Un modèle alternatif utilisant des valeurs ponctuelles de température et de concentration de nitrate a donné des résultats équivalents. L’approche proposée constitue donc une alternative simple et pratique à la mesure en continu de l’oxygène et permet une évaluation plus réaliste de l’état réel d’oxygénation d’une rivière que la prise de mesures ponctuelles.In rivers, dissolved oxygen concentrations typically show diel variations with maximum values during daytime and minimum values at night. The diel cycle must be taken into account when assessing the state of oxygenation of a watercourse. However, in water quality monitoring programs, dissolved oxygen concentrations are usually obtained from single measurements taken during daytime. The resulting data do not represent the real overall oxygen levels of a watercourse and thus can lead to erroneous conclusions regarding the oxygen status of a river. Continuous data collected over 24‑hour cycles are required for an accurate oxygen status assessment, but in practice, logistic and budget constraints do not allow such samplings. Modelling can be a convenient alternative to direct measurements. However, the water quality models that take into account the diel cycle of oxygen are generally complex to run. The purpose of this study was to review the information relating to the dynamics and the modelling of the diel variations in dissolved oxygen in rivers and to apply a simple model in a case study involving dissolved oxygen, nutrients, temperature and chlorophyll a data collected over 24‑hour cycles in the St. Charles River near Quebec City (Canada).Photosynthesis by algae, both benthic and planktonic, as well as by macrophytes, is an important and sometimes dominant factor in the oxygen budget of a river. Sediments and heterotrophic activity by bacteria, particularly in rivers receiving important loads of wastewaters, can be important sinks for oxygen. Temperature determines the solubility of oxygen, thereby directly influencing oxygen concentrations. Diel variations in oxygen thus reflect diel variations in temperature. Temperature also has an effect on biological processes such as respiration and photosynthesis. Reaeration varies not only with temperature, but also with the type of flow (laminar vs. turbulent) and current velocity. In rivers with important slopes and current velocities, reaeration can be sufficient to make up for oxygen losses due to high heterotrophic activity. Nevertheless, light is the first causative factor for the diel variations in oxygen, determining both autotrophic activity and water temperature. However, suspended matter in the water column reduces light penetration. Higher levels of suspended matter result in lower levels of photosynthesis and oxygen production. The sudden or large influx of runoff waters after heavy rain or snowmelt can also have an important impact on the oxygen budget of a river. Finally, chemical factors can have an influence on the diel variations in oxygen: nutrient inputs, in particular, can stimulate the rate of photosynthesis and oxygen production. Overall, oxygen dynamics are determined by the relative importance of biological, physical and chemical factors, which vary in time and space. However, biological processes often dominate over the other causative factors affecting diel variations in oxygen. In temperate climates, biological processes have a controlling function only during warmer months, with temperature and flow being the dominant controlling factors during colder months.Water quality models that take into account diel variations in oxygen are often designed to assess primary production and respiration. These models are based on either ODUM’s (1956) concept of oxygen curves or on direct and continuous measurements of dissolved oxygen. Periodic functions or Fourier series are also used to simulate diel variations in oxygen. The widely used USEPA QUAL2e model predicts diel variations in oxygen from different measurements including light intensity and from computation of the rates of photosynthesis and respiration. Several other modelling approaches use various combinations of indirect methods to predict the variations in oxygen, based on light intensity, algal biomass, primary production, and reaeration. Specific models are sometimes necessary due to particular regional characteristics or environmental issues.In general, water quality models designed for water management purposes are complex and require the measurement of a large number of parameters, which necessitates elaborate and costly logistics. Estimating the parameters controlling the oxygen budget in a river thus ends up being more time and labour consuming than the direct measurement of diel variations in oxygen. A simpler model leading to the estimation of the concentrations and the amplitude of the variations of dissolved oxygen was developed and applied to the St. Charles River.The St. Charles River flows from the Laurentians north of Quebec City to the St. Lawrence River. The vast upper watershed is mostly forested, but the lower part is heavily urbanized. Two stations were located in the upper watershed and one in the section of the river within Quebec City. Water quality was excellent at the upstream stations and poor at the downstream station, due to wastewater inputs and low flow rates. Sampling was carried out in the months of July and August of 1996 and 1997. Physico-chemical and light measurements were made every two hours for periods of 24 hours. Nutrient and chlorophyll a samples were collected every four hours. Small diel variations in oxygen (amplitude: 1.48 mg/L) were observed at the upstream station, while much larger ones (amplitude: 4.23 mg/L) were measured at the downstream station. Modelling of the diel variations in oxygen was carried out using a sine function. Oxygen concentrations over 24 hours were successfully predicted from average concentration of dissolved oxygen, amplitude, and phase of the cycle. Average oxygen concentrations and amplitude can be derived from physico-chemical and/or biological variables easily measured in standard water quality monitoring programs. Average oxygen concentrations showed a very strong correlation with temperature (r2 = 0.91) and amplitude of oxygen level variations was strongly correlated with average nitrate concentration (r2 = 0.58). These relations were used in the sine function and resulted in significantly correlated modelled and measured oxygen concentrations for six of the seven cycles. Overall correlation between modelled and observed values was high (r2 = 0.77). Modelled values obtained with single measurements of temperature (taken at 2:30 P.M.) and nitrate concentration (which shows no diel variation) were also highly correlated with observed data (r2 = 0.75). Absolute and relative bias as well as root-mean-square error also showed the validity and the equivalence of the two approaches.This study shows that simple models based on available water quality data may generate realistic oxygen values over 24‑hour cycles. These models would be a valuable diagnostic and decision making tool for the management of water quality in rivers

A quantum study of multi-bit phase coding for optical storage

We propose a scheme which encodes information in both the longitudinal and spatial transverse phases of a continuous-wave optical beam. A split detector-based interferometric scheme is then introduced to optimally detect both encoded phase signals. In contrast to present-day optical storage devices, our phase coding scheme has an information storage capacity which scales with the power of the read-out optical beam. We analyse the maximum number of encoding possibilities at the shot noise limit. In addition, we show that using squeezed light, the shot noise limit can be overcome and the number of encoding possibilities increased. We discuss a possible application of our phase coding scheme for increasing the capacities of optical storage devices.Comment: 8 pages, 7 figures (Please email author for a PDF file if the manuscript does not turn out properly

Water column gradients beneath the summer ice of a High Arctic freshwater lake as indicators of sensitivity to climate change

Ice cover persists throughout summer over many lakes at extreme polar latitudes but is likely to become increasingly rare with ongoing climate change. Here we addressed the question of how summer ice-cover affects the underlying water column of Ward Hunt Lake, a freshwater lake in the Canadian High Arctic, with attention to its vertical gradients in limnological properties that would be disrupted by ice loss. Profiling in the deepest part of the lake under thick mid-summer ice revealed a high degree of vertical structure, with gradients in temperature, conductivity and dissolved gases. Dissolved oxygen, nitrous oxide, carbon dioxide and methane rose with depth to concentrations well above air-equilibrium, with oxygen values at >150% saturation in a mid water column layer of potential convective mixing. Fatty acid signatures of the seston also varied with depth. Benthic microbial mats were the dominant phototrophs, growing under a dim green light regime controlled by the ice cover, water itself and weakly colored dissolved organic matter that was mostly autochthonous in origin. In this and other polar lakes, future loss of mid-summer ice will completely change many water column properties and benthic light conditions, resulting in a markedly different ecosystem regime

Genomic evidence of functional diversity in DPANN archaea, from oxic species to anoxic vampiristic consortia

DPANN archaea account for half of the archaeal diversity of the biosphere, but with few cultivated representatives, their metabolic potential and environmental functions are poorly understood. The extreme geochemical and environmental conditions in meromictic ice-capped Lake A, in the Canadian High Arctic, provided an isolated, stratified model ecosystem to resolve the distribution and metabolism of uncultured aquatic DPANN archaea living across extreme redox and salinity gradients, from freshwater oxygenated conditions, to saline, anoxic, sulfidic waters. We recovered 28 metagenome-assembled genomes (MAGs) of DPANN archaea that provided genetic insights into their ecological function. Thiosulfate oxidation potential was detected in aerobic Woesearchaeota, whereas diverse metabolic functions were identified in anaerobic DPANN archaea, including degradation and fermentation of cellular compounds, and sulfide and polysulfide reduction. We also found evidence for “vampiristic” metabolism in several MAGs, with genes coding for pore-forming toxins, peptidoglycan degradation, and RNA scavenging. The vampiristic MAGs co-occurred with other DPANNs having complementary metabolic capacities, leading to the possibility that DPANN form interspecific consortia that recycle microbial carbon, nutrients and complex molecules through a DPANN archaeal shunt, adding hidden novel complexity to anaerobic microbial food webs

Size-fractionated microbiome structure in subarctic rivers and a coastal plume across DOC and salinity gradients

Little is known about the microbial diversity of rivers that flow across the changing subarctic landscape. Using amplicon sequencing (rRNA and rRNA genes) combined with HPLC pigment analysis and physicochemical measurements, we investigated the diversity of two size fractions of planktonic Bacteria, Archaea and microbial eukaryotes along environmental gradients in the Great Whale River (GWR), Canada. This large subarctic river drains an extensive watershed that includes areas of thawing permafrost, and discharges into southeastern Hudson Bay as an extensive plume that gradually mixes with the coastal marine waters. The microbial communities differed by size-fraction (separated with a 3-μm filter), and clustered into three distinct environmental groups: (1) the GWR sites throughout a 150-km sampling transect; (2) the GWR plume in Hudson Bay; and (3) small rivers that flow through degraded permafrost landscapes. There was a downstream increase in taxonomic richness along the GWR, suggesting that sub-catchment inputs influence microbial community structure in the absence of sharp environmental gradients. Microbial community structure shifted across the salinity gradient within the plume, with changes in taxonomic composition and diversity. Rivers flowing through degraded permafrost had distinct physicochemical and microbiome characteristics, with allochthonous dissolved organic carbon explaining part of the variation in community structure. Finally, our analyses of the core microbiome indicated that while a substantial part of all communities consisted of generalists, most taxa had a more limited environmental range and may therefore be sensitive to ongoing change